Ultimate Drill Bit Size Conversion Chart: Metric, Fractional, Wire & Letter Sizes

Drill bit sizing should be simple. Then you open a drill index and find millimetres, fractions, number drills and letters all sitting next to one another.

A drawing might call for 9mm. An older machine manual specifies 23/64″. A tapping chart asks for a #7 drill, while another job calls for a Letter H. They are all describing diameter, but they use four different sizing systems to get there.

This drill bit size conversion chart brings those systems together so you can quickly compare metric, fractional, number and letter drill sizes without guessing at the nearest equivalent.

The one conversion worth remembering is:

1 inch = exactly 25.4mm

From that single relationship, any metric drill diameter can be converted into decimal inches, and any decimal-inch measurement can be converted back into millimetres.

For straightforward drilling in timber or sheet material, being a fraction of a millimetre away from a nominal size may not matter. In machining, tapping, reaming, clearance holes or close-tolerance fabrication, it can matter a great deal. A drill that looks “near enough” on a ruler may be several hundredths or tenths of a millimetre away from the size actually specified.

That is why this guide gives you the numbers rather than simply calling two drills equivalent.

Jawel Tools stocks a broad range of drill bits, sets for professional workshop, engineering and DIY use, including metric HSS and cobalt sets suited to many of the applications covered below. The wider Jawel range also includes electric power drills for workshop, trade and home use.

Fast Drill Bit Size Conversion Lookup

To convert between metric and imperial drill sizes, use 25.4mm per inch. Divide a metric measurement by 25.4 to obtain decimal inches, or multiply decimal inches by 25.4 to obtain millimetres.

The important word there is decimal. A fractional drill such as 3/8″ first needs to be understood as a decimal value — in this case 0.375″ — before it can be compared accurately with metric, number or letter drills.

Mathematical Conversion Formulas for Inches and Millimetres

The formulas are:

Decimal inches = millimetres ÷ 25.4

Millimetres = decimal inches × 25.4

A few everyday examples make this easier to see:

• 5mm ÷ 25.4 = 0.19685″
• 10mm ÷ 25.4 = 0.39370″
• 0.250″ × 25.4 = 6.35mm
• 0.500″ × 25.4 = 12.7mm

Those are conversions, not approximate nominal matches. The difficulty comes when you then try to choose a drill from another sizing series.

A 9mm drill, for example, converts to approximately 0.3543″. The nearest common 1/64″ fractional size is 23/64″, which measures 0.359375″ or approximately 9.128mm. That makes the fractional drill roughly 0.128mm larger than a true 9mm drill.

On a rough timber hole, that difference could be immaterial. On a hole being prepared for a specific thread, dowel, pin, interference fit or subsequent machining operation, it deserves attention.

Quick Comparison: The Four Drill Bit Sizing Systems

Sizing system	How sizes are written	Typical examples	How the series works
Metric	Millimetres	3mm, 6.5mm, 10mm	Diameter is stated directly in mm
Fractional inch	Fractions of an inch	1/8″, 1/4″, 3/8″	Common drills progress through fractional-inch sizes
Number drill	#80 to #1	#40, #21, #7	Larger number = smaller drill
Letter drill	A to Z	A, H, M, Z	Diameter increases from A through Z

The systems overlap. That is useful because it gives machinists a much finer choice of diameters than fractional drills alone, but it is also where mistakes begin.

A 1/4″ drill, for instance, is exactly 0.2500″ or 6.35mm. Letter drill E also has a nominal diameter of 0.2500″. A nearby 6.3mm or 6.4mm metric drill is close, but neither is mathematically identical.

In precision work, the decimal value is the common language between all four systems.

Understanding the Four Standard Drill Bit Sizing Systems

Before using a conversion table, it helps to understand why these different sizes exist. Metric and fractional systems are intuitive because the size designation relates directly to the diameter. Number and letter drills are different: their names are essentially labels attached to predetermined decimal-inch diameters.

1. Fractional Inch Drill Sizes

Fractional drill bits are expressed as fractions of one inch, commonly in increments based on 1/64″.

That gives familiar sizes such as:

• 1/16″
• 1/8″
• 3/16″
• 1/4″
• 5/16″
• 3/8″
• 1/2″
• 3/4″
• 1″

A 1/64″ increment equals 0.015625″, or approximately 0.3969mm. That is already a useful clue as to why metric, number and letter sizes are needed for closer diameter selection. Nearly four-tenths of a millimetre between successive basic fractional steps can be far too coarse for thread preparation or precision engineering.

