Spanner Size Chart: Metric & Imperial Table (Free PDF)

Note this is the width of the hex head across the flats, not the thread diameter. For example: An M10 bolt has a 10 mm thread but its head is 16 mm wide, it requires a 16 mm spanner.

ISO bolts and nuts currently has the same head dimensions for each thread size. The same spanner works for both ends of the joint. Before you select a spanner you can check an unknown bolt with a thread gauge in the coarse pitch column.

One Metric System blunder is the DIN column of legacy sizes. The previous DIN series was overtaken by ISO standards and four popular bolt head sizes shrank. M10 head from 17 mm down to 16; M12 head from 19 mm to 18; M14 head from 22 mm to 21; M22 head from 32 mm up to 34.

Fasteners are still made today in the old sizes for use in industrial applications and cars and trucks. When your 16 mm spanner turns freely on what you think is an M10 head, reach for a 17 mm spanner first rather than jumping to the conclusion that this is a worn-out bolt. Both sizes will be in a well-stocked toolbox.

AS 1252 is for high strength bolted assemblies. They're like the European EN 14399 family. Structural bolts has bigger heads because they can absorbs with more torque and they have a greater bearing surface.

For example, a M20 structural bolt needs a spanner 32 mm where a M20 commercial bolt would of need a 30 mm spanner. Never mix the two types. The bigger head is a visual clue to indicate structural grade bolts.

There are other families of bolts with even smaller heads, JIS small-head bolts and flange bolts for instance. Their heads will be one or two sizes smaller different than shown here.

The Imperial wrench sizing follows the ASME B18.2.1 standard which is easy enough to memorize. Basically the wrench size is approximately 1.5x the size of the bolt diameter. So if it’s a 3/8 inch bolt, the wrench will be 9/16 inch, if it’s a 1/2 inch bolt, it’ll be 3/4 inch. And you just extrapolate from there in ascending order on the size chart.

It’s all about the width across the flats of the head of the part. Hex nuts (standard) is the same size as their corresponding bolts, so you can wrench them with one wrench for both pieces.

I know a lot of folks don’t think it matters, but the big hex column does matter. Standard hex heads has 1/8 inch less width on their flat side than heavy hex heads do for the same diameter.

Fasteners under the F3125 standard include all the structural bolts like the ASTM A325 and A490 grades. They will always be heavy hex heads. For example, a 3/4 inch structural bolt is sized for a 1-1/4 inch wrench. It isn’t a 1-1/8 inch wrench.

The heavy hex nut is also thicker. Same wrench as the heavy head. If you’re around any anchor bolts, flanges or other steelwork, then you want the heavy hex sizes. Why do the sizes in an ironworker's spud wrench seem to jump around so weirdly?

This chart compares nearest metric and imperial spanners and indicates which one will fit the closest. The millimeter conversion column tells you what spanner size is nearest. You can see from this chart that some metric and imperial sizes are very close.

For example, a 3/4 inch head fits a 19 mm spanner just fine and can be worked safely. And a 1/2 inch head will have a little slop with a 13 mm spanner, which is exactly how corners gets rounded under torque.

In an emergency or if trying to identify a spanner, use the table for reference. For anything that needs to be torqued down tightly, use the proper system. It should of been used to avoid rounding.

The Whitworth system is strange, as the spanner stamp indicates the thread diameter instead of the head size. So what appears to be a 1/4 W spanner is actualy 0.525 inches, or 13.34 mm across flats. That’s a lot bigger then a current 1/4 inch AF wrench.

In fact, the markings are for thread diameter rather than across-flats measurements. They sized the tool for the bolt it was meant to fit; that’s why they expected you to know their convention. It’s a reminder of the early part of the last century when that was British standard.

What is more, each of these spanners overlaps with the next size BSF. This was because British Standard Fine had one-size-smaller heads than those on the same-diameter threads. That means authentic antique spanners will be marked both ways (eg 1/4 W and 5/16 BSF), since they are actually the same spanner for two different standards.

In fact, there are even other changes in hexagon sizes that occurred in various British standards from 1911-1940. To save on steel during the Second World War, head sizes was cut down once more. A 1/4 BSW hexagon from the war time is then equivalent to what would of been called 5/16 BSW by a pre-war spanner. You can therefore find very old, and wartime, hardware one spanner size off this table.

These are sizes you’ll encounter on machines from the steam age up through 1970s machines: agricultural equipment, machine tools, cars, and especially pre-1970s British motorcycle.

Whitworth hexagon spanners don’t snug onto metric or AF. You can force it, but that’s the quickest route to ruining a vintage fastener you can never replace. The millimetre column is there only to identify size, not substitute. A Whitworth-only set of sockets and wrenches isn’t optional if you’re working on old British machines with any regularity.