Article — Imperial to Metric Conversion Calculator
Imperial to Metric Conversion Calculator: Exact Factors
Every imperial-to-metric factor in this calculator is exact, not approximated. One foot equals 0.3048 metres; one pound equals 0.45359237 kilograms; one US gallon equals 3.785411784 litres; one Fahrenheit degree equals 5/9 of a Celsius degree, with a 32-point offset at the freezing point of water. The values were fixed by the 1959 International Yard and Pound Agreement and the General Conference on Weights and Measures.
That exactness matters. NASA lost the 327-million-dollar Mars Climate Orbiter in 1999 because of a unit confusion between pound-seconds and newton-seconds of impulse. Day-to-day imperial-to-metric work is lower stakes, but the same rule holds: use the exact factor, not a memorised approximation.
What is imperial to metric conversion?
Imperial-to-metric translation moves a measurement from the system inherited from the British Empire (inches, feet, miles, pounds, ounces, gallons, Fahrenheit) into the system used by the rest of the world (centimetres, metres, kilometres, kilograms, grams, litres, Celsius). The United States, Liberia, and Myanmar are the only countries that still use imperial in daily life; everyone else runs on metric.
The two systems have different physical anchors. The metre traces to the speed of light; the kilogram traces to Planck’s constant. The foot and the pound trace to historical English standards, stitched into the metric world by the 1959 treaty. Both systems are exact today, but the imperial side was made exact by definition, not by physics.
Imperial to metric formulas
Each imperial-to-metric pair has a multiplier you apply to convert.
ft × 0.3048 = m in × 2.54 = cmmi × 1.609344 = km yd × 0.9144 = mlb × 0.45359237 = kg oz × 28.3495 = ggal (US) × 3.78541 = L fl oz × 29.5735 = mL(°F - 32) × 5/9 = °CThe temperature formula is the only one that needs an offset; the others are pure multiplication. The 32-degree subtraction comes from the freezing point of water sitting at different numbers on the two scales (32 °F vs 0 °C).
Imperial to metric reference table
The values that appear in everyday imperial-to-metric work:
- 1 in = 2.54 cm (exact)
- 6 ft = 1.8288 m (standard adult height anchor)
- 1 mile = 1.609344 km (exact)
- 1 lb = 0.45359237 kg (exact)
- 1 oz = 28.3495 g (avoirdupois)
- 1 stone = 6.35029 kg = 14 lb
- 1 fl oz (US) = 29.5735 mL
- 1 US cup = 236.588 mL = 8 fl oz
- 1 US gallon = 3.78541 L (UK gallon is 4.54609 L)
- 1 short ton = 907.185 kg = 2000 lb
- 32 °F = 0 °C, 68 °F = 20 °C, 212 °F = 100 °C
Why imperial to metric factors are exact
The exactness comes from a single 1959 agreement. The International Yard and Pound Agreement, signed by the US, UK, Canada, Australia, New Zealand, and South Africa, defined the yard as 0.9144 metres and the pound as 0.45359237 kilograms. Every imperial unit derived from those two values inherits the exactness.
One inch is 1/36 of a yard, so 1 in = 2.54 cm exactly. One foot is 12 inches, so 1 ft = 0.3048 m exactly. One mile is 5280 feet, so 1 mile = 1609.344 m exactly. The US gallon is defined as 231 cubic inches, so it converts to litres exactly because the inch-to-centimetre factor is exact. The avoirdupois ounce is 1/16 of a pound, so 1 oz = 28.3495 g.
Before 1959, the US and the UK had subtly different pounds, differing by a few parts per million. The 1959 treaty eliminated the gap by picking a new common value, 0.45359237 kg. The previous US pound was 0.4535924277 kg; the previous UK pound was 0.45359243 kg. Both nations rounded to the new shared standard.
Imperial to metric temperature conversion
Fahrenheit to Celsius is the trickiest imperial-to-metric pair because of the offset. The forward formula is °C = (°F − 32) × 5/9. Reverse: °F = °C × 9/5 + 32. The 5/9 ratio matches the Celsius degree being larger than the Fahrenheit degree by 80 percent.
Mental shortcut: subtract 30 from Fahrenheit, then divide by 2. 70 °F → 40 / 2 = 20 °C (true: 21.1 °C). Close enough for travel and weather. The shortcut works to within 2 °C between 0 °F and 100 °F. Going metric to imperial: double Celsius, add 30. 25 °C → 50 + 30 = 80 °F (true: 77 °F).
The only temperature where the two scales agree is −40 °F = −40 °C. Below or above that point, the scales diverge. Body temperature is 98.6 °F = 37 °C; room temperature is 68-72 °F = 20-22 °C.
Imperial to metric mental shortcuts
Useful approximations for when no calculator is available:
For distance, 1 mile is roughly 1.6 km; multiply miles by 1.6 to get km. For mass, 1 lb is roughly 0.45 kg; multiply pounds by 0.45 for a quick kg. For volume, 1 US gallon is roughly 3.8 L. None of these are exact, but each is within one percent of the truth, which is plenty for travel planning, rough cost comparisons, or label-reading.
Common imperial to metric mistakes
A US gallon is 3.78541 L; a UK gallon is 4.54609 L — 20 percent larger. Fuel-economy comparisons that mix the two get nonsense answers. A 30 mpg US car is 36 mpg UK. The same physical efficiency reads differently because the gallon is different.
The second pitfall is confusing weight ounces with fluid ounces. Avoirdupois ounce is mass (28.35 g); fluid ounce is volume (29.57 mL). For water they happen to be close, but for honey, oil, or any non-water liquid the values diverge sharply.
Third: rounding the temperature factor. The °F − 30 divided by 2 trick is a shortcut, not the truth. For lab work or any task where 2 degrees matters, use the exact 5/9 formula.
Imperial to metric in cooking and travel
A US recipe asking for “1 cup flour” means 236.6 mL by volume, which is about 125 g by weight for all-purpose flour. European recipes specify weight directly, which is more reliable because flour packs differently in different cups. When converting a US recipe, weighing the flour rather than measuring its volume gives better results.
Travel essentials: temperature (Celsius outside the US), distance (km on signs), fuel (litres at the pump). A 350-mile drive is 563 km; a 100 °F day is 37.8 °C; a 12-gallon fill is 45.4 L.