Take the area around 1/4″ as an example. The neighbouring 1/64″ sizes are:

• 15/64″ = 0.234375″ = 5.953mm
• 1/4″ = 0.250000″ = 6.350mm
• 17/64″ = 0.265625″ = 6.747mm

There is nearly 0.4mm between each of those sizes. Number, letter and metric drills can fill many of those gaps.

Fractional drills remain especially familiar on equipment, drawings and fasteners specified in imperial units. If you routinely work on American machinery, older equipment, classic vehicles or imported components, a fractional drill set can still earn its keep.

2. Metric Drill Sizes

Metric drills are the easiest system to understand because the number printed on the bit is its nominal diameter in millimetres.

A 6.5mm drill bit is 6.5mm in diameter. There is no gauge number or fraction to decode.

Metric drills are widely used throughout UK engineering, automotive, fabrication and general workshop work. Depending on the range, drill sets may progress in 0.1mm, 0.5mm or other increments.

ISO 235:2016 covers dimensional series for parallel-shank stub drills, parallel-shank jobber drills and Morse-taper shank drills. ISO states that the standard includes dimensional tables in both millimetres and inches, and the 2016 edition remains current following its review and confirmation in 2022.

For a practical workshop set, Jawel Tools’ Sealey AK47251 HSS Split Point Fully Ground Drill Bit Set 25pc Metric covers 1–13mm in 0.5mm increments. The bits are fully ground HSS with 135° split points and are specified for materials including high-carbon steel, copper and plastics.

That 0.5mm spacing makes the set useful for general drilling, but it also illustrates why a conversion chart can still matter. If a drawing calls for 6.8mm, for example, neither 6.5mm nor 7mm should automatically be substituted simply because those are the nearest bits in a particular set.

3. Number Drill Bit Sizes: #1 to #80

Number drills — sometimes loosely referred to as wire-gauge drills — are commonly encountered from #80 through #1.

Their logic runs in the opposite direction to what many people expect:

The higher the drill number, the smaller the diameter.

At the commonly used ends of the range:

• #80 = 0.0135″ = 0.343mm
• #1 = 0.2280″ = 5.791mm

So #80 is tiny, while #1 is the largest of the normal #1–#80 series.

Unlike metric measurements or fractions, the number itself does not tell you the diameter. A #11 drill, for example, is 0.1910″, equivalent to approximately 4.851mm. You either know the size, read it from a chart, or measure/check the drill.

This apparently awkward system becomes much more useful once tapping enters the conversation.

Suppose the fractional sizes either side of a required hole are too far apart. Number drills provide intermediate diameters such as #7 at 0.2010″ / 5.105mm, #21 at 0.1590″ / 4.039mm, or #36 at 0.1065″ / 2.705mm. Those smaller steps are why number drills appear so frequently on imperial tap-drill charts.

A common workshop mistake is to read #40 as “larger” than #20 because 40 is the greater number. With number drills, that is backwards. Always check the diameter.

4. Letter Drill Bit Sizes: A to Z

Letter drill sizes continue into larger decimal-inch diameters where the common number-drill range finishes.

The normal series runs from:

A = 0.2340″ / 5.944mm

through to:

Z = 0.4130″ / 10.490mm

Unlike number drills, letter sizes become larger as you move through the alphabet.

Some useful reference points include:

• A = 0.2340″ / 5.944mm
• E = 0.2500″ / 6.350mm
• H = 0.2660″ / 6.756mm
• M = 0.2950″ / 7.493mm
• S = 0.3480″ / 8.839mm
• Z = 0.4130″ / 10.490mm

The real value of the letter series is the same as the number series: it fills diameter gaps.

Think of a conventional fractional drill index as a staircase with fairly wide steps. Number, letter and metric drills add extra steps between them. For general drilling you may not need that degree of choice; for tapping, machining and controlled fits, those intermediate diameters can be exactly what the drawing or thread chart requires.

Master Drill Bit Size Conversion Tables

The tables below are designed for lookup rather than theory. Where a “nearest fraction” is shown, it means the closest standard 1/64″ fractional value by diameter — not that the two sizes should automatically be treated as interchangeable.

If the hole has a specified tolerance, always work from the required diameter and permitted limits rather than choosing solely by nearest nominal size.

Metric Drill Sizes With Closest 1/64″ Fractional Equivalent

How to read the difference column: a positive figure means the listed fractional drill is larger than the metric size; a negative figure means it is smaller.

