Unit conversion is one of those things that sounds boring until you realize you've been doing it wrong. The stakes vary — a wrong temperature conversion on a recipe is a ruined dinner; a wrong unit in a chemistry calculation can be a failed lab report or worse. This reference covers the conversions that come up most often, along with the formulas and a few things people consistently get confused.
For calculations beyond what you can do mentally, the unit converter at CalcHub handles multi-step conversions instantly.
Length
| From | To | Multiply By |
|---|
| Inches | Centimeters | × 2.54 |
| Centimeters | Inches | × 0.3937 |
| Feet | Meters | × 0.3048 |
| Meters | Feet | × 3.2808 |
| Miles | Kilometers | × 1.60934 |
| Kilometers | Miles | × 0.6214 |
| Yards | Meters | × 0.9144 |
| Meters | Yards | × 1.0936 |
Quick mental checks:
- 1 inch ≈ 2.5 cm (close enough for rough estimates)
- 1 mile ≈ 1.6 km
- 100 meters ≈ 109 yards (slightly more than 100 yards)
- 6 feet ≈ 183 cm
Weight and Mass
| From | To | Multiply By |
|---|
| Pounds | Kilograms | × 0.4536 |
| Kilograms | Pounds | × 2.2046 |
| Ounces | Grams | × 28.3495 |
| Grams | Ounces | × 0.03527 |
| Stones | Kilograms | × 6.3503 |
| Kilograms | Stones | × 0.1575 |
| Metric tons | Pounds | × 2,204.62 |
Quick mental checks:
- 1 kg ≈ 2.2 lbs
- 70 kg person ≈ 154 lbs
- 1 pound ≈ 454 grams
Volume
| From | To | Multiply By |
|---|
| US fluid ounces | Milliliters | × 29.5735 |
| Milliliters | US fluid ounces | × 0.03381 |
| US cups | Milliliters | × 236.588 |
| Liters | US cups | × 4.2268 |
| US gallons | Liters | × 3.78541 |
| Liters | US gallons | × 0.2642 |
| UK pints | Liters | × 0.56826 |
| US pints | Liters | × 0.47318 |
Watch out: US and UK liquid measures differ. A US gallon is 3.785 L; a UK (imperial) gallon is 4.546 L. A US pint is 16 fl oz; a UK pint is 20 fl oz. This trips people up constantly in cooking and chemistry.
Temperature
Temperature is the one conversion that isn't a simple multiplication — there's an offset involved.
| Conversion | Formula |
|---|
| Celsius to Fahrenheit | °F = (°C × 9/5) + 32 |
| Fahrenheit to Celsius | °C = (°F − 32) × 5/9 |
| Celsius to Kelvin | K = °C + 273.15 |
| Kelvin to Celsius | °C = K − 273.15 |
| Fahrenheit to Kelvin | K = (°F − 32) × 5/9 + 273.15 |
Key reference points:
- Water freezes: 0°C = 32°F = 273.15 K
- Water boils: 100°C = 212°F = 373.15 K
- Room temperature: ~22°C = ~72°F = ~295 K
- Body temperature: 37°C = 98.6°F
- Absolute zero: -273.15°C = -459.67°F = 0 K
Memory trick for Celsius/Fahrenheit:
- 0°C = 32°F, 10°C = 50°F, 20°C = 68°F, 30°C = 86°F, 40°C = 104°F
- Notice each 10°C jump = 18°F jump. Memorize 20°C ≈ 68°F as your anchor.
Speed
| From | To | Multiply By |
|---|
| Miles per hour | Kilometers per hour | × 1.60934 |
| Kilometers per hour | Miles per hour | × 0.6214 |
| Miles per hour | Meters per second | × 0.44704 |
| Meters per second | Miles per hour | × 2.2369 |
| Knots | Kilometers per hour | × 1.852 |
| Knots | Miles per hour | × 1.15078 |
Quick check: 60 mph ≈ 97 kph (nearly 100 kph). 100 kph ≈ 62 mph.
Area
| From | To | Multiply By |
|---|
| Square feet | Square meters | × 0.0929 |
| Square meters | Square feet | × 10.7639 |
| Acres | Hectares | × 0.40469 |
| Hectares | Acres | × 2.47105 |
| Square miles | Square kilometers | × 2.58999 |
| Square kilometers | Square miles | × 0.3861 |
Pressure
| From | To | Multiply By |
|---|
| PSI | kPa | × 6.89476 |
| kPa | PSI | × 0.14504 |
| Atmospheres | kPa | × 101.325 |
| kPa | Atmospheres | × 0.009869 |
| Bar | PSI | × 14.5038 |
Tire pressure in the US is measured in PSI; in most of the rest of the world it's in kPa or bar. Standard car tire pressure of 32 PSI = 220 kPa = 2.2 bar.
Energy
| From | To | Multiply By |
|---|
| Calories (food) | Kilojoules | × 4.184 |
| Kilojoules | Calories | × 0.2388 |
| Watt-hours | Kilojoules | × 3.6 |
| BTU | Joules | × 1055.06 |
| Kilowatt-hours | BTU | × 3412.14 |
Note on calories: A "Calorie" on a food label (capital C) is actually a kilocalorie — 1000 small calories. When you see a food item listed as 250 Cal, that's 250 kcal or 1,046 kJ.
Common Mistakes to Avoid
Forgetting to square/cube for area and volume. If 1 foot = 0.3048 meters, then 1 square foot = 0.3048² = 0.0929 square meters, and 1 cubic foot = 0.3048³ = 0.02832 cubic meters. Multiply by the conversion factor once for length, twice for area, three times for volume.
Mixing US and UK measures. US tablespoon, cup, pint, quart, and gallon are all different from their UK counterparts. Always check which system is being used.
Confusing mass and weight. In everyday usage these are treated as equivalent, but technically mass (kg) and weight (N, a force) are different. For most conversions between kilograms and pounds, treating them as equivalent is fine. In physics problems involving gravitational differences, it isn't.
For unit conversions beyond what you can do mentally, CalcHub's unit converters cover all of the above categories and more — temperature, currency, cooking measurements, digital storage, and fuel efficiency. Faster than doing the mental arithmetic and no rounding errors to worry about.