How many millimetres is a 1/4 inch drill bit?

A 1/4 inch drill bit is exactly 6.35 mm, because one inch is 25.4 mm and a quarter of that is 6.35. So if a metric instruction calls for a 6.35 mm hole, a 1/4 inch bit is the same thing. The closest common metric bit is 6.5 mm, which is about 0.006 inch larger, so for a snug fit pick the 1/4 inch or a 6 mm bit instead of jumping to 6.5 mm.

What does a number drill like

Number drills run from #1 (largest, 0.228 inch) down to #80 (smallest, 0.0135 inch) and fill the gaps between fractional sizes. They are common as tap drills: #7 is 0.201 inch, the standard tap drill for a 1/4-20 UNC thread, sitting between 13/64 (0.203) and 3/16 (0.1875). When a thread chart says drill #7, grab the bit stamped 7, not a fraction.

What are letter drill sizes A to Z?

Letter drills pick up where the number drills end, covering the range from A (0.234 inch) up to Z (0.413 inch), again filling gaps between fractions. A letter F drill is 0.257 inch, the common tap drill for a 5/16-18 thread. If your index only has fractions, this chart shows the nearest fraction so you can substitute, but for a tapped hole the exact letter size matters because half a thousandth changes the thread engagement.

How do I convert a metric drill bit to a fraction?

Enter the millimetre value and the tool snaps it to the nearest 1/64 fraction and reports how close it is. For example 8 mm is 0.315 inch, which rounds to 5/16 (0.3125), about 0.0025 inch smaller. For wood and rough holes that difference is invisible; for a press fit or a bearing seat, use the exact mm value shown rather than the fraction, since rounding to 1/64 can be off by up to 0.0078 inch.

My bit is unmarked — how do I find its size?

Measure the shank with calipers and type that decimal inch or mm into the tool. It returns the nearest fraction, the closest number drill and the closest letter drill, with an exact or closest flag. A caliper reading of 0.250 inch flags exact at 1/4; a reading of 0.199 inch flags closest at number drill #8 (0.199) rather than #7 (0.201). Always trust the measured value over a faded stamp.

Why mix number, letter and fractional drills at all?

Together they give fine, even coverage of diameters that fractions alone leave gaps in. Between 3/16 (0.1875) and 1/4 (0.250) there are seven number drills, so you can hit almost any tap or clearance size. Buying a 115-piece set that combines all three systems means you rarely have to ream or settle for a loose hole. This chart is the decoder ring that tells you which of the three a given diameter belongs to.

Find the right tap drill before threading a hole

You need to cut a 1/4-20 thread but the index is a jumble of marks. The thread chart says use a #7 drill. Type number drill 7 and the tool confirms 0.201 inch, between 13/64 and 3/16, so you grab the right bit instead of guessing with a fraction and stripping the thread. Same flow for metric taps: enter the mm tap-drill and read the nearest fraction or number drill your set actually contains.

Match a metric plan to an imperial drill set

A European furniture plan calls for an 8 mm dowel hole, but your bits are all fractions and letters. Enter 8 mm and the tool snaps it to 5/16 (0.3125 inch, about 0.0025 inch under) and shows the closest letter drill too. For a glued dowel that 5/16 is perfect; for a press fit you can see the exact gap and decide whether to ream.

Identify an unmarked bit with calipers

A loose bit rolled out of the case with no legible stamp. Measure the shank at 0.272 inch, type it in, and the tool flags it exact as letter drill I (0.272). Now you can label it, file it back in the index, and know exactly what it cuts the next time a plan asks for that diameter.

Buy the right drill index without overlap

Before ordering a combined number-letter-fraction set, you want to know what each system actually covers. Punch a few sizes through the chart to see how the number drills fill the gap below 1/4 inch and letters bridge 1/4 to 1/2, so you choose a 115-piece index that earns its drawer space instead of three sets that overlap.

Drill Bit Size Chart: Fraction, Number, Letter, mm Converter

Convert between fractional inch, decimal inch, mm, number (#1-#80) and letter (A-Z) drill sizes — closest match, full chart, one-click copy — 100% in your browser

Runs locally
Category Format Converter
Best for Turning pasted content or local files into a handoff-friendly format.

I have the size asSize value

Equivalent sizes

Number, letter and fractional drills overlap on a single physical scale, so a result is the closest standard size, not always an exact match. For a press fit or tap, check the mm/inch value, not just the label.