The 25mm example shows why looking at the numbers can be revealing. 25mm and 63/64″ differ by only about 0.003mm, making them exceptionally close dimensionally. Compare that with 12.5mm and 1/2″, where the 1/2″ drill is 12.7mm — a full 0.2mm larger.

They may both look like sensible “nearest equivalents” on a chart, but they are not equally close.

A Practical Metric Drill Set for Workshop Use

For general engineering and workshop drilling, the Sealey AK47251 25pc metric HSS drill set covers 1mm through 13mm in half-millimetre increments. Its 135° split-point geometry is designed to help the cutting edges bite into the workpiece at the start of the hole, while the fully ground HSS construction is intended to reduce heat and friction.

For tougher materials, Jawel also stocks the Sealey AK4701 HSS Cobalt Split Point Drill Bit Set 19pc. It covers 1–10mm in 0.5mm increments and uses fully ground HSS containing 5% cobalt, with Sealey specifying the set for drilling harder materials. The 135° split point is also intended to bite quickly and reduce the need for a centre mark.

The distinction matters. Choosing the correct diameter is only half the job; the bit material and geometry also need to suit what you are drilling.

A perfectly sized drill that rapidly overheats, loses its edge or wanders across the workpiece is still the wrong tool for the job.

Fractional Drill Bit Sizes: 1/64″ to 1″

Fractional drills are easiest to compare when both their decimal-inch and metric values are visible. The table below progresses in 1/64″ increments.

The table makes one thing immediately obvious: a fractional drill can sit very close to a metric size in one part of the range and noticeably further away in another.

That is why a conversion chart should be treated as a measurement reference, not permission to substitute sizes blindly.

Number Drill Bit Size Chart: #80 to #1

Number drills give you a much finer selection of diameters than ordinary fractional drills. They are particularly useful in engineering, machining and thread preparation, where a small change in hole size can affect thread engagement or component fit.

Remember the numbering runs backwards:

#80 is the smallest standard number drill and #1 is the largest.

A useful workshop habit is to ignore the designation for a moment and look at the decimal value. That makes it much easier to compare a number drill with the nearest metric or fractional alternative.

For example:

• #7 = 0.2010″ / 5.105mm
• 13/64″ = 0.203125″ / 5.159mm
• 5.0mm = 0.19685″

All three are reasonably close to one another, but they are not the same size.

Letter Drill Bit Size Chart: A to Z

Letter drills cover decimal-inch diameters from 0.2340″ to 0.4130″, filling many of the spaces between common fractional sizes.

They are frequently encountered in machining and imperial thread work, particularly where a standard fractional drill would leave too large a jump between available diameters.

Some letter drills line up neatly with familiar fractional sizes. Letter E, for example, is exactly 0.2500″, the same nominal diameter as 1/4″.

Others sit between fractions. Letter H at 0.2660″ falls just above 17/64″ at 0.265625″. The difference is tiny — around 0.0095mm — but once again, whether that difference matters depends entirely on the job.

How to Choose the Right Drill Size Without Causing Tolerance Problems

A conversion chart tells you how sizes compare. It does not tell you automatically whether one size can replace another.

That decision depends on what the hole is actually doing.

A screw clearance hole, tapped hole, dowel bore and hole that will later be reamed may all begin with a drill bit, but they have very different requirements.

The “Nearest Size” Substitution Trap

One of the easiest mistakes to make is seeing two dimensions that look close and treating them as interchangeable.

Suppose a drawing specifies a 6.5mm hole. A 1/4″ drill is 6.35mm, giving a difference of 0.15mm.

For drilling a cable access hole through timber, that is unlikely to cause any meaningful problem. If the same hole forms part of a controlled fit between machined components, 0.15mm could be well outside the allowed tolerance.

Before substituting a drill size, consider:

• the specified hole tolerance
• whether a bolt or fastener needs clearance
• whether the hole will be tapped
• whether the hole will later be reamed
• whether a dowel, pin or bearing will be fitted
• the material being drilled
• the accuracy of the drilling machine and setup

There is also another practical point that charts cannot show: a drill does not always produce a hole exactly equal to its marked diameter.

Runout in the drill or chuck, poor workholding, worn cutting edges, incorrect feed, material behaviour and wandering at the start of the cut can all affect the finished hole.

When tolerances are tight, the drill is often only one stage in the process.

Drill Size for Tapped Holes

A tap needs enough material left inside the hole to form the thread.

Drill too large and you remove too much material before tapping. The result is reduced thread engagement.

Go too small and the tap must remove more material than intended, increasing cutting load and making binding or tap breakage more likely.

That is why tapping charts specify a particular tap drill size for each thread.

