You find a perfect jacket online, but the sizes are in centimetres. A recipe asks for 350°F and your oven shows Celsius. A friend says she ran 10 kilometres and you wonder, “Is that far?”
Most of the world uses the metric system, while the US (and, in some everyday uses, the UK) still relies on imperial or US customary units. The good news is that converting between them only needs a few numbers and one simple rule.
Quick Answer
To convert, multiply by the conversion factor. To go back the other way, divide by the same number (or multiply by its inverse).
- Inches to centimetres: × 2.54
- Miles to kilometres: × 1.609
- Pounds to kilograms: × 0.4536
- Fahrenheit to Celsius: (°F − 32) × 5/9
Length: Inches, Feet, Miles and Metric
| Imperial | Metric |
|---|---|
| 1 inch | 2.54 cm |
| 1 foot | 30.48 cm |
| 1 yard | 0.914 m |
| 1 mile | 1.609 km |
| Metric | Imperial |
|---|---|
| 1 cm | 0.394 inches |
| 1 metre | 3.281 feet (or 1.094 yards) |
| 1 km | 0.621 miles |
Example 1: A person who is 5’10” is 70 inches tall. 70 × 2.54 = 177.8 cm.
Example 2: A 10 km run is 10 × 0.621 = 6.21 miles.
Weight: Pounds, Ounces and Kilograms
| Imperial | Metric |
|---|---|
| 1 ounce | 28.35 grams |
| 1 pound | 0.4536 kg |
| Metric | Imperial |
|---|---|
| 1 gram | 0.0353 ounces |
| 1 kg | 2.205 pounds |
Example: 70 kg × 2.205 = 154.3 pounds. Going back, 180 pounds × 0.4536 = 81.6 kg.
Volume: Fluid Ounces, Gallons and Litres
| US customary | Metric |
|---|---|
| 1 fluid ounce | 29.57 ml |
| 1 gallon | 3.785 litres |
| Metric | US customary |
|---|---|
| 1 litre | 33.81 fluid ounces |
| 1 litre | 0.264 gallons |
Watch out: The UK imperial gallon is bigger than the US gallon. One imperial gallon equals about 4.546 litres. Always check which one a product or recipe uses.
Temperature: Celsius and Fahrenheit
Temperature doesn’t use a single multiplier. It uses a formula with two steps.
- Celsius to Fahrenheit: °F = (°C × 9/5) + 32
- Fahrenheit to Celsius: °C = (°F − 32) × 5/9
| Celsius | Fahrenheit | What it feels like |
|---|---|---|
| 0°C | 32°F | Freezing |
| 20°C | 68°F | Mild room temperature |
| 37°C | 98.6°F | Normal body temperature |
| 100°C | 212°F | Water boils |
| 177°C | 350°F | Common baking temperature |
Quick mental trick: To estimate Fahrenheit, double the Celsius number and add 30. For 20°C, that gives 70°F, which is close to the exact 68°F. To go back, subtract 30 and halve it.
Easy Mental Shortcuts
When you don’t need an exact figure, rounding makes life easier:
- Kilometres to miles: multiply by 0.6
- Miles to kilometres: multiply by 1.6
- Kilograms to pounds: multiply by 2.2
- Pounds to kilograms: divide by 2.2
- Inches to centimetres: multiply by 2.5
- Litres to US gallons: divide by 4 (roughly)
Step-by-Step: How to Convert Any Unit
- Identify what you are converting. For example, miles to kilometres.
- Find the conversion factor. 1 mile = 1.609 km.
- Multiply if you go from the unit with the larger factor to the smaller one. For 5 miles: 5 × 1.609 = 8.05 km.
- Reverse it by dividing. 8.05 ÷ 1.609 = 5 miles.
- Round sensibly. One or two decimal places are usually enough.
Common Mistakes to Avoid
- Mixing up US and UK gallons. They are different sizes.
- Confusing fluid ounces with weight ounces. One measures volume, the other weight.
- Rounding too early in a multi-step calculation.
- Using the temperature formula backwards. Subtract 32 first when going from Fahrenheit.
When Does Conversion Matter?
Unit conversions come up more often than people expect:
- Online shopping and sizing, especially when buying from another country.
- Shipping and customs, where weight and size affect cost.
- Cooking and baking, where small differences change results.
- Travel, from speed limits to weather forecasts.
- Fitness and health tracking, such as distance, weight and height.
Frequently Asked Questions
How do I convert metric to imperial? Multiply by the conversion factor for the unit, such as 0.621 for kilometres to miles, or 2.205 for kilograms to pounds.
How do I convert imperial to metric? Multiply by the factor in the other direction, such as 2.54 for inches to centimetres, or 0.4536 for pounds to kilograms.
What is the easiest way to convert Celsius to Fahrenheit? Double the Celsius number and add 30 for a quick estimate. Use the full formula, °F = °C × 1.8 + 32, for an exact answer.
Is imperial the same as US customary? Not exactly. Many units look similar, but some, like the gallon and the pint, differ in size.
Which countries still use imperial units? The US uses US customary units in daily life. The UK uses a mix, such as miles on road signs and metric for most other things.
Final Thoughts
Converting metric to imperial, and back again, comes down to one factor and one operation: multiply to go one way, divide to return. Keep a short list of the common factors handy, use mental shortcuts for quick estimates, and use exact numbers when accuracy counts.
Disclaimer: This article is for general information only. Conversion values are rounded. For medical, engineering or legal uses, please use official exact figures.