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What Spanner Sizes are Available?

Before buying your spanners make sure you are getting the right size. Most spanners come in a variety of sizes and size standards have changed over the years so there are a lot of spanner sizes to choose from. You can also buy full spanner sets that vary in sizes, pieces and styles which come in easy to carry cases.

Current Spanner Sizing Systems

Fasteners measured using the current AF (‘across flats’) imperial system relate directly to the spanner size. The measurement is taken between the two parallel sides of the head of the fastener. For example, a ¼” fastener fits a spanner with a ¼” head.


Metric fasteners often use a different size system to metric spanners and it can be confusing. The fastener size system relates to the size of their thread rather than the size of their heads. Some of the most common sizes are M6, M8, M10 and M12. ‘M’ stands for ‘metric’ and the number is the width of the threaded section of the fastener, in millimetres.

The heads of the fasteners are a standard size for each ‘M’ size but unfortunately, due to there being two standards in the UK, there is occasionally a little variation (usually 1 or 2mm). Usually, M6 fasteners require a 10mm spanner, M8 a 13mm spanner, M10 a 17mm spanner and M12 a 19mm spanner.


It is possible to use some metric spanners on imperial fasteners. It is not ideal but once in a while won’t hurt.

Old Spanner Sizing Systems

Before metrication and the AF imperial system, there was the British Standard Whitworth (W), British Standard Fine/British Standard Whitworth (BSF/BSW or BS) and the British Association (BA) sizing systems.


The spanners were named after the size of the fastener. For example, a ¼W fastener had a ¼” diameter thread size and the head was a standard size that would fit a ¼W spanner. You may come across some of these fastener sizes in older equipment and electronics (particularly pre-1970s). You may have a modern metric or imperial spanner that will fit the fastener or you may have to get a specific spanner sized to the old standards.

Scaffold spanners are the only spanners left that are commonly still made using the Whitworth standard. These come in 7/16W and 1/2W. See What is a scaffold spanner? for more information.

BA sizes are the most likely to crop up in old electronic equipment and if you are working with this sort of device you may wish to purchase sizes 2BA (jaw size of 8.2mm) and 4BA (jaw size of 6.3mm), as they are the most common. The largest BA size is 0BA which has a jaw size of 10.5mm.