Units, and the ones that are two units with one name CONVERT

The problem is never metres to feet

Nobody has ever been hurt converting metres to feet. Every unit accident in the record comes from the small set of units where the name does not identify the quantity: where two different amounts share one word, and everybody in the room is confident they know which one is meant.

• A gallon is two different volumes. Imperial is 4.546 litres, US is 3.785. That is a twenty per cent difference, and the word on both sides of the Atlantic is “gallon”.
• A ton is three different masses: the long ton, the short ton and the tonne, the last of which is also spelled “ton” by most people typing quickly.
• A fluid ounce differs between the two systems in the opposite direction to the gallon, which catches people who have learned that American measures are smaller and applied the rule.
• A mile at sea is not a mile on land, and the gap is about fifteen per cent.
• A calorie on a food label is a thousand calories. Both are written “calorie”.
• A mil is a thousandth of an inch to a machinist and a millimetre to almost everybody else.

So an ambiguous name is never silently resolved

Every unit whose name is shared is listed twice here, with its system attached, and the answer says in words which one it used. You cannot convert a gallon on this page; you convert an imperial gallon or a US gallon, and the result tells you which you picked.

That is the entire design. The arithmetic in a unit converter is trivial (it is a multiplication), and the value is not in the arithmetic. It is in refusing to guess, and in making the guess visible when one has been made. A converter with one entry called “gallon” has answered a question you did not ask and has not told you which one.

What to do when you are converting somebody else's number

The hard case is not choosing your own units. It is a figure on a drawing, a datasheet or an order from somebody who did not say which system they meant. Naming the unit here does not tell you what they meant.

Work it out from context before you convert. Where was the document written, and under which standard? What is the plausible range for the thing being measured? A fuel figure that comes out at half or double what it should is telling you which gallon was meant. Does another number in the same document cross-check it? And when a real decision rests on it, ask, because a one-line email is cheaper than the alternative.

Then write the unit in full in whatever you produce. “US gal” and “imp gal” cost three characters and stop the next person repeating the whole exercise. Most of these accidents happened because somebody along the chain wrote the ambiguous form.

Nothing you type here leaves the tab. Nearby: the guide to the units that have two meanings goes through the cases in detail, FIGURES handles significant figures and rounding, and EPOCH answers the same kind of question about a number that turns out to be a time.