A #21 drill bit measures 0.1590 inches — about 4.04 mm — and is the standard tap drill for 10-32 threads.
For the full breakdown, see our best 21 Drill Bit guide.
Nobody guesses this one correctly from the name alone. A #21 drill bit is a wire-gauge bit, so the “21” does not mean 21/64 of an inch or a decimal of 0.21. It is a position on a gauge scale that happens to equal 0.1590 inch. That translates to 4.04 mm, and across US shops the size is best known as the drill you reach for before tapping 10-32 threads.
#21 Drill Bit Size: Decimal And Metric Values
The #21 drill bit diameter is 0.1590 inch in decimal measure. Converted to metric, 0.1590 × 25.4 = 4.0386 mm, which is why charts list the size as either 4.04 mm or 4.039 mm depending on rounding. LittleMachineShop’s number drill size chart records the 0.1590-inch figure, and Newman Tools’ decimal equivalents table lists the same value, so both unit systems agree. Whenever you compare a #21 against fractional or metric bits, do it by decimal inches — that is the measurement every drill system shares.
Here is how the #21 sits against the bits it is most often confused with:
| Drill Designation | Decimal Inches | Metric (mm) |
|---|---|---|
| #20 (next size up) | 0.1610 | 4.09 |
| #21 | 0.1590 | 4.04 |
| #22 (next size down) | 0.1570 | 3.99 |
| 5/32 in. | 0.1563 | 3.97 |
| 4.0 mm metric bit | 0.1575 | Send us your feedback Tell us what you thought about this post — we read every note. |