#20 drill bit — 4.089 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.

Threads this bit taps

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.

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.