Identify Any Screw or Bolt Size

Measure two things: the major (outside) diameter of the threaded part and the thread pitch — the distance between adjacent threads. Together they give the screw size, for example M4 × 0.7 mm. This tool measures both, on screen or from a printed gauge.

Thread pitch is the distance from one thread crest to the next. Two screws can share the same diameter but have different pitches (coarse or fine), and they will not fit the same nut or hole. Matching the pitch is what makes an identification reliable.

Metric screws are sized by diameter in millimetres (M3, M4, M5). Imperial machine screws use a gauge number and threads-per-inch (#6-32, #8-32). This tool reports the closest match in both systems so you can decide which one your screw belongs to.

After you calibrate the screen against a bank card, on-screen measurement is good to roughly a quarter of a millimetre — enough to identify standard screw sizes. For the most exact reading, print the paper gauge and verify the 50 mm bar with a ruler.

Yes. The "Measure on screen" mode needs only a calibrated display and any bank or ID card for reference. The printable PDF is an option for people who prefer a physical gauge.

The PDF is vector artwork at exact millimetre scale. Print it at 100% / Actual size (never "Fit to page") and measure the built-in 50 mm bar to confirm. If the bar is not 50 mm, reprint with the correct scale.