About the Drill Bit Size
Decimal drill bit measurements, the kind stamped on a bit's shank or listed in a machinist's reference table, don't always correspond to a size a tape measure or standard fractional bit set displays. This calculator takes a decimal inch value and finds the closest standard fractional inch size in 1/64-inch increments, along with the equivalent in millimeters, so you can match a decimal spec to the bit actually in your case.
How It Works
Enter a drill bit size as a decimal number of inches. The calculator multiplies that value by 64 and rounds to the nearest whole number, giving a count of 64ths of an inch. It then reduces that fraction to its simplest form using the greatest common divisor of the numerator and 64, and also reports the millimeter equivalent by multiplying the original decimal by 25.4.
Formula & Methodology
Take the number-7 drill bit as an example, listed at 0.201 inches in reference tables. Multiplying by 64 gives 12.864, which rounds to 13. Since 13 shares no common factor with 64, which equals 2 to the 6th power, and 13 is prime, the fraction doesn't reduce and stays as 13/64 inch, the nearest standard fractional size to that decimal spec.
Examples
Number 7 bit for a 1/4-20 tap
A 0.201 inch decimal spec, the standard drill size for tapping a 1/4-20 thread, rounds to 13/64 inch and converts to 5.11 millimeters.
A quarter-inch letter-F bit
A 0.257 inch decimal spec, close to a letter-F bit, rounds to 16/64, which reduces to exactly 1/4 inch, and converts to 6.53 millimeters.
Advantages
- Converts a decimal spec straight to the nearest fractional size in reduced form, so the result reads as a clean fraction rather than an unreduced one.
- Adds the millimeter equivalent in the same step, useful when comparing a US fractional bit set against a metric one.
- Handles the reduction automatically using the greatest common divisor, avoiding manual simplification errors on fractions that do reduce, like 16/64 down to 1/4.
Common Mistakes
- Assuming every decimal drill spec lands exactly on a clean fraction, when many number and letter bits fall between 64ths and only approximate a fractional equivalent.
- Mixing up which direction to round, when the calculator always rounds to the mathematically nearest 64th rather than always rounding up or down.
- Treating the millimeter output as a standard metric bit size, when it's simply the exact conversion of the decimal inch value, not a size that necessarily exists in a metric bit index.
Edge Cases to Watch For
- The result is always rounded to the nearest 1/64 inch. Number bits, like #1 to #80, and letter bits, A to Z, that fall between fractional sizes will be matched to the closest 64th, not returned as their own number or letter designation.
- Because rounding happens before the fraction is reduced, two decimal values that are very close to each other can round to the same 64ths count and therefore report the identical fractional size.
- The millimeter conversion uses the exact 25.4 mm/inch factor and is not itself rounded to a standard metric drill bit size, since metric bits are sold in their own separate size series.
Common Use Cases
- Machinists and toolmakers translating a decimal spec from a reference chart into the fractional bit they need to pull from a drawer.
- Hobbyists and DIYers who only have a fractional bit set on hand and need to match it to a tap drill size given in decimal inches.
- Anyone comparing a US fractional drill index against metric bits, using the millimeter conversion to bridge the two systems.