Most of the world measures in metric, while a few countries — the United States chief among them — still use imperial units in daily life. If you cook, travel, shop online or read a spec sheet, sooner or later you have to convert between them. Here is the short version.
Why two systems?
The metric system (officially the SI) is built around powers of ten: a kilometer is 1,000 meters, a kilogram is 1,000 grams. That makes it easy to scale up and down. The imperial system grew out of older customary units — inches, feet, pounds and gallons — with less tidy relationships between them (12 inches to a foot, 16 ounces to a pound).
The conversions you actually need
You don’t have to memorise everything. These handful of factors cover the vast majority of everyday conversions:
| From | To | Multiply by |
|---|---|---|
| Inches | Centimeters | 2.54 |
| Feet | Meters | 0.3048 |
| Miles | Kilometers | 1.60934 |
| Pounds | Kilograms | 0.453592 |
| Ounces | Grams | 28.3495 |
| Gallons (US) | Liters | 3.78541 |
To go the other way, divide instead of multiply. Rather than doing it by hand, drop the numbers into our unit converter, which covers length, weight, temperature, volume, area, speed and digital storage.
Temperature is the exception
Length and weight are linear — one factor does the job. Temperature is different because the scales don’t share a zero point, so you use a formula: °F = (°C × 9/5) + 32, and to reverse it, °C = (°F − 32) × 5/9.
Common mistakes to avoid
- Mixing up ounces. A fluid ounce measures volume; a weight ounce measures mass. They are not the same thing.
- US vs UK gallons. A US gallon (3.785 L) is smaller than a UK/imperial gallon (4.546 L).
- Troy vs standard ounces. Gold and silver use the heavier troy ounce — see how many grams are in an ounce.
Convert anything instantly
Seven categories, metric and imperial, updated as you type.
Open the unit converter