Drill bit size chart: number, letter, fractional and metric sizes in decimal inches and millimetres

01What do the four drill size systems measure?

All four systems name the cutting diameter of a twist drill; they differ only in how that diameter is labelled. Fractional sizes are the familiar inch fractions printed on most US bits. Metric sizes are labelled directly in millimetres, which is how Starrett's UK range, Fisch and most bits sold in Europe are labelled. Number sizes, also called wire gauge sizes, and letter sizes are older US designations that fill the gaps between neighbouring fractional sizes, which is why tap drill charts lean on them so heavily.

A drill size chart is therefore a translation table. Find your bit in whichever system it is labelled in, read across to the decimal inch column, and you can compare it with any other bit, fastener or thread on the same scale. The millimetre column does the same job for readers who work in metric. The four tables below are split by system so each one stays short enough to scan: number sizes first, then letter sizes, then fractional sizes, then the common metric jobber sizes. If you have a bit in your hand and only want its decimal, go straight to the table for the label printed on its shank. If you are trying to match a hole to a screw, a thread or a fitting, read the sections further down before you pick a size, because the right answer is often not in this chart at all. All four tables also come as one PDF sheet, in US Letter and A4, among the site's printable charts.

02Number drill sizes (#80 to #1)

Number drills cover the smallest end of the range, from #80 at 0.0135 in up to #1 at 0.228 in. The steps between them are uneven: at the small end neighbouring sizes differ by a few ten-thousandths of an inch, while near #1 the jumps are larger. The decimal values below agree in AmesWeb and Wikipedia for all 80 sizes; the millimetre values are Wikipedia's three-decimal conversions.

03Letter drill sizes (A to Z)

Letter drills pick up where number drills stop. Letter A is 0.234 in, just above #1, and the series climbs to letter Z at 0.413 in. Unlike the number series, the letters run in the direction you would expect: A is the smallest and Z the largest. AmesWeb, Wikipedia and Little Machine Shop all give the same decimals for every letter.

04Fractional drill sizes up to one inch

Fractional drills step up in 1/64 in increments from the smallest fraction to one inch, and they overlap both the number and letter ranges. Because a fraction converts to an exact decimal, the millimetre column here is an exact conversion rather than a rounded one. The sources consulted did not tabulate fractional sizes above one inch, so they are not listed; for larger holes in wood or sheet material, a hole saw or step bit usually takes over.

05Common metric drill sizes (DIN 338 jobber)

Metric twist drills are labelled by their diameter in millimetres. The table lists the common high-speed steel jobber sizes in Starrett's catalogue, which are made to DIN 338, with the inch conversion and the overall length Starrett gives for each. It is a list of common sizes, not the full metric series: finer metric steps exist but were not tabulated by the sources consulted, so they are left out rather than guessed.

06How do the four systems fit together?

They form one continuous scale once you convert to decimal inches. The number series covers the small end and stops at #1 (0.228 in). The letter series begins immediately above it with A (0.234 in) and ends at Z (0.413 in), so number and letter sizes never overlap each other. Fractional sizes run across both ranges and beyond, and metric sizes sit among all three.

That arrangement is the whole point of the older systems. A fractional set jumps in 1/64 in steps, which is too coarse when a thread or press fit needs a hole a few thousandths larger or smaller. Number and letter sizes slot in between the fractions, so a machinist can choose a drill that lands close to the ideal diameter instead of rounding to the nearest sixty-fourth. For general wood and DIY work, a fractional or metric set is usually all you need; number and letter drills earn their place when you start tapping threads or fitting parts.

For a slower walkthrough of the labels, the guide to reading drill bit sizes explains each system step by step, with examples of how the markings appear on real bits and cases.

07How do you read a decimal drill size?

Read the decimal inch value as thousandths of an inch, and compare sizes digit by digit from the left. A #7 drill at 0.201 in and letter F at 0.257 in share the same first decimal digit, so the second digit decides it: F is the larger of the two. Machinists often say "thou" for a thousandth of an inch and quote drill sizes that way, which is how the decimal column is normally read aloud.

