How to Convert Weight Units

The conversions themselves are exact

There is a common belief that metric-to-imperial conversion is inherently approximate — that 2.2 pounds to the kilogram is a rounding of something messier underneath.

It isn't. Since the international yard and pound agreement of 1959, the avoirdupois pound has been defined as exactly 0.45359237 kilograms. It is not a measurement anybody made; it is a definition several countries agreed on, and everything else in the imperial column follows from it:

Unit	Exact definition
1 pound	0.45359237 kg
1 ounce	1/16 lb = 0.028349523125 kg
1 stone	14 lb = 6.35029318 kg
1 US short ton	2000 lb = 907.18474 kg

So a conversion adds no error of its own. If your answer is imprecise, the imprecision arrived with the number you started from.

Enter a Weight, tell it what it's Measured in, and everything else fills in — Kilograms, Grams, Milligrams, Metric tonnes, Pounds, Ounces, Stone and US short tons together:

An ounce is two different weights

This is the one that costs real money.

The ounce you get on a kitchen scale is the avoirdupois ounce: 28.349523125 grams, one sixteenth of a pound. The ounce that gold, silver and platinum are priced in is the troy ounce: 31.1034768 grams.

Troy is about 9.7% heavier. A price quoted per troy ounce, read as if it were an ordinary ounce, overstates how much metal you're getting by roughly a tenth — and because both are just called "an ounce" in conversation, nothing about the sentence warns you.

That's why the converter above offers only the avoirdupois ounce. Two different units sharing a name in a single dropdown is precisely how the mistake gets made, so the dropdown declines to make it available.

A ton is three different weights

Worse, and more common in ordinary documents:

Name	Pounds	Kilograms
US short ton	2000	907.18474
UK long ton	2240	1016.0469088
Metric tonne	—	1000 exactly

The short ton is about 9% lighter than a metric tonne. The long ton is about 1.6% heavier than a tonne, and about 12% heavier than the short ton.

Freight quotes, shipping limits and older engineering documents do not reliably say which one they mean. When a source just says "ton", that's a question to resolve before the number goes into anything — not a rounding difference you can absorb.

10 st 7 lb is not 10.7 stone

Stone is used for body weight in the UK and Ireland, and it carries exactly the same trap that feet and inches do.

A stone is 14 pounds. So 10 st 7 lb is 10.5 stone, because seven pounds is half a stone — not 10.7. The wrong version looks entirely plausible, which is what makes it survive.

The safe habit is the same one that works for length: convert to the smallest unit first, do the arithmetic there, and convert back once at the end. Work in pounds, not in stone-and-pounds.

A fluid ounce is not an ounce

A fluid ounce measures volume. An ounce measures weight. They're different quantities that happen to share half a name.

They coincide only for something with roughly the density of water, and even then only in the US system — the UK fluid ounce is a different size again. A recipe calling for 8 fl oz of honey does not want 8 oz on a scale; honey is about 1.4 times as dense as water, so you'd be short by nearly a third.

If a recipe mixes both, the giveaway is what's being measured: liquids in fluid ounces, solids in ounces. When in doubt, weigh it — weight is unambiguous and volume depends on how tightly the flour was packed.

The ones worth memorising

• Kilograms to pounds: double it and add a tenth. (2.2 against the true 2.20462.)
• Pounds to kilograms: halve it, then take off a tenth.
• Ounces to grams: multiply by 28. The true figure is 28.35, so this runs slightly light.
• Grams to ounces: divide by 28, or by 30 if you just want the shape of the answer.

Those are fine for shopping and cooking. For anything that gets ordered, shipped, dosed or paid for, use the exact figures — they're definitions, so there's no reason to accept an approximation.

Weight or mass?

Strictly, they're not the same thing. Mass is how much matter something contains; weight is the force gravity applies to it. A kilogram of flour has the same mass on the moon and about a sixth of the weight.

Every unit on this page converts mass, which is what kitchen scales, bathroom scales, shipping labels and recipes all mean when they say weight. The distinction starts to matter in physics problems and off the surface of the earth, and essentially nowhere else in daily life.