The four series
How drills are sized
- Fractional
- Inch sizes named by their diameter, in 1/64 in steps to 1/2 in and 1/32 in steps above. Most woodworking twist, brad-point, spade and Forstner bits in the US are sold in these sizes.
- Metric
- Named by their diameter in millimetres, usually in 0.5 mm steps in this range. Common on European tools, dowels and hardware such as 5 mm shelf pins and 35 mm hinge cups.
- Number
- Wire-gauge sizes from #80 (0.0135 in) up to #1 (0.2280 in). The higher the number, the smaller the bit. They fill the gaps between fractional sizes below 1/4 in.
- Letter
- A (0.234 in) to Z (0.413 in), carrying on above the number series. The letters run the right way: A is smallest.
Number and letter drills are machinists’ sizes. In woodworking they mostly matter when a plan or a piece of hardware names one, or when the fractional bit you have is just too tight or too loose.
Matching a size across systems
Find the size you were given, read its decimal inches, and look for the closest bit you own in another series. The chart gives decimal inches to three or four places for every size, so the comparison is a matter of reading down a column.
| Bit | Decimal inches | Millimetres | Difference from 6 mm |
|---|---|---|---|
| 15/64 in | 0.2344 | 5.95 | 0.05 mm small |
| 6 mm | 0.2362 | 6.00 | — |
| Letter B | 0.238 | 6.05 | 0.05 mm large |
| 1/4 in (letter E) | 0.2500 | 6.35 | 0.35 mm large |
A plan calls for a 6 mm hole and you have only inch bits. 15/64 in is the nearest fractional size, a hair small; 1/4 in is a third of a millimetre over — loose for a 6 mm dowel. Letter E and 1/4 in are the same size.
What this chart does not cover
- Screw and tap drill sizes. Which bit to use for a pilot hole, clearance hole or tapped thread depends on the screw, the material and the wood’s density; this chart gives only the bits’ diameters.
- Actual hole size. A bit cuts slightly over its nominal size, more so in a hand drill or a worn bit. Drill a test hole in scrap when the fit matters.
- Large boring bits. Forstner, spade and hole-saw sizes above 1 in follow the maker’s range and are not listed.
Sources
The number and letter series are standardised in ASME B94.11M; the standard itself was not reviewed, and the sizes are as published in the two charts above. Fractional and metric bits are named by their diameter, so their decimal and millimetre values are exact conversions (1 in = 25.4 mm), computed rather than copied.
- 1.
Drill Size Chart. FastenerMart.
Used for: Decimal-inch sizes of the number (#1–#80) and letter (A–Z) drill series.
- 2.
Number Drill Sizes. Little Machine Shop.
Used for: A size-by-size cross-check of the number drill series; all 80 sizes agree with FastenerMart.
About this chart
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