Definitions
Mass
Mass is an intrinsic property of matter. It measures the amount of material in an object and its resistance to acceleration (inertia). Mass does not change with location: a 70 kg person has 70 kg of mass on Earth, on the Moon, on Mars, and in deep space.
- SI unit: kilogram (kg)
- Common units: grams (g), pounds (lb — technically a mass unit), ounces (oz)
- Symbol: usually m
Weight
Weight is a force. It measures the gravitational attraction between an object and a nearby large mass (usually a planet). Weight depends on both the object's mass and the strength of the local gravitational field.
- SI unit: newton (N)
- Common units: pounds-force (lbf), kilogram-force (kgf)
- Symbol: usually W or Fg
- Formula:
W = m × g, where g is the local gravitational acceleration
The Critical Difference
Because weight is a force, the same object can have different weights in different gravitational environments while its mass remains constant:
| Location | g (m/s²) | Mass (kg) | Weight (N) | Weight as "kgf" |
|---|---|---|---|---|
| Earth (standard) | 9.80665 | 70 | 686.5 | 70.0 |
| Earth (equator) | 9.7803 | 70 | 684.6 | 69.8 |
| Earth (pole) | 9.8322 | 70 | 688.3 | 70.2 |
| Moon | 1.62 | 70 | 113.4 | 11.6 |
| Mars | 3.71 | 70 | 259.7 | 26.5 |
| Deep space (far from any body) | ≈ 0 | 70 | ≈ 0 | 0 |
The mass is 70 kg everywhere. The weight in newtons changes by a factor of six between Earth and Moon. This is why astronauts on the Moon can jump higher — their muscles are the same, but the weight their legs must overcome is much less.
Everyday Confusion
In everyday life, people use "weight" to mean "mass". A doctor says "your weight is 70 kg". A supermarket sign says "weight: 2 lb". This is technically incorrect — kg and lb are mass units, not weight units — but it is so widespread that it has become standard usage in cooking, medicine, commerce, and travel.
In physics, engineering, and any context where forces are being calculated, the distinction matters. When a structural engineer says "the weight of the beam", they mean the force the beam exerts on its supports, and they will use newtons or pound-force in the calculation.
What M-Convert Does
M-Convert's mass category is officially titled "Mass & Weight Converter" to reflect common usage. The conversions themselves are between mass units (kg, g, lb, oz, st, t). We do not convert mass to force because that requires a specific gravitational context.
If you need to convert mass to weight (or vice versa), you need the local gravitational acceleration:
W = m × g m = W / g
For Earth surface calculations, use the standard gravity value g₀ = 9.80665 m/s² exactly.
Which Units Are Mass and Which Are Weight?
- Mass units: kilogram, gram, milligram, metric ton, pound (avoirdupois), ounce, stone, carat, atomic mass unit.
- Force/weight units: newton, kilonewton, pound-force (lbf), kilogram-force (kgf), dyne.
The trick: "pound" without qualification is a mass unit; "pound-force" (lbf) is a force unit. Similarly, "kilogram" is mass but "kilogram-force" (kgf) is force. In US customary engineering, the term "pound" in a technical context sometimes means pound-force, which is a common source of confusion. Always check the context.
The Formula for Weight
Weight is mass times gravitational acceleration:
Weight (N) = Mass (kg) × g (m/s²)
Where g varies:
- Standard gravity on Earth: g₀ = 9.80665 m/s² (exact by definition).
- Sea level at the equator: about 9.7803 m/s².
- Sea level at the poles: about 9.8322 m/s².
- Moon surface: about 1.62 m/s².
- Mars surface: about 3.71 m/s².
Why This Matters
Physics education
Students routinely confuse mass and weight because everyday language does. Correcting this early prevents errors in later mechanics problems where the distinction is essential (for example, calculating acceleration, momentum, or the tension in a cable).
Space travel
Spacecraft are designed around mass budgets (mass is what is expensive to launch), but the forces the spacecraft experiences depend on weight in whatever gravitational field it currently occupies. Both are needed for the full picture.
International commerce
Most countries have officially adopted SI and use mass units (kg) for trade. The US still uses mass units labeled as "weight" (lb), but the underlying physical quantity is still mass. Modern international trade agreements are in mass.
Health
Medical dosing is based on mass (kg), not force. A scale measures weight (force) and divides by the local g to display mass (kg or lb). The distinction is embedded in how scales work.