Wrench, Socket and Allen Key Sizes by Bolt Standard

Once you know a bolt's thread designation, the spanner, socket or Allen key it needs follows directly from the standard it was made to — but the three major systems in everyday use (Metric, British Whitworth/BSF, and American Unified) each define across-flats sizes independently, so identical-looking thread designations can call for completely different tools. The charts below let you go straight from bolt size to the correct drive size, for both external hex heads (spanner or socket) and internal hex sockets (Allen key), without needing to measure a sample first.

Jump to Metric spanner sizes, Whitworth & BSF spanner sizes, Unified (UNC/UNF) spanner sizes, Metric Allen key sizes, and Unified Allen key sizes.

If you already know a bolt or nut's across-flats measurement and want to identify which standard it belongs to, our Spanner Size Comparison Chart cross-references every common jaw size against its Metric, AF and Whitworth equivalents. This page works the other way round — from a known bolt size to the tool you need — and adds Allen key sizing, which the comparison chart doesn't cover.

WRENCH, SPANNER & SOCKET SIZES (EXTERNAL HEX)

These tables apply to hexagon-head bolts, screws and nuts — the head sits proud of the surface and is driven from the outside, using an open-ended spanner, ring spanner, or socket. The size quoted is the across-flats (A/F) dimension: the distance between two parallel faces of the hexagon, measured with the jaws (or socket) square across the head rather than across the corners.

ALLEN (HEX) KEY SIZES (INTERNAL HEX)

Socket head cap screws, socket button screws, socket countersunk screws and grub/set screws are driven from an internal hexagonal recess in the head, using a hex key (Allen key) or hex bit rather than a spanner. Because the key sits inside the head rather than around it, key sizes are smaller than the equivalent external spanner size for the same nominal bolt diameter, and follow their own standard series.

RELATED PAGES

• Spanner Size Comparison Chart
• Metric Bolt Dimensions
• Imperial Bolt Dimensions
• Unified Bolt Dimensions
• Screw Head Types
• Screw Driving Types

FREQUENTLY ASKED QUESTIONS

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

Metric, AF and Whitworth across-flats dimensions are all defined independently of one another, so numbers that look close rarely match exactly. Using the wrong system tends to sit at a slight angle across the flats, concentrating load on the corners of the head and rounding it off under torque rather than gripping the full face. Always match the exact size and system where possible — see our Spanner Size Comparison Chart if you need to cross-reference a measured jaw size against all three systems.

Is a hex key the same as an Allen key?

Yes — "Allen key" is a genericised trademark (originally the Allen Manufacturing Company) for what is more generically called a hex key or hexagon wrench: an L-shaped or T-handled hexagonal bar that engages an internal hexagonal recess in a screw head.

Why is the hex key size smaller than the spanner size for the same nominal bolt diameter?

A socket head sits within the fastener's own body diameter rather than extending outward around it, so the internal hex recess is necessarily smaller than an external hex head of the same nominal thread size would be. Socket head screws are typically made from higher-tensile alloy steel to compensate, so they can still achieve comparable or greater clamping force despite the smaller drive.

What does A/F mean?

A/F stands for "Across Flats" and refers to the distance measured between two parallel faces of a hexagon head, nut, or key — as opposed to across the corners, which gives a larger measurement. Spanner, socket and hex key sizes are always quoted as the A/F dimension.

My bolt is a size that isn't in any of these tables — what's going on?

It may be a British Association (BA) fastener, which uses its own numbered sizing system distinct from BSW/BSF, a specialist or non-standard fastener, or a worn/rounded head that no longer measures true to its original size. Re-measure carefully across the flats with calipers and check the result against all four standards before concluding it's non-standard.