Unit Conversion Reference: Length, Mass, Temperature, and Area
Quick Answer
Length: 1 in = 2.54 cm, 1 ft = 0.3048 m, 1 mi = 1.609344 km. Mass: 1 lb = 0.45359237 kg, 1 oz = 28.349523125 g. Temperature: °F = °C × 9/5 + 32; K = °C + 273.15. Area: 1 acre = 4046.8564224 m², 1 ha = 10000 m². Use our Unit Converter for instant conversion in your browser.
Introduction
Unit conversion multiplies a quantity by a ratio equal to 1 (e.g. 2.54 cm / 1 in) to change units without changing the quantity. For most physical quantities this is a simple multiplication; for temperature it is an affine transform (multiply then add) because the zero points differ. This reference lists exact factors for the four most-converted categories: length, mass, temperature, and area.
Step by Step
-
Length — distance and dimension
Metric base: 1 m. 1 km = 1000 m, 1 cm = 0.01 m, 1 mm = 0.001 m. Imperial: 1 in = 2.54 cm (exact), 1 ft = 12 in = 0.3048 m (exact), 1 yd = 3 ft = 0.9144 m, 1 mi = 5280 ft = 1609.344 m (exact). Nautical: 1 nmi = 1852 m (exact).
-
Mass / weight
Metric base: 1 kg. 1 g = 0.001 kg, 1 t (metric ton) = 1000 kg. Imperial: 1 lb = 0.45359237 kg (exact), 1 oz = 1/16 lb = 28.349523125 g (exact), 1 stone = 14 lb = 6.35029318 kg. US short ton = 2000 lb; metric tonne = 1000 kg ≈ 2204.62 lb.
-
Temperature — an affine transform
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. Note: never say 'degrees Kelvin' — it is just kelvin.
-
Area — two-dimensional extent
Metric: 1 m², 1 ha = 10000 m², 1 km² = 1000000 m². Imperial: 1 in² = 6.4516 cm² (exact), 1 ft² = 0.09290304 m² (exact), 1 acre = 43560 ft² = 4046.8564224 m² (exact), 1 mi² = 640 acres = 2.5899934473296 km². 1 hectare ≈ 2.47105 acres.
-
Apply the conversion correctly
For multiplicative units: target = source × factor. For temperature: apply the affine formula (multiply then add/subtract). Always carry units through the calculation and cancel them — this catches factor-direction mistakes (multiplying when you should divide).
Examples
Miles to kilometers
Input: 26.2 mi (a marathon)
Output: 26.2 × 1.609344 = 42.1649 km
Pounds to kilograms
Input: 150 lb
Output: 150 × 0.45359237 = 68.0389 kg
Celsius to Fahrenheit
Input: 100 °C (boiling water)
Output: 100 × 9/5 + 32 = 212 °F
Acres to square meters
Input: 2.5 acres
Output: 2.5 × 4046.8564224 = 10117.14 m² (≈ 1.0117 ha)
Common Problems
- Confusing US short ton (2000 lb) with metric tonne (1000 kg) and UK long ton (2240 lb): they differ by ~10%. Always state which ton is meant in cross-border trade.
- Forgetting temperature is affine: '20 °C is 68 °F' is correct, but multiplying 20 by 9/5 without adding 32 gives 36, which is wrong. Temperature conversion requires the offset, not just a factor.
- Using 2.2 as an exact lb-to-kg factor: the exact value is 0.45359237 kg/lb (1/0.45359237 ≈ 2.20462262). Using 2.2 introduces ~0.2% error, which compounds in large quantities.
- Squaring linear factors incorrectly for area: 1 ft = 0.3048 m, so 1 ft² = 0.3048² = 0.09290304 m², not 0.3048 m². Area factors are the square of length factors; volume factors are the cube.
Tips
- Carry units through every step and cancel them like algebra — 'dimensional analysis' catches most conversion mistakes before you compute a number.
- Memorize a few anchor conversions to sanity-check results: 1 in ≈ 2.5 cm, 1 kg ≈ 2.2 lb, 1 mi ≈ 1.6 km, 1 L ≈ 1.06 US qt.
- For temperature, remember the three anchor points: 0 °C = 32 °F (freezing), 100 °C = 212 °F (boiling), −40 °C = −40 °F (the point where the two scales cross).
- Use our Unit Converter for one-off conversions and keep this reference for the formulas — knowing the factor lets you convert in any tool or spreadsheet without internet.