187 drill bits — four series in one table
Metric, fractional-inch, number and letter bits side by side. Which inch drill sits closest to 6.8 mm, how many millimetres a #7 is, and which screw thread each one taps.
The sizes you reach for when tapping
A tap drill is the thread’s outside diameter minus its pitch, which lands between the round numbers: 2.5 mm for M3, 3.3 for M4, 6.8 for M8. That is exactly why drill sets carry those odd sizes.
Each series names its sizes differently
Metric bits state the millimetres outright, inch bits state sixty-fourths. Number and letter bits carry names inherited from old wire gauges, where no formula links the label to the size.
metric (mm)
fractional inch
number
letter
A #7 has to be looked up
In the number series #1 is the fattest and higher numbers get thinner — but by no fixed step. That a #7 is 5.1 mm and a #21 is 4.04 mm can only be looked up. Letters A to Z continue upward from there.
Crossing from one series to another
Use this when the only bits at hand belong to another series. For tapping, prefer the slightly smaller bit over the slightly larger one — a hole can be opened up later, never closed.
How to read this
- Inch bits are counted in sixty-fourths — 16/64 reduces to 1/4.
- Higher numbers are thinner; later letters are thicker, with A picking up above #1.
- An inch is 25.4 mm, so multiply the inch value by 25.4 to get millimetres.
- For tapping, drill with the bit nearest to the thread’s outside diameter minus its pitch.
Frequently asked questions
Q. How many millimetres is a #7 drill?
5.1 mm. The number series inherits its names from old wire gauges, so no formula connects the number to the size — you have to look it up.
Q. Can I tap a metric thread with an inch bit?
If one is close enough, yes. Prefer the slightly smaller bit over the slightly larger: a hole can be opened up, never closed.
Q. Why do drill sets include odd sizes like 3.3 mm?
Because that is the tap drill for M4. Tap drills are outside diameter minus pitch, which always lands between the round numbers.
Q. How do letter and number drills meet?
The numbers top out at #1 (5.79 mm) and the letters start at A (5.94 mm), running up to Z (10.49 mm). The two series join right there.
Q. Why sixty-fourths?
Halving an inch again and again lands on 1/64. That is why a bit marked 16/64 is simply called 1/4.