Unit Conversion: Length, Weight, Temperature and More

Master unit conversion for length, mass, temperature, area, and volume — with formulas and practical engineering examples.

5 min read Convert · Science · Engineering 6 sections + FAQ

Unit conversion is one of the most fundamental mathematical operations in science, engineering, cooking, and daily life. The metric system and imperial system coexist — and switching between them requires precise formulas that are easy to get wrong.

Whether converting kilometers to miles, Celsius to Fahrenheit, or kilograms to pounds, the key is knowing the exact conversion factor. Approximations work for rough estimates; precision matters for engineering, medicine, and international commerce.

Metric vs imperial systems

The metric system (SI — Système International) uses base-10 relationships between units. A kilometer is exactly 1000 meters. A kilogram is exactly 1000 grams. The imperial system, used primarily in the United States, uses arbitrary relationships: 12 inches per foot, 3 feet per yard, 5280 feet per mile. The metric system is used by every country for science and by most countries for everyday measurement. The imperial system is dominant only in the US, Myanmar, and Liberia.

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Length conversion

Key length conversions: 1 inch = 2.54 cm (exact). 1 foot = 30.48 cm. 1 yard = 0.9144 m. 1 mile = 1.60934 km. 1 km = 0.621371 miles. 1 nautical mile = 1.852 km (exact). For quick mental math: multiply miles by 1.6 for km (3% error), or multiply km by 0.6 for miles (4% error). For precision, use the exact factors.

Mass and weight

Mass conversions: 1 kilogram = 2.20462 pounds. 1 pound = 453.592 grams. 1 ounce = 28.3495 grams. 1 stone (UK) = 14 pounds = 6.35029 kg. 1 metric ton = 1000 kg = 2204.62 pounds. 1 US ton (short ton) = 2000 pounds = 907.185 kg. Weight vs mass: weight is force (depends on gravity), mass is intrinsic. At Earth's surface, 1 kg of mass weighs approximately 9.81 newtons. In everyday use, mass and weight are used interchangeably.

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Temperature formulas

Temperature conversions require formulas, not just multiplication: Celsius to Fahrenheit: (C × 9/5) + 32. Fahrenheit to Celsius: (F - 32) × 5/9. Celsius to Kelvin: C + 273.15. Kelvin to Celsius: K - 273.15. Key reference points: 0°C = 32°F = 273.15K (water freezes). 100°C = 212°F = 373.15K (water boils). 37°C = 98.6°F = 310.15K (body temperature). -40°C = -40°F (the only point where the two scales meet).

Area and volume

Area conversions: 1 m² = 10.7639 sq ft. 1 hectare = 10,000 m² = 2.47105 acres. 1 acre = 4046.86 m². 1 sq mile = 2.58999 km². Volume conversions: 1 liter = 0.264172 US gallons. 1 US gallon = 3.78541 liters. 1 UK gallon = 4.54609 liters. 1 cubic foot = 28.3168 liters. 1 fluid ounce (US) = 29.5735 ml. 1 cup (US) = 236.588 ml.

Practical conversion tips

For speed: km/h to mph, multiply by 0.621. For cooking: 1 teaspoon = 5ml, 1 tablespoon = 15ml, 1 cup = 240ml (US). For fuel efficiency: km/l to mpg, multiply by 2.825. For screens: 1 inch = 2.54cm at any DPI. For altitude: meters to feet, multiply by 3.281. Use mnemonic devices for common conversions you need frequently — the exact factor is always worth double-checking for precision work.

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Frequently Asked Questions

What is the difference between mass and weight? +
Mass is the amount of matter in an object (measured in kilograms). Weight is the gravitational force on that mass (measured in newtons). At Earth's surface, 1 kg of mass weighs 9.81 N. In everyday language, "weight" typically means mass.
What is the exact conversion between miles and kilometers? +
1 mile = 1.60934 km (exact: 1.609344). For quick mental conversion, multiply miles by 1.6 for km, or multiply km by 0.625 for miles. These approximations have about 3% error.
How do I convert Fahrenheit to Celsius? +
Subtract 32, then multiply by 5/9: (F - 32) × 5/9 = C. Example: 72°F → (72-32) × 5/9 = 40 × 0.556 = 22.2°C.
What is the difference between a US gallon and a UK gallon? +
A US gallon is 3.785 liters. A UK (imperial) gallon is 4.546 liters — about 20% larger. Fuel economy ratings (mpg) are not directly comparable between US and UK specifications.
What is an SI unit? +
SI (Système International) units are the modern metric system standard. The seven base SI units are: meter (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (substance), candela (luminosity).
Why does the US still use the imperial system? +
Historical inertia and cost of conversion. The US came close to metrication in the 1970s but it was never mandated for consumer use. Science, medicine, and military in the US already use metric.
How do I convert area units? +
Area conversions involve squaring the linear conversion factor. 1 foot = 0.3048 m, so 1 sq ft = 0.3048² = 0.0929 m². For land: 1 hectare = 10,000 m² = 2.471 acres. 1 acre = 43,560 sq ft.