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Common unit conversions

These are the conversions people search for most often — at the grocery store, in the kitchen, on a road trip, or while reading a recipe written in a different country. Each section explains the relationship between the units, gives the exact formula, and walks through a worked example so you can do the math by hand when you need to.

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Length: meters, feet, miles, and kilometers

The metric and imperial systems handle distance very differently. The meter is defined by the speed of light — specifically the distance light travels in 1/299,792,458 of a second — while the foot has been pinned to a metric value since 1959, when the international yard was set at exactly 0.9144 meters. From that single agreement every other length unit follows.

  • 1 inch = 2.54 cm (exact)
  • 1 foot = 0.3048 m (exact)
  • 1 yard = 0.9144 m (exact)
  • 1 mile = 1.609344 km (exact)

Worked example. A 5K race is 5 kilometers, or 5,000 meters. To convert to miles, divide by 1.609344: 5 ÷ 1.609344 ≈ 3.107 miles. That's why a 5K is often described as "just over three miles."

Weight: grams, kilograms, ounces, and pounds

One pound is defined as exactly 0.45359237 kilograms — a number adopted internationally in 1959. From there, an ounce is one sixteenth of a pound, and a US ton is 2,000 pounds (different from the metric tonne of 1,000 kg, and different again from the UK long ton of 2,240 pounds).

  • 1 pound = 453.59237 g (exact)
  • 1 ounce = 28.349523125 g (exact)
  • 1 stone = 6.35029318 kg (UK body-weight unit)
  • 1 metric tonne = 1,000 kg

Worked example. A 10-pound bag of flour weighs 10 × 0.45359237 ≈ 4.536 kg. Reverse it: a 5-kg bag weighs about 11.02 pounds.

Temperature: Celsius, Fahrenheit, and Kelvin

Temperature is the only common category that can't be converted with a simple multiplier. The three scales have different zero points: 0 °C is the freezing point of water, 0 °F is roughly the freezing point of brine, and 0 K is absolute zero — the coldest temperature physically possible.

  • °F = °C × 9/5 + 32
  • °C = (°F − 32) × 5/9
  • K = °C + 273.15

Worked example. A 350 °F oven equals (350 − 32) × 5/9 ≈ 176.7 °C. Most recipes round to 175 °C or 180 °C, both of which are close enough for baking.

Volume: liters, gallons, cups, and milliliters

The litre is defined as one cubic decimeter (1,000 cm³). US and UK volume units share names but differ in size: a US cup is 240 mL by nutrition labelling rules but 236.588 mL by the legal definition, and the UK cup of 284 mL is rarely used in modern recipes.

  • 1 US gallon = 3.785411784 L (exact)
  • 1 Imperial gallon = 4.54609 L (exact)
  • 1 US fluid ounce ≈ 29.5735 mL
  • 1 US cup ≈ 236.588 mL

Worked example. A 2-liter soda bottle holds 2 ÷ 3.785411784 ≈ 0.528 US gallons, which is just over half a gallon — the rough rule of thumb most Americans use.

Area: square meters, acres, and hectares

Area units scale as the square of length, which is why the conversion factors look unusual. A square mile contains 1,609.344² ≈ 2,589,988 square meters. The acre — originally the area a yoke of oxen could plow in a day — is fixed at 4,046.8564224 m² in the international system.

  • 1 hectare = 10,000 m² = 2.47105 acres
  • 1 acre = 4,046.8564224 m² = 43,560 ft²
  • 1 square mile = 640 acres

Mental-math shortcuts

When you don't have a calculator, these rounded factors get you within about 2% of the exact answer:

  • Meters to feet: multiply by 3.28 (or roughly 3⅓)
  • Kilometers to miles: multiply by 0.62 (or divide by 1.6)
  • Kilograms to pounds: multiply by 2.2
  • Liters to US gallons: divide by 3.8
  • °C to °F: double the Celsius value and add 30 (rough)