Unit Converter
Convert length, weight, temperature, volume and more between metric and imperial, with exact factors marked and the US and imperial split handled.
One conversion here reverses the size of its own gaps
Almost every unit on this page is a straight multiplication: a metre is always 3.28 feet, so equal steps stay equal steps. Fuel economy is the exception, and it produces an answer most people get backwards. Over 10,000 miles:
| Upgrade | Gain in mpg | Fuel saved |
|---|---|---|
| 16 → 20 mpg | +4 | 125 gallons |
| 20 → 25 mpg | +5 | 100 gallons |
| 50 → 100 mpg | +50 | 100 gallons |
| 34 → 50 mpg | +16 | 94 gallons |
| 10 → 11 mpg | +1 | 91 gallons |
| 40 → 50 mpg | +10 | 50 gallons |
Going from 16 to 20 mpg saves 125 gallons. Going from 34 to 50 mpg — four times the improvement on paper — saves only 94. The table is not sorted by the mpg column at all, and cannot be: miles per gallon puts distance on top, so the fuel you actually burn is one divided by it. Reciprocals drop away steeply and then flatten out.
| Starting at | Same car in L/100km | What +1 mpg saves |
|---|---|---|
| 10 mpg | 23.52 | 90.9 gallons |
| 15 mpg | 15.68 | 41.7 gallons |
| 20 mpg | 11.76 | 23.8 gallons |
| 25 mpg | 9.41 | 15.4 gallons |
| 30 mpg | 7.84 | 10.8 gallons |
| 40 mpg | 5.88 | 6.1 gallons |
| 50 mpg | 4.70 | 3.9 gallons |
| 60 mpg | 3.92 | 2.7 gallons |
The same +1 mpg is worth 33 times more on a truck than on a hybrid. That is why replacing the worst vehicle in a fleet saves far more fuel than making a good one better, and why "I gained 16 mpg" means nothing until you know where it started.
Which is why the rest of the world measures it upside down
Litres per 100 km puts fuel on top and distance underneath — the reciprocal, measured directly. That makes it linear, so one step is worth the same everywhere:
| L/100km | Same car in mpg | Litres per 10,000 miles | What −1 L/100km saves |
|---|---|---|---|
| 4 | 58.8 | 644 | 161 litres |
| 6 | 39.2 | 966 | 161 litres |
| 8 | 29.4 | 1287 | 161 litres |
| 10 | 23.5 | 1609 | 161 litres |
| 12 | 19.6 | 1931 | 161 litres |
That last column never moves. You can subtract, compare and average L/100km figures and get sensible answers. Doing the same with mpg does not work: two cars driven the same distance, one at 10 mpg and one at 50, average 16.7 mpg between them, not 30. The honest figure is total miles over total gallons — the harmonic mean — and it lands 13.3 points below the obvious answer because the thirsty car burns most of the fuel. Convert both to L/100km first and the plain average is correct again.
This is also why the conversion between them is 235.215 divided by the number rather than a multiplication, and why fuel labels in the United States now print gallons per 100 miles next to the mpg figure.
How to use
- Choose the category and units.
- Type a value in either box.
- Check whether US or imperial is meant for volumes.
- Note which conversions are exact rather than rounded.
Frequently asked questions
Which conversions are exact?
More than people expect. An inch is exactly 2.54 cm, a pound exactly 0.45359237 kg, and a mile exactly 1.609344 km — all defined by international agreement in 1959 rather than measured. Temperature conversions are exact too, since both scales are defined.
Why do US and imperial volumes differ?
Because they diverged after American independence. The US kept the older English wine gallon while Britain redefined its own in 1824, leaving the imperial gallon about 20 per cent larger. Weights and lengths were harmonised; volumes never were, which is why a pint differs across the Atlantic.
What is the most expensive unit error on record?
The Mars Climate Orbiter, lost in 1999 because one team supplied thrust data in pound-force seconds while the receiving software expected newton-seconds. It is the standard cautionary example, and unit errors in medicine are a more frequent if less spectacular category.
Why does converting back and forth change my number?
Rounding. Each conversion rounds to the displayed precision, so a round trip through an intermediate unit accumulates error. Converting once from the original figure, rather than chaining conversions, avoids it.
Are pounds a unit of mass or force?
Both, confusingly, depending on context. The pound-mass and pound-force are distinct units that happen to be numerically equal at standard gravity, which is why engineering texts distinguish them explicitly and everyday use does not.
Is a metric ton the same as a ton?
No, and there are three. A metric tonne is 1,000 kg, a US short ton is 2,000 pounds — about 907 kg — and a UK long ton is 2,240 pounds, about 1,016 kg. They differ by up to 10 per cent, which matters a great deal in freight quotes.
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