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Work out weight as a force from mass and gravity — in newtons, kilonewtons, kgf and lbf — and see what the same mass weighs on other planets. For kilograms to pounds, use the weight converter.
| Property | Value |
|---|
In everyday speech these words are interchangeable. In physics and engineering they are not, and the distinction matters as soon as gravity varies or forces are being calculated.
| Mass | Weight | |
|---|---|---|
| What it is | How much matter an object contains | The force gravity exerts on that matter |
| Unit | Kilogram (kg) | Newton (N) |
| Changes with location | No — the same everywhere | Yes — depends on local gravity |
| In free fall | Unchanged | Effectively zero |
| Measured with | A balance, by comparison | A spring scale or load cell |
| SI status | Base quantity | Derived — it is a force |
So when someone says they weigh 70 kilograms, they are strictly stating their mass. Their weight is about 686 newtons. Everyone understands what is meant, and in daily life the shorthand is harmless — because gravity on Earth's surface barely varies.
This is where the distinction stops being pedantic. Take a person with a mass of 70 kg:
| Location | Gravity (m/s²) | Weight (N) | Scale would read |
|---|---|---|---|
| Pluto | 0.62 | 43 | 4.4 kg |
| Moon | 1.625 | 114 | 11.6 kg |
| Mars | 3.721 | 260 | 26.6 kg |
| Venus | 8.87 | 621 | 63.3 kg |
| Earth | 9.80665 | 686 | 70 kg |
| Jupiter | 24.79 | 1,735 | 177 kg |
| Sun's surface | 274 | 19,180 | 1,956 kg |
A bathroom scale does not measure mass. It measures the force you exert on it, then divides by an assumed value of gravity and displays the result in kilograms.
Even on Earth, gravity is not perfectly uniform. It is slightly weaker at the equator than at the poles, because of Earth's rotation and its equatorial bulge, and it drops slightly with altitude. The variation is around 0.5% from pole to equator — small enough that scales ignore it, but real enough that laboratories calibrate for local gravity.
The value 9.81 appears everywhere, but the defined constant — standard gravity, written g₀ — is exactly 9.80665 m/s². It was fixed by international agreement as a conventional reference value rather than a measurement of any particular place.
| Location | Approximate gravity |
|---|---|
| Standard (defined) | 9.80665 m/s² |
| North or South Pole | ~9.832 m/s² |
| Sydney | ~9.797 m/s² |
| Melbourne | ~9.800 m/s² |
| Darwin | ~9.781 m/s² |
| Equator, sea level | ~9.780 m/s² |
| Top of Mount Everest | ~9.77 m/s² |
Australia spans a fair range of latitude, so gravity in Darwin is measurably lower than in Hobart. For trade-approved scales this matters, and instruments are calibrated by gravity zone. For everything else, the difference is invisible.
Three units for the same quantity, and each turns up in different places.
| Unit | Equals | Where you meet it |
|---|---|---|
| Newton (N) | 1 kg accelerated at 1 m/s² | The SI unit — physics, engineering, structural loads |
| Kilonewton (kN) | 1,000 N | Building loads, crane ratings, structural design |
| Kilogram-force (kgf) | 9.80665 N | Older engineering, some tools and tyre pressures |
| Pound-force (lbf) | 4.44822 N | American engineering, aviation thrust ratings |
A quick sense of scale: one newton is roughly the weight of a small apple. A kilonewton is about the weight of 100 kg — which is why structural loads in kN map neatly onto tonnes if you divide by about ten.
| Field | Why the distinction matters |
|---|---|
| Structural engineering | Loads are forces. A 500 kg load applies about 4.9 kN, and beams are rated in kN |
| Lifting and rigging | Slings and shackles are rated by working load limit — a force, not a mass |
| Aviation | Thrust in kN or lbf; aircraft weight changes as fuel burns but must be balanced against lift |
| Spaceflight | Mass determines the fuel needed to accelerate; weight is irrelevant in orbit |
| Physics education | Confusing the two is the single most common error in mechanics problems |
| Trade measurement | Goods are sold by mass, so scales must be calibrated for local gravity |
Astronauts on the International Space Station are often described as being in zero gravity. Gravity there is about 90% of its value at Earth's surface — the station is only around 400 km up, which is barely any distance compared with Earth's radius.