Common drill bit chart

Size	Inch	mm
1/16	0.0625	1.587
#43	0.0890	2.261
3/32	0.0938	2.381
#36	0.1065	2.705
1/8	0.1250	3.175
#29	0.1360	3.454
5/32	0.1563	3.969
#21	0.1590	4.039
3/16	0.1875	4.762
#7	0.2010	5.105
7/32	0.2188	5.556
A	0.2340	5.944
1/4	0.2500	6.350
F	0.2570	6.528
9/32	0.2813	7.144
5/16	0.3125	7.938
Q	0.3320	8.433
T	0.3580	9.093
3/8	0.3750	9.525
Z	0.4130	10.490
7/16	0.4375	11.112
1/2	0.5000	12.700

What this tool does

A drill bit size chart and converter for DIY, woodworking and machine shop work. American drill sets mix four numbering systems on one physical scale: fractional inches in 1/64 steps, the wire gauge number drills #1 through #80, the letter drills A through Z, and plain metric millimetres. This tool takes any one of those and shows you the rest at once, plus the exact decimal inch and the millimetre value so you can match a bit to a hole, a tap or a dowel without a wall chart. Type 1/4 and read 6.35 mm. Type a number drill #7 and see it lands at 0.201 inch, the standard tap-drill for a 1/4-20 thread. Type 6 mm and find the nearest fraction and the closest number drill. Because the four systems interleave, a result is flagged exact when it lands on a real size and closest when it rounds to the nearest standard bit. Every value copies with one click, and the share link reopens your exact conversion. No upload, no account, no network call — it all runs as plain JavaScript in your browser tab.

Tool details

Input: Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy + Preview; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 11 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Format Converter · Operations; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Drill Bit Size Chart fits into your work

Use it when the main problem is getting content from one practical format into another.

Conversion jobs

Turning pasted content or local files into a handoff-friendly format.
Previewing a conversion before you use it in a larger workflow.
Cleaning small format mismatches without opening a full editor.

Conversion checks

Try a small sample first when the source format is messy.
Check character encoding, separators, and line endings after conversion.
Keep the source until the converted output has been reviewed.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Find the right tap drill before threading a hole
You need to cut a 1/4-20 thread but the index is a jumble of marks. The thread chart says use a #7 drill. Type number drill 7 and the tool confirms 0.201 inch, between 13/64 and 3/16, so you grab the right bit instead of guessing with a fraction and stripping the thread. Same flow for metric taps: enter the mm tap-drill and read the nearest fraction or number drill your set actually contains.
Match a metric plan to an imperial drill set
A European furniture plan calls for an 8 mm dowel hole, but your bits are all fractions and letters. Enter 8 mm and the tool snaps it to 5/16 (0.3125 inch, about 0.0025 inch under) and shows the closest letter drill too. For a glued dowel that 5/16 is perfect; for a press fit you can see the exact gap and decide whether to ream.
Identify an unmarked bit with calipers
A loose bit rolled out of the case with no legible stamp. Measure the shank at 0.272 inch, type it in, and the tool flags it exact as letter drill I (0.272). Now you can label it, file it back in the index, and know exactly what it cuts the next time a plan asks for that diameter.
Buy the right drill index without overlap
Before ordering a combined number-letter-fraction set, you want to know what each system actually covers. Punch a few sizes through the chart to see how the number drills fill the gap below 1/4 inch and letters bridge 1/4 to 1/2, so you choose a 115-piece index that earns its drawer space instead of three sets that overlap.

Common pitfalls

Reading a number drill as a fraction. A
Trusting a faded stamp over a measurement. Stamps wear off and bits get mixed between sets. If the label and your caliper reading disagree, the caliper wins. Type the measured decimal in and use the exact-or-closest flag rather than the worn marking.
Treating the snapped fraction as exact for a press fit. Rounding any diameter to the nearest 1/64 can be off by up to 0.0078 inch. That is fine for wood and clearance holes, but for a bearing seat or reamed hole use the exact mm or inch value the tool shows, not the rounded fraction.

FAQ

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Drill Bit Size Chart: Fraction, Number, Letter, mm Converter

Equivalent sizes

Common drill bit chart

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Drill Bit Size Chart fits into your work

Conversion jobs

Conversion checks

Good next steps

Real-world use cases

Find the right tap drill before threading a hole

Match a metric plan to an imperial drill set

Identify an unmarked bit with calipers

Buy the right drill index without overlap

Common pitfalls

FAQ

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