It’s an argument as old as time. Imperial? Or Metric? For some of us, it depends on our age and what we were taught at school. For others, it depends on what country we were raised in. But whatever the reason, two distinctive set of measurements persist – and this can make things a little complicated when it comes to hand tools.

In this article, we take a closer look at the two types of measurement and the relationship between them.

1. You can’t really compare them

You might think that it’s as easy as finding a comparison chart to convert metric to imperial or vice versa. In truth, the two measurements aren’t comparable. This is because imperial is measured in inches and there’s very little crossover between imperial measurements and standard metric ones.

What this means in terms of hand tools is that there’s no exact equivalent for a 9/16” spanner in metric units. So, if you’re working on something that requires a metric tool, then you can’t just swap an imperial tool in and hope for the best. You need to go metric to get the required accuracy and avoid any potential damage.

2. What you need depends on what you’re working on

If we look at cars, then imperial fixings were generally used in the UK on vehicles up to the early 1990s. While newer cars tend to use metric as standard, imperial fixings still appear on American vehicles, aircrafts and other specialist equipment.
It’s therefore important to keep the project that you’re working on in mind as that’s likely to dictate if you need to head down the imperial or metric route with your hand tools – or a mixture of both.

3. Knowing your terms can help you to find the right tool

As well as the ‘imperial’ and ‘metric’ we all know and love, imperial is commonly referred to as ‘A/F’. This stands for ‘Across Flats’ and indicates that distance between the jaws of the open end of a spanner or the flat edges of a standard bolt. Equally, you might come across ‘SAE’, which stands for ‘the Society of Automotive Engineers’. This is an older term that was used on primarily US cars produced during the 1970s. SAE sockets are sized in inches and fractions of inches.

4. Knowing your fittings can also help

If the bolt you want to loosen doesn’t specify an imperial or metric measurement, then it could be tricky to know which type of tool you need. A simple rule is that if the bolt has lines on its head then it’s considered imperial. If it’s got numbers on the head then it’s metric and the higher the number, the stronger the bolt.

Another notable difference is that metric fasteners are coarser than their imperial counterparts. These are important points to be aware of if you come across an unfamiliar or unidentified fixture or fitting.

5. Comparison charts do exist

Okay, so while we said that you can’t really compare imperial and metric measurements, you can find comparison charts online and we’ve included an example below for a wrench.

Standard wrench Standard wrench
Size in inches Inches decimal Size in millimetres Inches decimal
5/32” 0.156 4 0.157
7/32” 0.219 6 0.236
5/16” 0.313 8 0.315
13/32” 0.406 10 0.394
15/32” 0.469 12 0.472
9/16” 0.563 14 0.551
5/8” 0.625 16 0.630
11/16” 0.688 18 0.709


As you can see though, the comparison isn’t 100% accurate and therefore you run the risk of using a tool that’s not quite fit for the job at hand. We therefore always recommend using imperial hand tools for imperial fixings and fittings, and metric tools for metric ones.

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