Drill Size Comparison Chart

Size	Decimal inch	Millimeters
#60	0.0400	1.016
#50	0.0700	1.778
#40	0.0980	2.489
1/8	0.1250	3.175
#30	0.1285	3.264
9/64	0.1406	3.572
5/32	0.1563	3.969
3/16	0.1875	4.763
1/4	0.2500	6.350
5/16	0.3125	7.938
3/8	0.3750	9.525
1/2	0.5000	12.700

This is a comparison chart, not a tap drill or clearance hole specification. Choose a drill for the actual fastener, thread, fit, and material.

Sage pointing while explaining something

Use the chart as a starting point

Reference values help compare sizes and plan a job. Product specifications, codes, tolerances, and the actual material still control the final choice.

Sage's guided lesson

Numbers, letters, fractions: why drills use all three

A drill index that mixes number, letter, and fractional sizes looks confusing until you know why each system exists. This lesson explains where the sizes overlap, how to read the chart quickly, and where the numbering habit actually came from.

Sage dressed as an ancient Greek philosopher

Three systems, one drawer

Why a drill index has numbers, letters, and fractions

Fractional drills cover common inch sizes in steps of a 64th. Number drills fill in the small end of the range, from #80, the smallest, up to #1, the largest of that set. Letter drills pick up where the numbers stop, running A through Z toward larger diameters. Metric drills are a separate system entirely, sized in whole and half millimeters.

None of these systems were designed together. They grew up in different shops for different jobs, and the overlap is exactly why the chart is useful: it lets you cross-reference a size no matter which system a print or tap chart happens to use.

Sage's rule

If a print calls out a number or letter size you don't recognize, look up its decimal inch value before you touch the drill press. Never assume "bigger letter" or "bigger number" always means bigger hole; check the actual decimal.

Using the chart

How to read and use this chart

Find the size you already know in whichever column you have. If a fastener spec gives you a fraction, read across to the decimal inch and millimeter columns to confirm the exact value before ordering or chucking a bit.

Quick conversion habit

decimal inch × 25.4 = millimetersmillimeters ÷ 25.4 = decimal inch

When a drill size falls between two chart rows, the decimal inch value is what matters for clearance and tap drill calculations, not which named system it belongs to.

I know this is off topic, but this reminds me of a joke I heard when teaching this to one of my interns, What do you call a bear stuck in the rain with no umbrella? Wet. Yeah, I don’t see the connection either, but I think it is cute!

Before you chuck up a bit

Tips and common mistakes

Worn or resharpened bits can run slightly undersize. Measure a suspect bit with calipers instead of trusting the stamped size.
Number and letter drills are easy to mix up by eye. Keep the index sorted and labeled rather than guessing from the shank stamp.
A tap drill size is not the same thing as a clearance drill size for the same fastener. Use the Tap Drill Calculator or the Fastener Hole Reference to keep the two straight.

Do one reality check

Metric and inch drills that look close, like a 5mm and a 13/64 in bit, are not interchangeable for a precision fit. Confirm the decimal value before substituting one for the other.

Where the numbering came from

A little history

Numbered drill sizing grew out of 19th century wire gauge practice, where a higher gauge number meant a thinner wire. Twist drill makers borrowed that logic directly, which is why a #80 drill is tiny and a #1 drill is comparatively large. Letter sizes were added later to extend the range upward without renumbering everything that already existed in machine shops.

Fractional inch drills existed alongside these systems the whole time because they matched the way shop prints were already dimensioned. No single body ever unified the three, so the mixed system in this chart is the practical result of a century of shops solving the same problem in parallel.

That drills it home

Class is over. It is officially cheese fries time.

You now know why a drill index has three different numbering habits crammed into one drawer, and how to cross-reference between them without guessing. That earns cheese fries, Sage's favorite way to celebrate a finished lesson.

Quick answers

Drill size questions

Why do smaller drill numbers mean bigger holes?

Numbered drills borrowed their scale from wire gauge, where a higher gauge number always means a thinner wire. Twist drill makers kept that convention, so #1 is a comparatively large drill and #80 is one of the smallest.

What is the difference between a letter drill and a number drill?

Both fill gaps between fractional inch sizes. Number drills, 1 through 80, cover the smaller end of the range, and letter drills, A through Z, continue upward into larger diameters.

Can I substitute a metric drill for a close inch size?

Only if the decimal inch values are close enough for the tolerance the job needs. Compare the actual decimal or millimeter values in the chart rather than assuming a similar-looking size is equivalent.

Why does my drill measure slightly smaller than its stamped size?

Resharpening removes a small amount of material from the cutting edges over time, which can leave a bit running under its original diameter. Measure worn bits with calipers when a hole size matters.