#21 drill bit — 4.039 mm

Closest in other series

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.

One step up and down

number

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.

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.