The fourth decimal place matters in the number series. Several number sizes carry a significant fourth digit, and one popular online chart, Little Machine Shop, rounds six of them (#69, #28, #25, #18, #10 and #5) to three decimals. AmesWeb and Wikipedia carry the full four-decimal values, and this page uses those. If two charts disagree in the last digit for one of those six sizes, this is why.

For metric work, use the millimetre column as given rather than converting by hand. The fractional table shows exact conversions, the number and letter tables show three-decimal conversions, and the metric table shows Starrett's diameters with their inch equivalents. Mixing a rounded value from one chart with an exact value from another is an easy way to end up a size off.

08Which sizes are close equivalents across systems?

Only a few pairs are true equivalents; most are near neighbours that differ in the third or fourth decimal place. Reading the decimal columns of the four tables above side by side gives these matches:

• Letter E and 1/4 in are identical at 0.250 in.
• #12 and 4.8 mm show the same decimal to four places in these tables, with 3/16 in just below both.
• 4 mm and #22 differ only in the fourth decimal place, with 4 mm the larger.
• 3.2 mm sits just over 1/8 in.
• 2.5 mm falls between #40 and #39, much nearer #40.
• 5 mm falls between #9 and #8, nearer #9. The #7 drill (0.201 in) is larger than both, and a little smaller than 13/64 in.
• 6 mm falls between 15/64 in and letter B, roughly midway; letter A is just under 15/64 in.
• 6.5 mm is just under letter F (0.257 in).
• 7 mm is just under letter J.
• 8 mm is almost exactly letter O and a little over 5/16 in.
• 10 mm falls between 25/64 in and letter X.
• 12 mm falls between 15/32 in and 31/64 in, nearer 15/32 in.

A near neighbour is fine for a clearance hole or a hole in wood, where a few thousandths make no practical difference. It is not fine for a tapped thread, a dowel or a bearing seat, where the drill size is specified for a reason. In those cases, use the size the thread or part calls for, and buy that exact bit rather than substituting the nearest one from a different system.

09Does a bigger drill number mean a bigger drill?

No. Number drills run backwards: #1 is the largest at 0.228 in and #80 the smallest at 0.0135 in. The letter series runs the other way, from A as the smallest to Z as the largest. Mixing the two directions up is an easy mistake when reading a number drill chart, and it can put you dozens of sizes away from the drill you wanted.

A related trap is the screw gauge. A #8 wood screw and a #8 drill are unrelated: the screw number describes the screw's diameter in a separate gauge system, while the drill number is a wire gauge size. For a #8 screw in hardwood, WOOD Magazine's chart gives a 7/64 in pilot hole, and for softwood 3/32 in, neither of which is a #8 drill. Always look up the screw in a pilot chart rather than matching numbers.

10When should you use a tap drill or pilot hole chart instead?

Use a tap drill chart whenever the hole will be threaded, and a pilot hole chart whenever a wood screw will be driven into it. This chart tells you what each drill measures; those charts tell you which drill a particular job calls for, and the answer is often a number or letter size you would not pick by eye.

For threads, the tap drill for a 1/4-20 UNC thread is a #7 drill, and for 5/16-18 UNC it is letter F. That is why both appear in the At a glance panel on this page. Metric threads are simpler: the tap drill for M6 coarse is 5.0 mm, which appears in the metric table above, but the tap drill for M8 coarse is 6.8 mm, which is not among the common jobber sizes listed here. That gap is a good example of why you look up the thread rather than guess from the nearest drill. Tap drill charts are also built around a stated thread percentage; Little Machine Shop lists 75% / 50% thread drills and recommends the larger, lower-percentage drill for steel, stainless and iron because it is easier to tap. The full metric tap drill sizes and UNC/UNF list covers every thread from the smallest machine screws up.