For a common metric thread, a useful workshop approximation is:

Tap drill diameter ≈ major thread diameter − thread pitch

For example, an M8 × 1.25 thread gives:

8.00 − 1.25 = 6.75mm

A standard tapping chart will then guide you towards the appropriate readily available drill size for the required thread and engagement.

Treat this formula as a quick sense-check, though, rather than a replacement for the correct thread data. Thread percentage, material and application can change the preferred drill size.

Drill Size for Clearance Holes

A clearance hole has a different job. The fastener should pass through the hole without its threads engaging the material.

This means the hole must be larger than the fastener’s nominal outside diameter.

Clearance sizes are often divided into categories such as:

• close fit – limited clearance with more accurate positioning
• normal fit – general assembly work
• loose fit – easier assembly or situations where alignment is less precise

If you are choosing between two near-equivalent drills for a general clearance hole, the larger drill may often make more practical sense because the bolt needs room to pass cleanly.

That logic does not carry over to a tapped hole. Increasing a tap drill size simply to make drilling easier can reduce the amount of thread available afterwards.

Drill Bit Materials: HSS vs Cobalt vs Carbide

Choosing the right diameter is important, but drill material determines how well the tool copes with the material being cut.

There is no single “best” drill bit material for every job. A bit designed for general workshop drilling may be forgiving and affordable, while a harder cutting tool designed for demanding production work can be less tolerant of vibration or poor setup.

High-Speed Steel (HSS)

High-Speed Steel is the everyday workhorse of the drill cabinet.

Good-quality HSS drills are suitable for a wide range of workshop materials, including many steels, non-ferrous metals, plastics and general-purpose applications. They offer a useful balance between cutting performance, toughness and cost.

For regular drilling where the material is not exceptionally hard, HSS is often the sensible starting point.

Jawel Tools’ Sealey AK47251 HSS Split Point Fully Ground Drill Bit Set contains 25 metric drills from 1–13mm in 0.5mm increments, giving useful coverage for day-to-day engineering and workshop work.

Cobalt HSS

Cobalt drill bits are still based on high-speed steel, but the alloy contains cobalt to improve hot hardness and wear resistance.

That becomes useful when drilling tougher materials where an ordinary drill can rapidly lose its edge as temperatures rise.

The Sealey AK4701 HSS Cobalt Split Point Fully Ground Drill Bit Set available from Jawel Tools contains 5% cobalt HSS drills from 1–10mm in 0.5mm increments.

For stainless steel and other demanding metals, however, buying a cobalt drill is only part of the solution. Cutting speed, feed pressure, lubrication and keeping the cutting edges engaged all have a large effect on tool life.

Let a drill rub instead of cut and heat builds quickly.

Solid Carbide

Carbide sits at the harder, more specialised end of drill tooling.

Solid carbide drills can maintain a cutting edge at high speeds and perform extremely well in rigid CNC and production environments. Their hardness is also their weakness: carbide is much less forgiving of shock, vibration, poor alignment and interrupted cuts than HSS.

Drop an HSS drill and it will normally survive.

Treat a small carbide drill the same way and you may be shopping for another one.

For manual drilling and general maintenance work, HSS or cobalt is therefore often the more practical choice. Carbide comes into its own where machine rigidity, accuracy, cutting conditions and production requirements justify it.

Why Drill Point Geometry Matters

Two drill bits can have the same diameter and still behave very differently at the start of a hole.

The point angle and cutting-edge geometry influence how the drill enters the work, how much force it requires and how readily it tries to wander from the intended position.

118° vs 135° Drill Points

Traditional general-purpose twist drills commonly use a point around 118°.

A 135° point is flatter and is frequently found on drills intended for harder materials. When combined with a split-point design, it can improve self-centring and help the drill begin cutting with less tendency to skate across the surface.

Both of the Sealey metric drill sets mentioned above use 135° split points.

That is especially useful when accuracy matters because the hole can only be as well positioned as its starting point. If the drill wanders before biting, the diameter may be correct but the hole location is already wrong.

A centre punch, rigid workholding and correctly aligned drilling machine remain valuable even with a split-point drill.

Getting More Accurate Holes in the Workshop

The number etched on the drill is only one part of accurate drilling.

