Metric vs Imperial: The Complete Guide to Understanding Both Systems
Confused about metric vs imperial units? This complete guide explains the differences, which countries use which system, and how to convert between them easily.
What Are the Metric and Imperial Systems?
Two major measurement systems dominate the world today: the metric system and the imperial system. Understanding the difference between them is essential for travellers, students, cooks, scientists, and anyone who works across international borders. This guide breaks down everything you need to know about both systems — and how to convert between them quickly.
The Metric System (SI Units)
The metric system, formally known as the International System of Units (SI), is the global standard for measurement. It was developed in France during the 1790s and has since been adopted by almost every country in the world as the official system of measurement for science, industry, and everyday life.
The metric system is built on powers of 10, which makes it extremely easy to convert between units. You simply multiply or divide by 10, 100, or 1000.
Core Metric Units
- Length: millimetre (mm), centimetre (cm), metre (m), kilometre (km)
- Weight: milligram (mg), gram (g), kilogram (kg), tonne (t)
- Volume: millilitre (ml), litre (L)
- Temperature: Celsius (°C)
- Speed: kilometres per hour (km/h)
Which Countries Use Metric?
Almost every country in the world officially uses the metric system — including all of Europe, Africa, Asia, South America, and Australia. In total, over 95% of the world's population uses metric as their primary system of measurement.
The Imperial System
The imperial system originated in Britain and was the standard measurement system throughout the British Empire. Unlike metric, imperial units do not follow a consistent base — conversions between units require memorising specific numbers like 12 (inches in a foot), 3 (feet in a yard), and 5,280 (feet in a mile).
Core Imperial Units
- Length: inch (in), foot (ft), yard (yd), mile (mi)
- Weight: ounce (oz), pound (lb), stone (st), ton
- Volume: fluid ounce (fl oz), pint (pt), quart (qt), gallon (gal)
- Temperature: Fahrenheit (°F)
- Speed: miles per hour (mph)
Which Countries Use Imperial?
Only three countries in the world have not officially adopted the metric system: the United States, Liberia, and Myanmar. The United Kingdom officially uses metric but still uses imperial units informally for road distances (miles), beer (pints), and body weight (stones and pounds).
Metric vs Imperial: Side-by-Side Comparison
| Measurement | Metric Unit | Imperial Unit | Conversion |
|---|---|---|---|
| Short distance | centimetre (cm) | inch (in) | 1 inch = 2.54 cm |
| Medium distance | metre (m) | foot (ft) | 1 foot = 0.3048 m |
| Long distance | kilometre (km) | mile (mi) | 1 mile = 1.609 km |
| Small weight | gram (g) | ounce (oz) | 1 oz = 28.35 g |
| Body weight | kilogram (kg) | pound (lb) | 1 kg = 2.205 lb |
| Liquid volume | litre (L) | gallon (gal) | 1 US gal = 3.785 L |
| Temperature | Celsius (°C) | Fahrenheit (°F) | °F = (°C × 1.8) + 32 |
| Speed | km/h | mph | 1 mph = 1.609 km/h |
Why Does the US Still Use Imperial?
The United States came close to adopting the metric system in the 1970s when Congress passed the Metric Conversion Act of 1975. However, the conversion was made voluntary rather than mandatory, and public resistance meant it never became widespread in everyday life. The US uses metric extensively in science, medicine, the military, and manufacturing — but daily life, road signs, and retail still run on imperial units.
Key Conversion Formulas to Memorise
- Kilometres to miles: multiply by 0.621 (or divide by 1.609)
- Kilograms to pounds: multiply by 2.205
- Litres to US gallons: multiply by 0.264
- Celsius to Fahrenheit: multiply by 1.8, then add 32
- Centimetres to inches: divide by 2.54
Which System Is Better?
From a scientific and practical standpoint, the metric system is considered superior because of its base-10 structure, which makes calculations far simpler. You never need to remember that there are 5,280 feet in a mile or 16 ounces in a pound — you simply move the decimal point. This is why every scientific publication in the world uses metric units exclusively.
That said, imperial units are deeply embedded in American and British culture, and familiarity with both systems remains a valuable skill for anyone operating internationally.
Convert Between Systems Instantly
Use our free converters to switch between metric and imperial units without any manual calculation:
- Length Converter — metres, feet, inches, kilometres, miles
- Weight Converter — kilograms, pounds, ounces, grams
- Volume Converter — litres, gallons, pints, fluid ounces
- Temperature Converter — Celsius, Fahrenheit, Kelvin
- Speed Converter — km/h, mph, knots