For wood screws, pilot hole size depends on the screw gauge and on the wood. Hardwood takes a larger pilot than softwood for the same gauge, which is why a single drill size cannot serve both. The chart of pilot hole sizes for screws gives the pilot, shank and countersink sizes by gauge and wood type.

11What drill bit sizes do common taps need?

Common taps need these drills: a #7 for 1/4-20, letter F for 5/16-18, 5/16 in for 3/8-16, and 6.8 mm for M8 coarse. The short table below pulls the most-asked taps together, inch and metric, coarse and fine, so you can find the drill and then look up its decimal in the tables above. Fine-pitch metric taps take a larger drill than coarse ones of the same diameter, so check the pitch before you drill.

For any tap not in this short list, the full tap drill size chart covers every UNC and UNF thread up to one inch, metric coarse and fine threads, and the alternative drill for hard metals.

12Which drill length do you need?

Most general-purpose drilling uses jobber-length bits, the medium length sold in standard sets. Wikipedia describes three common length series: stub (also called screw-machine) bits, which are short and stiff; jobber bits, which are the most common; and taper or long-series bits for deep holes. A shorter bit flexes less and wanders less when starting; a longer one reaches through thicker stock.

In metric, jobber drills follow DIN 338, according to Starrett's catalogue, and the overall lengths in the metric table above are Starrett's figures for that standard. Inch jobber, stub and taper lengths were not available from the sources consulted, so they are not given here; check the maker's listing if overall length matters for your job.

13When doesn't the drill size chart give you the answer?

The chart gives the diameter a drill cuts, not whether it fits your tool or suits your job. Four situations need something else:

• The bit will not fit the chuck. Chuck size names the largest shank a chuck accepts, not the largest hole it can drill: a 1/2 in chuck takes shanks up to that size, and larger-cutting bits need a reduced shank. The guide to drill chuck capacity covers keyed, keyless and hex holders.
• The hole is larger than one inch. The tables stop at one inch because the sources stop there. For bigger holes, see hole saw sizes for pipe and conduit or hole saw kits; for thin sheet, a single step bit covers many sizes, and the step bit picks compare current models.
• The hole is for a thread or screw. Use the tap drill or pilot hole chart, as above.
• The drill size is right but the hole is not. Ordinary twist drills tend to wander on a flat surface, which puts the hole off position. In metal a spotting drill helps, in wood a brad-point bit holds its line, and split points reduce the drift. The page on why a drill bit wanders goes through the fixes.

14What mistakes do people make with drill size charts?

Most errors come from reading the right chart the wrong way. The ones that cost the most time are:

• Reading the number series upwards. #60 is smaller than #6, not larger.
• Treating screw gauge numbers as drill numbers. A #10 screw does not want a #10 drill.
• Substituting a near metric size for a tap drill. A near neighbour leaves too much or too little thread; buy the size the chart specifies.
• Comparing a rounded chart with a four-decimal one. For the six number sizes that Little Machine Shop rounds, two correct charts can disagree in the last digit.
• Assuming a set covers every size. Fractional sets step in sixty-fourths; number and letter sizes between them are usually sold singly or in a separate index.

Drill index cases and drill gauge plates are labelled in these same four systems, so the tables on this page double as a key for reading them. When a bit's markings have worn off, its decimal can be matched against the tables to identify it.

15Once you know the size, which type of bit should you use?

The material decides the bit type, and the size chart applies to all of them. A general-purpose twist drill has a point angle of 118 degrees and works in wood, metal and plastic. Starrett's catalogue describes split-point HSS drills ground to 135 degrees as self-centering, needing less feed pressure and wandering less, which helps in steel. For wood, brad-point bits keep the hole in line, and larger holes call for spade, Forstner or auger bits, which are compared in the guide to Forstner, spade and auger bits.

For an overview of every family, from cobalt and carbide to masonry and glass bits, see the main kinds of drill bits. The drill bit guides hub groups the buyer guides by material, the reference charts section collects the other size tables on this site, and the tool accessory lookup shows which accessories fit a specific drill model.

16Questions readers ask