For better results:

1. Mark the position accurately. Use appropriate measuring and marking tools rather than judging the location by eye.
2. Secure the workpiece. A drill vice or suitable clamps reduce movement and improve both accuracy and safety.
3. Check the drill condition. Chipped, blunt or uneven cutting edges can produce poor holes.
4. Minimise runout. Check that the drill is seated correctly and the chuck is in good condition.
5. Use suitable speed and feed. Bigger drills generally require lower rotational speed than small drills in the same material.
6. Use cutting fluid when appropriate. This can reduce heat and improve cutting conditions in many metal-drilling applications.
7. Measure critical work afterwards. If the actual finished diameter matters, do not assume the drill marking guarantees the resulting hole size.

Jawel Tools also stocks workshop equipment such as drill vices and supporting tools that help make the drilling setup more stable. A rigid setup often makes a bigger difference than people expect; even a premium cutting tool will struggle to produce a controlled hole if the workpiece is moving underneath it.

Choosing a Drill Size: Quick Workshop Checklist

Before reaching for the nearest bit in the box, check:

• What diameter is actually specified?
• Is the dimension metric, fractional, number or letter?
• What is the exact decimal or millimetre equivalent?
• Does the drawing give a tolerance?
• Is the hole for clearance, tapping, reaming or a fitted component?
• What material are you drilling?
• Does the drill material suit the workpiece?
• Can your machine and setup realistically hold the required accuracy?

This takes seconds once it becomes habit and can prevent a surprising number of mistakes.

Frequently Asked Questions

What is a #11 drill bit size in inches and millimetres?

A #11 drill bit measures 0.1910 inches in diameter, which converts to approximately 4.851mm.

Because number drills use predefined decimal diameters rather than an obvious mathematical sequence, the #11 designation itself cannot be reliably converted without a drill size chart.

What fractional drill bit is closest to 9mm?

A 9mm drill equals approximately 0.3543″. The nearest standard 1/64″ fractional size is 23/64″, which measures 0.359375″ or approximately 9.128mm.

That makes the 23/64″ drill about 0.128mm larger than a true 9mm drill.

Whether that substitution is acceptable depends on the tolerance and purpose of the hole.

How do number drill bit sizes work?

Number drills normally run from #80 to #1. The numbering is inverse, so the diameter increases as the number becomes smaller.

A #80 drill is just 0.0135″ / 0.343mm, while a #1 drill measures 0.2280″ / 5.791mm.

How do letter drill sizes work?

Letter drills run from A through Z, with the diameter increasing through the alphabet.

Letter A measures 0.2340″ / 5.944mm, while Letter Z measures 0.4130″ / 10.490mm.

They provide additional decimal-inch diameters between many common fractional sizes.

How do you convert millimetres to inches?

Divide the millimetre measurement by 25.4.

For example:

8mm ÷ 25.4 = 0.31496″

Rounded to four decimal places, that is approximately 0.3150″.

How do you convert inches to millimetres?

Multiply the decimal-inch measurement by 25.4.

For example:

0.375″ × 25.4 = 9.525mm

Because 0.375″ equals 3/8″, a 3/8″ drill has a nominal metric diameter of 9.525mm.

Is a metric drill bit interchangeable with the nearest imperial size?

Sometimes for non-critical work, but not automatically.

A close imperial size can still be larger or smaller than the specified metric diameter. The difference should be compared with the permitted tolerance and the purpose of the hole before making a substitution.

What is the difference between a tap drill and a clearance drill?

A tap drill creates the hole that will later be threaded using a tap. Its diameter is smaller than the thread’s major diameter so material remains for the thread profile.

A clearance drill produces a hole large enough for the fastener to pass through without the threads engaging the material.

Using a clearance-hole size where a tap drill is required can leave too little material to form a strong thread.

Is a 135° split-point drill better than a standard drill?

Not in every application, but it can offer useful advantages.

A 135° split point tends to bite into the work more readily and resist wandering, which is particularly useful in harder materials and where accurate starting position matters. Drill material, machine rigidity, cutting speed and feed remain equally important.

Final Thoughts: Measure First, Then Choose the Drill

Drill bit conversion is simple mathematics. Choosing the right drill for a real job is where judgement comes in.

Metric, fractional, number and letter drill sizes overlap, but “close” does not always mean “equivalent”. Converting everything back to either millimetres or decimal inches gives you a common reference point and makes the differences immediately visible.

For general workshop jobs, that may simply confirm the nearest available drill is perfectly adequate. When tapping threads, preparing a hole for reaming or working to an engineering drawing, those same few hundredths of a millimetre can become important.

Use the charts above as the starting point, then consider the hole tolerance, application, workpiece material and tooling before you make the cut.

Need to replace worn bits or build a more useful workshop selection? Explore the drill bit sets and workshop tooling available from Jawel Tools, including HSS and cobalt options for general engineering, fabrication and maintenance work.