The world uses two main measurement systems: the metric system (used by most countries) and the imperial system (used mainly by the United States). Understanding both — and knowing how to convert between them — is essential in today's connected world.
Which Countries Use Which System?
Metric System
The metric system (officially called the International System of Units, or SI) is used by 95% of the world's population, including all of Europe, Asia, Africa, South America, and Australia.
Imperial System
The imperial system is primarily used by the United States. Myanmar and Liberia have historically used imperial units but are transitioning to metric. The UK uses a mix — metric for most official purposes, but imperial for road distances (miles) and some everyday uses.
Length Conversions
| Metric | Imperial |
|---|---|
| 1 millimeter (mm) | 0.0394 inches |
| 1 centimeter (cm) | 0.394 inches |
| 1 meter (m) | 3.281 feet / 1.094 yards |
| 1 kilometer (km) | 0.621 miles |
Weight Conversions
| Metric | Imperial |
|---|---|
| 1 gram (g) | 0.035 ounces |
| 1 kilogram (kg) | 2.205 pounds |
| 1 metric ton (tonne) | 2,204.6 pounds |
Volume Conversions
| Metric | US Imperial |
|---|---|
| 1 millilitre (ml) | 0.034 fl oz |
| 1 litre (L) | 2.113 pints / 0.264 gallons |
| 1 cubic meter (m³) | 264.2 gallons |
Why Does the US Use Imperial?
The United States adopted its system of measurements before the metric system was standardised (late 18th century). Several attempts to switch to metric have been made — including the Metric Conversion Act of 1975 — but none resulted in a complete transition due to the enormous cost and resistance to change.
Advantages of the Metric System
- Decimal-based — everything is in multiples of 10, making calculation easy
- Universal — used in science, medicine, and international trade worldwide
- Logical — 1,000 metres = 1 kilometre; 1,000 grams = 1 kilogram
Advantages of the Imperial System
- Familiar — deeply embedded in American culture and infrastructure
- Human-scale — some argue feet and inches feel more intuitive for human measurements
- Still in use — essential knowledge for working with US-made products and documentation