Spanner Size comparison Chart

A bolt head or nut's "size" refers to its across-flats (AF) dimension — the distance between two opposite flat faces — which is what determines the correct spanner or socket to use. Metric, imperial (AF), and Whitworth/BSF sizes don't map neatly onto each other, so a spanner that's close in number but from the wrong system will often fit poorly enough to round off the corners of the fastener rather than grip it properly. The chart below cross-references the common sizes so you can find the nearest equivalent, or confirm the exact spanner needed for a given bolt or nut size.

Dimensions shown are in mm, if you wish to convert to Inches, please use our universal converter.

RELATED PAGES

• Bolt Grades and Markings
• Fastener Abbreviations
• Engineering Glossary — Spanner Jaw Size

FREQUENTLY ASKED QUESTIONS

How do I measure a bolt head or nut to find its spanner size?

Measure across the flats (the distance between two parallel faces), not across the corners. Across-corners measures larger than across-flats and will lead you to the wrong size spanner.

Why doesn't a metric spanner fit an imperial bolt of a "similar" size?

Metric and imperial across-flats dimensions are defined independently and rarely coincide exactly — a spanner that's close but not exact will sit at a slight angle on the flats, concentrating load on the corners and rounding them off under torque. Always match the exact size where possible.

What does "AF" mean on a spanner?

AF stands for "Across Flats" and simply confirms the size marked is the across-flats dimension, which is the standard way of sizing spanners and sockets.

My bolt doesn't match any standard spanner size — what's going on?

It may be a Whitworth/BSF fastener (which uses its own across-flats sizing, distinct from AF), a specialist or non-standard fastener, or simply worn/rounded enough that it no longer measures true to size. Re-measure carefully across the flats and check it against the Whitworth column as well as metric and AF.