Bolt & Nut Size Finder

Hold a bolt against the screen and match its thickness to find its metric size (M6, M8 …) or inch size (1/4 in …), its length, and the hex key or wrench that fits. Use it to identify a screw before you buy a replacement.

How to use

1. On your first visit, press "Calibrate screen" and match the screen to a card or a coin.
2. On "1 Diameter", stand the bolt with its threads on the guide line and match the yellow bar; the "Size found" and other candidates appear.
3. On "2 Length", lay the bolt down and measure it. Countersunk heads are measured with the head; other heads from under the head.
4. On "3 Head and tool", choose the "Head type" to see the wrench, hex key or Phillips driver size.
5. With only a nut, measure its width across flats on "Nut only". If you know the size, pick it on "Size table" to see a real-size drawing.

How it works

• Metric threads M1.6 to M36 and inch threads #0 to 1 in (UNC and UNF) are included. Each value was checked in October 2026 against at least two agreeing sources.
• A screen reading is off by about 0.3 mm, so all sizes within that range (for example M8 and 5/16 in) are shown as candidates.
• For metric threads the tap drill is the diameter minus the pitch. Safety-related values such as tightening torque are left out.

Questions and answers

Why does my M8 bolt measure 7.8 mm?

That is normal. Real threads are made 0.1–0.2 mm under their nominal size, so an M8 bolt usually measures about 7.8–7.9 mm.

What size wrench fits an M8 bolt?

13 mm for ISO hex bolts and nuts. For M10, both 16 mm (ISO) and 17 mm (older DIN) heads are in use, so check both values on "3 Head and tool".

How can I tell a metric thread from an inch thread?

Hold the threads against the real-size combs on "Thread pitch", or try a nut of a known size. The right nut turns on by hand all the way.