Metric and imperial are two different ways of measuring the same world. Most countries use the metric system for almost everything, while the United States still uses inches, feet and pounds day to day — and the UK uses a mix of both. If you buy tools, parts or materials online, you will run into both systems constantly. This guide explains how each works, how they are linked, and the handful of numbers worth memorising.
How the Metric System Works
The metric system (officially the International System of Units, or SI) is built on powers of ten. For length, the base unit is the meter, and prefixes move the decimal point:
| Unit | Symbol | In meters | In inches |
|---|---|---|---|
| Millimeter | mm | 0.001 m | 0.03937 in |
| Centimeter | cm | 0.01 m | 0.3937 in |
| Meter | m | 1 m | 39.37 in |
| Kilometer | km | 1,000 m | 39,370 in (0.621 miles) |
Because everything is based on ten, converting within the system is just moving a decimal point: 1,250 mm = 125 cm = 1.25 m.
How Imperial (US Customary) Length Works
Imperial units grew out of older everyday measures, so the steps between them are uneven:
| Unit | Equal to | In millimeters |
|---|---|---|
| Inch | — | 25.4 mm (exact) |
| Foot | 12 inches | 304.8 mm |
| Yard | 3 feet / 36 inches | 914.4 mm |
| Mile | 5,280 feet | 1,609,344 mm (1.609 km) |
Inches are also split into fractions — halves, quarters, eighths and sixteenths — rather than decimals, which is why tape measures and drill bits carry markings like 7/16″. Our tape measure guide explains how to read them.
The Link Between the Two: 25.4
Since the international yard and pound agreement of 1959, the inch has been defined as exactly 25.4 millimeters. It is not a measured or rounded value — it is the definition. That makes every length conversion exact:
- Inches to millimeters: multiply by 25.4
- Millimeters to inches: divide by 25.4
- Feet to meters: multiply by 0.3048
Numbers Worth Memorising
- 1 inch = 25.4 mm
- 1 foot = 304.8 mm (about 30 cm)
- 1 meter = 39.37 inches (a little over 3 feet 3 inches)
- 100 mm ≈ 4 inches (actually 3.94 in)
- 1 mile ≈ 1.6 km
Where You Will Meet Both Systems
- Tools and hardware: wrenches, sockets, drill bits and bolts come in both. See our wrench size chart and metric bolt guide.
- Screens and wheels: TV, monitor and phone screens and bicycle and car wheels are usually described in inches, even in metric countries.
- Paper: A4 is metric; US Letter is in inches. See paper sizes in inches and mm.
- 3D printing and electronics: almost entirely metric. See 3D printer filament and nozzle sizes.
- Weather: rainfall is reported in mm in most countries and inches in the US. See rainfall mm to inches.
Tips for Working Across Both Systems
- Always write the unit next to every number. “12” could be millimeters, centimeters or inches.
- Convert once, at the start, and work in one system for the whole job rather than switching back and forth.
- Keep decimals until the end. Round only the final answer — rounding along the way adds errors.
- Do not assume “close” sizes are the same. 25 mm is not 1 inch, and 6 mm is not 1/4 inch.
- Use a two-scale tool. A tape measure or caliper that shows both mm and inches removes most conversion mistakes.
Frequently Asked Questions
Which countries use the imperial system?
The United States uses US customary units for most everyday measurements. Liberia and Myanmar are often cited as the other main exceptions. The UK uses metric officially but still uses miles, pints and feet and inches in everyday life.
Is imperial the same as US customary?
For length, yes — inches, feet, yards and miles are the same. They differ for volume: a UK (imperial) pint and gallon are larger than US ones.
Why is an inch exactly 25.4 mm?
It was agreed internationally in 1959 so that the US, UK and other countries would all use an identical inch, tied precisely to the metric system.
Related: MM to inches converter · MM to Inches chart (printable) · All measurement guides