Free Printable Bolt & Thread Gauge

A print-at-home paper tool that identifies a mystery fastener with no calipers and no login: lay a bolt on the sheet to read its thread pitch, shank diameter, and wrench/socket size. Print at 100% (Actual Size) on US Letter or A4, verify the ruler, and the gauge is accurate to the millimetre.

Print it at 100% — or the measurements will be wrong

This only works if your printer does NOT scale the page. In the print dialog choose "Actual Size" or "100%", never "Fit to Page". Then check the printed ruler: the 100 mm mark must measure exactly 100 mm and the 4-inch mark exactly 4 inches. If either is off, reprint at 100%. Paper reads coarse pitches and common sizes well; for fine pitches or precise diameters, confirm the reading with calipers or a steel gauge.

How to use the gauge

1. Print page 1 at 100% and verify the rulers. 2. Thread pitch: lay the threaded part across each comb until the teeth drop into the grooves with no gaps — the label is your pitch (mm) or threads-per-inch. 3. Shank diameter: slide the plain (unthreaded) part between a labelled pair of lines until it just fits. 4. Wrench size: settle the bolt head into the smallest hex outline it fills flat — that is your socket size.

What's on the gauge

Thread-pitch combs for metric (0.5, 0.7, 0.8, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5 mm) and SAE/UNC (8 to 24 threads per inch); shank-diameter slots from M3 to M16 and #8 to 1/2 inch; a wrench/socket across-flats hex gauge from 8 to 19 mm and 7/16 to 3/4 inch; and precise mm and inch rulers that double as the print-scale check. Page two adds size-to-pitch-to-socket reference tables for metric coarse and fine threads and for SAE UNC and UNF.

Letter and A4 — same scale

Two versions are provided: US Letter (8.5 x 11 in) and A4 (210 x 297 mm). The gauges print at identical physical scale in both — only the surrounding paper differs — so use whichever your printer is loaded with. Both are free to download, print, and share.

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