Metric vs Imperial Units: The Complete Guide
Updated: July 2026
Two measurement systems dominate the world: metric and imperial. Knowing how they differ — and how to convert between them — saves confusion whether you are cooking, travelling, studying or working. This guide breaks down both systems and the most common conversions.
What is the metric system?
The metric system (officially the SI) is a decimal system based on units of ten. Its core units are the metre (length), gram (mass) and litre (volume). Because everything scales by powers of ten (millimetre, centimetre, metre, kilometre), conversions within the system are simple. It is used by almost every country in the world.
What is the imperial system?
The imperial system uses units like inches, feet, miles, pounds and gallons. Unlike metric, its units do not scale by ten (12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile), which makes conversions harder. It is mainly used in the United States, with some use in the UK.
Why does the US still use imperial?
The US never fully switched to metric, largely for historical and cost reasons — changing road signs, manufacturing and everyday habits would be expensive and disruptive. Science, medicine and the military in the US do use metric, but daily life still runs on imperial.
The most useful conversions
- 1 inch = 2.54 cm
- 1 foot = 30.48 cm
- 1 mile = 1.609 km
- 1 pound = 0.4536 kg
- 1 gallon (US) = 3.785 litres
Our converters handle all of these instantly, in both directions.
Frequently asked questions
Which is better, metric or imperial?
Metric is generally easier because it scales by powers of ten, making conversions simple. Imperial persists mainly for historical reasons in the US. For science and international work, metric is the standard.
Why does the US use imperial units?
The US never completed a switch to metric, mostly due to the cost and disruption of changing infrastructure and habits. However, US science, medicine and the military do use metric.
How do I convert inches to centimetres?
Multiply the number of inches by 2.54 to get centimetres. For example, 10 inches = 25.4 cm. Our length converter does this instantly.