Unit and Temperature Converter

Converting a unit is a multiplication by one factor — provided you keep families separate and know the exact definitions. Temperatures are the exception: they convert through an affine function with an offset.

Conversion factors worth knowing

Some factors are exact by international definition: 1 inch is exactly 2.54 cm, 1 pound exactly 0.45359237 kg, 1 foot 0.3048 m. Others are conventions: the US gallon is 3.785411784 litres, the imperial gallon 4.54609 litres.

Converting temperatures

The Celsius, Fahrenheit and Kelvin scales are not proportional: neither the origin nor the step is shared. A simple multiplication is therefore wrong.

Useful anchors: 0 °C is 32 °F and 273.15 K; 100 °C is 212 °F; −40 is the only value identical on both the Celsius and Fahrenheit scales.

Area and volume: the classic traps

Area conversions square the linear factor; volume conversions cube it. One square metre is 10,000 cm² — not 100 cm². One cubic metre is 1,000 litres, and one litre is exactly 1 dm³. Mixing these up produces errors of a factor 100 or 1,000, frequent in dosing, painting or land calculations.

• 1 hectare = 10,000 m² = 0.01 km².
• 1 are = 100 m²; 1 centiare = 1 m².
• 1 acre = 4,046.86 m² ≈ 0.4047 hectare.

FAQ

• How many centimetres in an inch?

  Exactly 2.54 cm, by international definition since 1959. A foot is 30.48 cm (12 inches) and a yard 91.44 cm.

• How do I convert Celsius to Fahrenheit?

  Multiply by 1.8 then add 32. The other way, subtract 32 then divide by 1.8. For kelvins, add 273.15 to the Celsius value.

• Are US and imperial gallons the same?

  No. The US gallon is 3.785 litres, the imperial one 4.546 litres — about 20% more. The confusion is common in recipes and fuel consumption.

• Why isn't 1 m² equal to 100 cm²?

  Because the conversion spans two dimensions. 1 m = 100 cm, so 1 m² = 100 × 100 = 10,000 cm². For volumes the factor is cubed: 1 m³ = 1,000,000 cm³.