Unit conversion goes wrong far more often through ambiguity than through arithmetic. A ton is three different quantities, a gallon is two, horsepower is two, and a calorie means one of two things depending on whether the C is capitalised. Every one of those is a name collision rather than a hard sum, and no amount of decimal places protects against picking the wrong one.
Knowing which collisions exist is most of the skill.
Which names are overloaded?
Five, and they cover the overwhelming majority of real conversion errors.
| Name | Meanings |
|---|---|
| Ton | US short 2,000 lb · UK long 2,240 lb · metric tonne 1,000 kg |
| Gallon | US 3.785 L · imperial 4.546 L |
| Horsepower | Mechanical ~746 W · metric ~735 W |
| Calorie | 4.184 J · Calorie (kcal) 4,184 J |
| Ounce | Avoirdupois 28.35 g · troy 31.10 g · fluid ounce, a volume |
The metric tonne and the US short ton differ by about 10 per cent, which is small enough to look like a rounding and large enough to matter on a freight invoice. The two horsepowers differ by 1.4 per cent, which is small enough that it usually does not — except in a specification comparison where it is the whole difference being argued about.
Why is the calorie the worst of them?
Because the distinction is a capital letter and it is routinely lost. A calorie is 4.184 joules; a Calorie — which is what food packaging means — is a kilocalorie, 4,184 joules. A factor of a thousand hangs on the case of one character.
In practice food labelling in most of the world now uses kcal alongside kJ, which removes the ambiguity entirely. Where the older usage survives, the context resolves it: nothing edible contains 200 calories in the small sense.
The macro article works entirely in the food sense, which is the one anyone reading a label wants.
What is the difference between energy and power?
A rate and a quantity, and confusing them is the commonest error in this area. A kilowatt is a rate; a kilowatt-hour is an amount of energy — a kilowatt sustained for an hour.
That is why a 3 kW heater and 3 kWh are not the same statement: the first describes what it draws, the second what it consumed. A kilowatt-hour is 3.6 million joules, which is the bridge between the electrical and scientific families.
The same distinction runs through everything: watts and watt-hours, litres per hour and litres, miles per hour and miles. The electricity article is built on it.
Why do area factors look unfamiliar?
Because they are the square of the length factors. An inch is 2.54 cm, so a square inch is 6.4516 cm² — the same relationship, squared, and the resulting number bears no visual resemblance to the one it came from.
The two worth memorising are that a hectare is 10,000 square metres and roughly 2.47 acres, because those are the units most land is described in and neither is intuitive from the other.
Volume compounds it again by cubing, which is why a cubic metre is a thousand litres and a cubic foot is 28.3 — and why the scaling article matters as much as the conversion factor does.
What is not visible in a pressure figure?
Whether it is gauge or absolute. A tyre gauge reads zero at atmospheric pressure, so a tyre at 32 psi gauge is at about 46.7 psi absolute — and the two are used in different fields without either being labelled.
For tyres the numbers worth carrying are that 1 bar is about 14.5 psi, and a typical car tyre runs around 2.2 bar or 32 psi. For anything scientific or industrial the gauge-versus-absolute question has to be answered before the conversion means anything.
Why are there two families of megabyte?
Because the industry never resolved a genuine ambiguity. SI prefixes count in thousands, so a terabyte is 10¹² bytes. Binary prefixes count in 1,024s, so a tebibyte is 2⁴⁰ bytes — about 10 per cent more.
Drive manufacturers use the SI meaning and several operating systems report the binary one, which is why a "1 TB" drive shows as about 931 GB. Neither is lying; they are using different definitions of the same prefix.
The gap widens with scale: about 2.4 per cent at kilobytes, 4.9 at megabytes, 7.4 at gigabytes and 10 at terabytes. The file size article covers what that does to a recording-time estimate.
Questions people ask
Are the imperial units defined by the metric ones? Since 1959, yes. An inch is exactly 25.4 mm and a pound exactly 0.45359237 kg by international agreement, so every imperial conversion is exact rather than measured.
Which ounce does a recipe mean? Avoirdupois for weight, and a fluid ounce for volume — and the two are not related. A fluid ounce of flour and an ounce of flour are different quantities.
Is a knot a nautical mile per hour? Yes, and since a nautical mile is one minute of latitude, one knot covers one minute of latitude per hour. That relationship is why the unit survives at sea and in the air.
Why is temperature different? Because the scales do not share a zero, so a conversion needs an offset as well as a factor — and a temperature difference takes the factor alone, which the delta article covers.
The factors are exact; the names are not. The unit converter shows a whole category at once, and the length, weight, volume, area, speed, energy, power, pressure, temperature and data storage converters each handle one family with its collisions listed rather than hidden.