Crucially, mass is completely unaffected. Pushing a floating 200 kg piece of equipment in orbit still takes exactly as much force as it would to accelerate it on Earth — which is why astronauts move heavy objects slowly and deliberately. Weightless does not mean effortless.
What is the difference between weight and mass?
Mass is how much matter an object contains, measured in kilograms, and it is the same everywhere in the universe. Weight is the force gravity exerts on that mass, measured in newtons, and it changes with location. A 70 kg person has a mass of 70 kg on Earth and on the Moon, but weighs about 686 N here and only 114 N there.
How do I convert kilograms to newtons?
Multiply the mass in kilograms by the local gravitational acceleration. Using standard gravity of 9.80665 m/s², a mass of 70 kg gives 686.47 N. For rough work, multiplying by 10 gets you within 2%.
How much would I weigh on the Moon?
About one sixth of your Earth weight. Lunar gravity is 1.625 m/s² against Earth's 9.80665, so a 70 kg person weighs 114 N on the Moon compared with 686 N here — and a bathroom scale would display around 11.6 kg. Your mass is unchanged at 70 kg.
How much would I weigh on Mars?
Roughly 38% of your Earth weight. Martian gravity is about 3.721 m/s², so a 70 kg person weighs 260 N there against 686 N here, and a scale would read about 26.6 kg. Mass remains 70 kg.
What is standard gravity?
Standard gravity, written g₀, is defined as exactly 9.80665 m/s². It is a conventional reference value fixed by international agreement rather than a measurement of any particular location. Actual gravity varies from about 9.78 m/s² at the equator to 9.83 at the poles.
Does gravity change across Australia?
Yes, slightly. Gravity is weaker nearer the equator and at altitude, so it is measurably lower in Darwin (about 9.781 m/s²) than in Hobart. The variation is under 0.5% and irrelevant for everyday purposes, but trade-approved scales are calibrated by gravity zone because selling goods by mass demands that precision.
What is a newton?
The SI unit of force — the force needed to accelerate one kilogram at one metre per second squared. For a physical sense of scale, one newton is roughly the weight of a small apple, and a kilonewton is about the weight of 100 kg.
What is kilogram-force?
One kilogram-force is the weight of one kilogram under standard gravity, equal to 9.80665 newtons. It is numerically equal to the mass in kilograms on Earth, which makes it convenient and is a major reason mass and weight get confused. It is not an SI unit, but it persists in older engineering documentation and on equipment plates.
Is there gravity in space?
Yes. At the International Space Station's altitude of around 400 km, gravity is still about 90% of its value at Earth's surface. Astronauts float because the station is in continuous free fall around Earth — everything inside falls together, so nothing presses against anything. The correct term is microgravity.
If astronauts are weightless, is everything easy to move?
No, and this is the practical consequence of mass being unchanged. Weight disappears in orbit but inertia does not, so pushing a 200 kg module still requires the same force it would on Earth to accelerate it. Astronauts move large objects slowly and deliberately because stopping them takes just as much effort as starting them.
Why does my bathroom scale show kilograms?
Because it measures the force you apply, divides by an assumed value of gravity, and displays the result as mass. It works fine on Earth. Take it to the Moon and it would read about a sixth of your actual mass — a balance scale, which compares against known masses, would read correctly anywhere.
What is the SI unit of mass?
The kilogram. Since 2019 it has been defined using the Planck constant rather than a physical prototype, which means it can be reproduced in any suitably equipped laboratory instead of depending on a single metal cylinder kept in France.
The whole distinction comes down to one sentence: mass is a property of the object, weight is a force acting on it. Move the object and its mass is unchanged; move it somewhere gravity differs and its weight changes immediately.
In everyday Australian life the shorthand is harmless, because gravity here barely varies. It stops being harmless in engineering, physics and anything leaving the planet.
If what you actually need is kilograms to pounds or stone, that is the weight converter. For other measurement types, the unit converter handles ten categories in one tool.