Inch fractions and decimals — the workshop cheat sheet
Every 64th of an inch has an exact decimal and an exact millimetre value. Once memorised, they let you read any inch drawing without a calculator.
Fractional inches survive because drills, taps, wrenches and structural bolts are still manufactured to fractional sizes. A modern machinist reads 3/8-16 UNC as automatically as 10-32 metric. This is the reference table that makes it fluent.
The exact factors
1 in = 25.4 mm (exact) > 1/64 in = 0.015625 in = 0.396875 mm (exact) > 1/32 in = 0.03125 in = 0.79375 mm > 1/16 in = 0.0625 in = 1.5875 mm > 1/8 in = 0.125 in = 3.175 mm > 1/4 in = 0.25 in = 6.35 mm > 3/8 in = 0.375 in = 9.525 mm > 1/2 in = 0.5 in = 12.7 mm > 5/8 in = 0.625 in = 15.875 mm > 3/4 in = 0.75 in = 19.05 mm > 7/8 in = 0.875 in = 22.225 mm
Reading a fraction fast
Every fraction n/64 in has an exact terminating decimal because 64 is a power of 2, and 25.4 mm converts every one to an exact decimal in mm too. There is no rounding in the fraction system — the imprecision only enters when someone rounds to two decimal places on a drawing.
Drill and wrench sizing
US drill sets step by 1/64 up to 1/2, then by 1/32. A 'letter drill' set fills in between fractional sizes with 26 letters (A = 0.234 in through Z = 0.413 in). A 'number drill' set covers below 1/4 in with 80 sizes. Metric equivalents rarely land within 0.1 mm of a fractional size, which is why a US shop keeps both drill cabinets.
Worked example — a 3/8-16 tap
A 3/8-16 UNC tap needs a tap drill of about 5/16 in = 0.3125 in = 7.938 mm. The closest metric drill (8 mm = 0.315 in) is 0.062 mm larger — enough to reduce thread engagement from 75% to about 68%, which is still functional but marginally weaker. In critical joints, use the fractional drill.
The five-second recap
n/64 inch × 0.396875 = mm. n/16 inch × 1.5875 = mm. Every fractional inch has an exact mm equivalent — the only rounding in imperial machining is voluntary.
Try the converters mentioned in this article
FAQ
Why 1/64 and not 1/100?+
Historical binary halving with dividers. A machinist can bisect a line by eye to 1/64, but not to 1/100.
Are metric fasteners taking over?+
In consumer electronics and cars, yes. In US construction and aerospace, fractional-inch bolts still dominate.
What is a 'plus one' fractional size?+
One 64th above the nominal. A 3/8+1 drill is 25/64 in = 0.3906 in — used for clearance holes where a plain 3/8 is too tight.
This article was written by The Turbo Unit Converter engineering desk (Manufacturing & tooling) and last reviewed on 2026-06-28 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.