Screw size converter
Convert numbered machine and wood screw sizes to diameters in millimeters and inches, and back.
That's about the same as a common wood screw (#8).
What it means
How thick is that screw?
Major (outside) thread diameter. The number gives the thickness only; length and thread pitch are separate (8-32 × 1/2" is a #8, 32 threads per inch, half an inch long).
Every unit
That screw in every unit
Learn
Three things worth knowing
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13 thousandths a step
Each number adds 0.013 in (0.33 mm). A #8 is just over 4 mm, close to an M4; a #10 is close to M5.
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Wire gauges again
Screw numbers grew out of the wire gauges screws were cut from. The American Screw Company tidied them into this straight-line formula in the 19th century.
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8-32 is not 8 mm
In 8-32 the 8 is the screw number (4.17 mm) and 32 the threads per inch, not a size in millimeters.
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Background
About screw numbers
Small US screws are sized by number, and the number is a formula: diameter = 0.060 + 0.013 × n inches. A #0 is 0.060 in (1.52 mm), a #8 is 0.164 in (4.17 mm), a #10 is 0.190 in (4.83 mm).
From ¼ inch up, screws are sized in fractions instead. Metric screws need no conversion: an M4 is 4 mm across.