Your bathroom scale says 70 kg. Strictly speaking, that is wrong — the scale is measuring a force, and it is reporting a mass. The reason nobody minds is that the conversion between the two is a constant on Earth's surface, so the scale can do the division for you and never be noticeably wrong.
That shortcut works right up until it does not. This guide is about knowing which situation you are in.
The distinction in one paragraph
Mass is how much matter something contains. It does not change if you move it. Its SI unit is the kilogram. Weight is the force gravity exerts on that mass. It changes with where you are. Its SI unit is the newton. The relationship is W = m × g, where g is the local gravitational acceleration — about 9.807 m/s² at Earth's surface.
So a 70 kg person has a weight of about 687 newtons on Earth, about 114 N on the Moon, and 0 N in orbit. The mass is 70 kg in all three places.
Why the confusion is built into the units
English uses "weight" for both concepts, and the unit names make it worse. The pound exists in two forms:
- Pound-mass (lb) — a unit of mass, exactly 0.45359237 kg.
- Pound-force (lbf) — a unit of force: the weight of one pound-mass at standard gravity, about 4.448 N.
At Earth's surface these are numerically equal, which is precisely why they get muddled. A 10 lb object weighs 10 lbf. In a physics or engineering calculation, substituting one for the other silently introduces a factor of 32.174 — the number of feet per second squared in standard gravity — and the error is large enough to be obvious only if you happen to be checking.
The metric system dodged this by giving force its own name. A kilogram is never a force; a newton is never a mass. That is one of the practical advantages of a coherent unit system, and it is why engineering calculations are so much less error-prone in SI.
Does the distinction matter to you?
Work down this list. Most people stop at the first item.
| Situation | Does it matter? |
|---|---|
| Body weight, luggage, groceries, shipping | No. Everything stays on Earth; kilograms and pounds are fine. |
| Cooking and nutrition | No. Recipes and labels mean mass throughout. |
| Structural and mechanical engineering | Yes. Loads are forces; specify newtons or pounds-force explicitly. |
| Physics problems | Yes, always. This is usually the whole point of the question. |
| Spacecraft, aviation, high-precision metrology | Yes. g varies with altitude and latitude by enough to measure. |
| Trade measurement and legal metrology | Yes, in the standards. Scales are calibrated for local gravity. |
How scales actually work
A spring or load-cell scale measures force and divides by an assumed g. Move it somewhere with different local gravity and it reads differently for the same object. Gravity varies across the Earth's surface by roughly 0.5% — less at the equator, more at the poles, less at altitude. That is why commercial scales used in trade are calibrated for the gravity zone they will operate in, and why NIST Handbook 44 governs how weighing devices used in commerce must perform.
A balance scale is different: it compares your object against known masses, so gravity cancels out on both sides. A balance gives the same answer on the Moon. This is why laboratory mass standards are compared on balances rather than measured on load cells.
The 2019 redefinition of the kilogram
Until 2019 the kilogram was the last SI unit still defined by a physical object — the International Prototype of the Kilogram, a platinum-iridium cylinder near Paris. Its mass was, by definition, exactly one kilogram, so if it drifted, the kilogram drifted with it. Comparisons with national copies suggested that it had.
The 2019 revision replaced it by fixing the Planck constant at exactly 6.626 070 15 × 10⁻³⁴ J s. The kilogram is now derived from that, which makes it reproducible in any suitably equipped laboratory rather than dependent on one artefact. For everyday purposes nothing changed — the new definition was chosen to match the old one as closely as measurement allowed.
Getting it right in writing
- Say "mass" when you mean the amount of matter, even if colloquial English would say weight.
- Write lbf, not lb, when you mean a force. In SI, use newtons — there is no ambiguity to resolve.
- Never write "kgf" in new work. The kilogram-force (about 9.807 N) is a deprecated unit that still appears in older machinery specifications.
- In a calculation involving F = ma, check your units before you check your arithmetic. A mass-force mix-up is the most common source of an answer that is out by about a factor of ten.