Identifying the size of my Nut

1 Measure across the flats

The distance across the two parallel flats is the spanner size. Measure it with calipers or a ruler, or pick the imperial spanner that fits.

Overview

Calipers

Ruler

MEASURING GUIDE

How to measure a hex nut

A hex nut is sized three ways: across the flats (AF), the spanner size; the height through the body; and the inside across the thread. An M8 nut is AF 13mm, height 6.5mm, M8 thread.

Thread M8 AF 13mm Height 6.5mm Inside ≈6.8mm

MEASURING GUIDE

Measure with a caliper

Close the caliper's external (lower) jaws onto two opposite flats, the spanner size. That is the across-flats (AF) reading. An M8 nut is 13mm AF.

Thread M8 AF 13mm Tool Vernier caliper External jaws

MEASURING GUIDE

Measure with a ruler

Lay a steel rule flat across the middle of the nut, line one flat up to zero and read across to the opposite flat, the across-flats (AF) size. An M8 nut is 13mm AF.

Thread M8 AF 13mm Tool Steel rule Across flats

Getting the across-flats (AF) right is the best place to start. It is the spanner size, measured across the two parallel flats.

Spanner size (mm, imperial in brackets) Select a spanner size

Outside diameter mm

2 Measure the hole

MEASURING THE HOLE

Measure the hole, not the thread

The hole always measures smaller than the thread size. An M10 nut measures about 8.5mm through the hole, not 10mm. Measure at the narrowest point. This is what separates sizes that share a spanner.

Select a spanner size in Step 1 to see the likely hole sizes, or measure below.

Prefer to measure it yourself? With calipers or a hole gauge, measure straight across the hole at its narrowest point and type the reading here in millimetres. We will match it to the closest stocked sizes, so even an approximate figure helps.

3 Measure the height

MEASURING THE HEIGHT

Measure the overall height

Stand the nut on a flat surface and measure straight up the side, from the bottom face to the top. Height is a strong clue to the type of nut: a dome or coupler is tall, a thin lock nut is short.

Measure with calipers or a ruler against the side of the nut. An approximate figure still helps.

4 Measure the outside diameter

Measure across the outside of the threaded body with calipers - the widest point of the external timber thread. This sizes the insert, the same as the allen key does.

4 Coarse or fine?

Coarse vs fine is hard to tell by eye, even for experienced hands - so we work it out from your measurements where we can, and say so in the result. When a size genuinely comes in both, use the checks below to confirm.

Count the threads through the nut (easier on bigger nuts)

COUNTING THREADS

Count the ridges through the nut

Look through the nut and count the thread ridges from one face to the other. The nut's thickness divided by the pitch sets how many you see, so a coarse thread shows fewer, wider ridges and a fine thread shows more. Hard on small nuts, clearer on bigger ones.

Identify the size in Steps 1 and 2, then enter the nut thickness to see the coarse vs fine ridge count.

Threads you counted threads

i Pick your exact cage nut

5 If you have the matching bolt, check it

A bolt's thread is usually easier to read than a nut's, and a bolt and its nut are always the same size. So if you have the bolt, it is the fastest way to confirm.

6 Check the bolt's head markings (optional)

If you have the bolt, the stamp on its head is a strong clue to the thread family. It will not tell coarse from fine, but it does separate metric from imperial, and Whitworth from Unified.

Look at the top of the bolt head. Which of these does it most look like?

Lines from the centreUnified UNC / UNF A letter (R, S, T)Whitworth / BSF Numbers (8.8, 10.9)Metric A2 or A4Stainless (metric) No clear markingsNot sure - keep all