Mass and weight aren't actually the same thing
Everyday speech treats them as interchangeable, but physics draws a sharp line: mass is how much matter an object contains, while weight is the force gravity exerts on that mass. A 10 kg object has a mass of 10 kg everywhere in the universe — on Earth, on the Moon, floating in deep space — but its weight changes dramatically depending on local gravity.
That same 10 kg object weighs about 98 newtons on Earth and roughly a sixth of that on the Moon, purely because lunar gravity is weaker — nothing about the actual amount of matter changed. This calculator works entirely in mass units (kg, lb, stone, tons); for the gravity-dependent force side of the equation, that's what a dedicated weight conversion handles instead.
Units that don't mean what they sound like
A few mass units carry hidden traps for anyone converting by hand:
| Unit | Equals |
|---|---|
| 1 stone | 14 lb (6.35 kg) |
| 1 US (short) ton | 2,000 lb (907.2 kg) |
| 1 metric ton (tonne) | 1,000 kg (2,204.6 lb) |
| 1 lb | 0.453592 kg |
Stone — still the everyday unit for body weight in the UK and Ireland — divides into 14-pound blocks rather than any round metric number, which is exactly why "11 stone 6" doesn't convert to a tidy kilogram figure without doing the arithmetic. And three completely different "tons" (US short ton, UK long ton, metric tonne) exist side by side, differing by as much as 10% from each other — worth double-checking on anything involving freight, shipping, or industrial quantities.
Why a gold ounce and a sugar ounce aren't the same size
The ounce you'd use to weigh flour (the avoirdupois ounce, about 28.35 g) is not the same ounce used to price gold, silver, or other precious metals (the troy ounce, about 31.1 g) — a nearly 10% difference. Precious metals have used the troy system since medieval trade fairs, and it's stuck around specifically in that industry ever since, which is why a "one ounce" gold coin is noticeably heavier than a one-ounce coin of ordinary metal would be under the everyday system.
Mass, density, and volume
Mass, density, and volume are linked by one formula — mass equals density times volume — which is why this calculator can solve for any one of the three given the other two. For a deeper look at density itself, including how it's used to identify materials and why objects float or sink, see the Density Calculator. And if you need weight as a force rather than mass — including what a given mass would weigh on another planet — the Weight Calculator covers that.