Thread Pitch Gauge — Pitch, Radii and Feelers Online

An online stand-in for the machinist's thread pitch gauge, radius template set and feeler gauges. Pitch is measured as the length of ten threads on a calibrated screen — the eyeball error is divided by ten, so neighbouring pitches stop blurring together. The true-size printable sheet carries 60° and 55° profile combs, radius arcs from R1 to R25 and an honest paper-feeler table.

How to measure a thread

Calibrate the screen

Hold a bank card against the frame and match its width — from then on the on-screen scale shows true millimetres.

Count ten threads

Lay the bolt along the scale, crests against the marks. Note the first crest, count ten gaps and drag the marker to the eleventh crest.

Read the answer

The tool divides the length by ten and names the pitch, TPI, flank angle and the sizes that carry it, with the closest match of the other system alongside.

In doubt — print the sheet

The true-size combs press against the thread until the crests line up. Print strictly at 100%; the 50 mm bar checks the scale with a ruler.

Identify any thread pitch from ten threads at once

Features

Ten threads instead of one

The difference between 1.25 and 1.27 mm pitch is a quarter millimetre over ten threads but mere hundredths over one. Measuring the ten-thread span turns a ±0.5 mm eyeball error into ±0.05 mm of pitch — enough to separate any two standard pitches.

Metric, inch and pipe threads

Seventeen metric pitches from 0.35 to 4 mm and sixteen inch rows from 8 to 40 TPI, including BSP pipe sizes. The printed profiles carry the true flank angle — 60° for metric and UN, 55° for pipe — so crest alignment is visible to the eye.

The answer comes with fastener sizes

The tool does not stop at "1.25 mm": it names where the pitch lives — M8 coarse, M10 and M12 fine — and shows the closest pitch of the other system with its exact value, so metric and inch stop getting confused.

Radius arcs with nothing to cut out

Nested quarter circles from R1 to R25: set the rounded corner into the crosshair and the arc it follows names the radius. No cutting a half-millimetre-accurate semicircle with scissors — the comparison works right on top of the sheet.

An honest paper feeler gauge

A sheet of 80 g/m² office paper is about 0.10 mm, and the table converts stacks into gaps. The limits are printed next to it: ±10% spread, verify a ten-sheet stack with a ruler, and gaps under 0.1 mm are steel-blade territory, not paper.

1:1 print with a verification bar

The sheet carries a 50 mm bar to check with a ruler and a ruler for counting ten threads. Screen mode calibrates against a bank card like every measuring tool in this family, so a millimetre on screen equals a millimetre in metal.

Why this beats holding up a ruler

A ruler answers "about a millimetre and a quarter" — fasteners need to know whether that is 1.25 or 1.0. Over a single thread the difference is thinner than the ruler's own line. This tool builds the measurement so the eyeball error is divided by ten, reports how confident the answer is, and honestly names the runner-up when the measurement lands between two pitches.

Questions and answers

Why measure ten threads and not one?

Because on a screen or ruler the eye is reliable to about half a millimetre. Over one thread that is half a pitch of error — useless. Over ten threads the same error becomes 0.05 mm of pitch, while neighbouring standard pitches differ by at least 0.1–0.25 mm. That tenfold gain is exactly why real pitch gauges compare several threads at once.

How is M8 × 1.25 different from 1/4-20 by eye?

Hardly at all — that is the trap: 1.25 and 1.27 mm are indistinguishable over one thread, and the 8 vs 6.35 mm diameters hide under plating and wear. Over ten threads the spans are 12.5 vs 12.7 mm and the tool separates them confidently. Such a pair threads together for a couple of turns, then binds.

How do I measure an internal thread — a nut or a tapped hole?

A comb cannot reach inside a nut. Two honest ways: thread in a bolt of known pitch that runs freely and measure that bolt; or press a paper strip against the end face of the thread, take the crest imprint and count ten gaps on the scale. Direct internal measurement is a job for plug gauges.

What is the 55° angle on some combs?

Those are BSP pipe threads (G sizes) and the old Whitworth standard: their flank angle is 55° instead of the 60° of metric and American UN threads. Pipe pitches are defined in threads per inch — 28, 19, 14 and 11 TPI — and the sheet draws them as separate combs at their own angle.

Does a paper feeler replace a steel set?

Only partly, and the sheet says so outright. 80 g/m² paper gives steps of roughly 0.1 mm with about ±10% consistency — fine for a valve-cover gap or a rough spark-plug check. Gaps under 0.1 mm, and any measurement decided by hundredths, need steel feeler blades.

How do I read the radius chart?

For an inside fillet: set the corner into the crosshair and see which arc the fillet follows. For a rod or an outside radius: lay the part on the sheet over the arcs and match the silhouette. The arcs run from R1 to R25, starting in 0.5 mm steps.

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Who is this tool for?

🔌 Electronics Enthusiasts

Published 14 Aug 2026 Author: The Supreme