Trying to match your spanners and bolts can be a confusing and frustrating process. You hear a lot of phrases and words being thrown about, ‘M5’ this and ‘M12’ that, and you may not understand everything being said. This guide is here to help you, and hopefully give you a better understanding and put a stop to the confusion once and for all!
First of all, there are lots of types of spanners and sizings which you will need to identify for the intended task, whether it’s plumbing, fixing a car or DIY. So before buying a spanner make sure you know exactly what you need! Most lists of spanner sizes are divided into 2 rows, Nominal Size and Spanner size.
There are a variety of spanners to choose from, such as:
All of these spanners can be used for different professional and DIY tasks and each has their own benefits.
Combination spanners have either a doubled ended traditional spanner, or a spanner and a ratchet. The old method of how you used to measure the size of a spanner used to be; the spanner size was named after the fastener size.
For example if you were using a ¼’’ fastener size on a ¼’’ thread, the spanner would be called a ¼’’ Wide spanner. However the gap in the spanner would be 13.34mm wide, and ¼’’ is 6.35mm. Seems confusing, right? This method throws people off and only really applies if you’re using equipment and tools from pre 1970’s.
| Fastener Size | Spanner Size |
| M2 | 4mm |
| M3 | 5.5mm |
| M4 | 7mm |
| M5 | 8mm |
| M6 | 10mm |
| M7 | 11mm |
| M8 | 13mm |
| M10 | 17mm |
| M12 | 19mm |
| M14 | 22mm |
| M16 | 24mm |
| M18 | 27mm |
| M20 | 30mm |
| M22 | 32mm |
| M24 | 36mm |
| M27 | 41mm |
| M30 | 46mm |
| M33 | 50mm |
| M36 | 55mm |
| M39 | 60mm |
Luckily it’s easier now and the imperial system relates directly to the spanner size. The measurement is taken between the two parallel sides of the head of the fastener.
For example, a ½” fastener fits a spanner with a ½’’ head. Metric fasteners often use a different sizing system to metric spanners, which could cause confusion, as the fastener size relates to the thread size rather than the head size.
| Metric Size | Imperial Size |
| 8mm | 5/16'' |
| 19mm | 3/4'' |
| 22mm | 7/8'' |
| 24mm | 15/16'' |
| 27mm | 1 1/16'' |
You may have noticed that some bolt sizes are labelled as “M2” or “M4”.
The M stands for metric and the number after the ‘M’ refers to the width of the thread on the fastener in millimetres.
For example, an M10 Bolt equates to Metric 10mm, which is the width of the thread on the bolt. This method works for all sizes of M bolts from M1.6 all the way to M160.
