How to Convert Units: A Complete Conversion Guide
A comprehensive reference for converting length, weight, volume, temperature, speed, and area units between metric and imperial systems using our conversion calculator.
Related Calculator
Use the Conversion Calculator to apply what you learn in this guide.
Why Unit Conversion Matters
The world uses two primary measurement systems: the International System of Units (SI/metric), used by most countries, and the Imperial system, used primarily in the United States and a few other nations. Scientific and engineering work universally uses SI units, but everyday commerce, cooking, travel, and engineering projects often require bridging both worlds.
Getting unit conversion wrong has real consequences — from a recipe that fails to a NASA Mars orbiter lost in 1999 due to a metric/imperial mismatch costing $327 million.
The Conversion Factor Method
Every unit conversion can be performed by multiplying by a conversion factor — a fraction equal to 1 where the numerator and denominator express the same quantity in different units.
Example: Convert 5 miles to kilometres.
- Conversion factor: 1 mile = 1.60934 km
- Calculation: 5 × 1.60934 = 8.047 km
Example: Convert 68°F to Celsius (temperature requires a formula, not just multiplication — see below).
Reference Tables
Length
| From | To | Multiply By |
|---|---|---|
| Kilometres (km) | Miles (mi) | 0.621371 |
| Miles (mi) | Kilometres (km) | 1.60934 |
| Metres (m) | Feet (ft) | 3.28084 |
| Feet (ft) | Metres (m) | 0.3048 |
| Centimetres (cm) | Inches (in) | 0.393701 |
| Inches (in) | Centimetres (cm) | 2.54 |
| Metres (m) | Yards (yd) | 1.09361 |
| Nautical miles | Kilometres (km) | 1.852 |
Weight (Mass)
| From | To | Multiply By |
|---|---|---|
| Kilograms (kg) | Pounds (lb) | 2.20462 |
| Pounds (lb) | Kilograms (kg) | 0.453592 |
| Grams (g) | Ounces (oz) | 0.035274 |
| Ounces (oz) | Grams (g) | 28.3495 |
| Metric tonnes (t) | Short tons (US) | 1.10231 |
| Stone (st) | Kilograms (kg) | 6.35029 |
Volume
| From | To | Multiply By |
|---|---|---|
| Litres (L) | US Gallons (gal) | 0.264172 |
| US Gallons (gal) | Litres (L) | 3.78541 |
| Millilitres (ml) | US Fluid ounces (fl oz) | 0.033814 |
| US Fluid ounces (fl oz) | Millilitres (ml) | 29.5735 |
| Litres (L) | UK Pints (pt) | 1.75975 |
| Cubic metres (m³) | Cubic feet (ft³) | 35.3147 |
Temperature
Temperature cannot be converted with a simple multiplication factor — it requires additive offsets.
| Conversion | Formula |
|---|---|
| Celsius → Fahrenheit | °F = (°C × 9/5) + 32 |
| Fahrenheit → Celsius | °C = (°F − 32) × 5/9 |
| Celsius → Kelvin | K = °C + 273.15 |
| Kelvin → Celsius | °C = K − 273.15 |
| Fahrenheit → Kelvin | K = (°F − 32) × 5/9 + 273.15 |
Key reference points:
| °C | °F | K | Description |
|---|---|---|---|
| −273.15 | −459.67 | 0 | Absolute zero |
| 0 | 32 | 273.15 | Water freezes |
| 100 | 212 | 373.15 | Water boils |
| 37 | 98.6 | 310.15 | Normal body temperature |
Speed
| From | To | Multiply By |
|---|---|---|
| km/h | mph | 0.621371 |
| mph | km/h | 1.60934 |
| m/s | km/h | 3.6 |
| km/h | m/s | 0.277778 |
| Knots | km/h | 1.852 |
| mph | m/s | 0.44704 |
Area
| From | To | Multiply By |
|---|---|---|
| Square metres (m²) | Square feet (ft²) | 10.7639 |
| Square feet (ft²) | Square metres (m²) | 0.092903 |
| Hectares (ha) | Acres | 2.47105 |
| Acres | Hectares (ha) | 0.404686 |
| Square kilometres (km²) | Square miles (mi²) | 0.386102 |
| Square miles (mi²) | Square kilometres (km²) | 2.58999 |
Step-by-Step Guide: Using the Conversion Calculator
Step 1 — Select Your Unit Category
Choose from Length, Weight, Volume, Temperature, Speed, or Area. The input and output unit dropdowns will update to show relevant options.
Step 2 — Choose Your "From" and "To" Units
Select the unit you are converting from in the left dropdown and the unit you want to in the right dropdown.
Step 3 — Enter the Value
Type your number in the input field. The result updates instantly without needing to press a button.
Step 4 — Copy or Use the Result
Click the copy icon to copy the result to your clipboard, or use the displayed value directly in your work.
Frequently Asked Questions
Q: What is the difference between mass and weight? A: Mass (kg, g) measures the amount of matter in an object and is constant regardless of location. Weight is a force (measured in newtons) that depends on gravity. In everyday usage, "weight" and "mass" are used interchangeably — scales in shops measure mass, not force.
Q: Why are there two types of gallons (US and UK)? A: Historically, Britain and the US developed separate standards. The US gallon is 3.785 litres, while the Imperial (UK) gallon is 4.546 litres — about 20% larger. This difference is significant in fuel economy comparisons.
Q: What is the SI system? A: The International System of Units (Système International d'Unités) is the modern form of the metric system, agreed upon internationally in 1960. It has seven base units including the metre (length), kilogram (mass), and second (time).
Q: Can I convert compound units like km/h to m/s? A: Yes. Convert each component separately. Since 1 km = 1,000 m and 1 hour = 3,600 seconds, dividing km/h by 3.6 gives m/s. For example, 90 km/h ÷ 3.6 = 25 m/s.