What Is Weight?
Weight is the gravitational force acting on an object. Unlike mass, which is an intrinsic property measured in kilograms, weight is a force measured in newtons (N). Your weight changes depending on the strength of the local gravitational field — you weigh less on the Moon and more on Jupiter.
In everyday language, “weight” is often used interchangeably with “mass,” but in physics they are fundamentally different quantities. A bathroom scale actually measures the normal force (which equals weight when stationary), not mass directly.
Weight as a Force
The Weight Formula (W = mg)
Where W is weight in newtons, m is mass in kilograms, and g is gravitational acceleration in m/s². On Earth’s surface, g ≈ 9.81 m/s².
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Mass vs Weight
| Property | Mass | Weight |
|---|---|---|
| SI unit | kilogram (kg) | newton (N) |
| Type | Scalar | Force (vector, directed downward) |
| Changes with location? | No | Yes (depends on g) |
| Can it be zero? | No (always > 0) | Yes (in free fall / deep space) |
| Measured with | Balance (comparison) | Spring scale or force sensor |
Weight on Different Planets
Surface gravity varies dramatically across the solar system. The same mass experiences very different weight on each body.
| Body | g (m/s²) | 70 kg weight (N) | Relative to Earth |
|---|---|---|---|
| Mercury | 3.70 | 259 | 0.38× |
| Venus | 8.87 | 621 | 0.90× |
| Earth | 9.81 | 687 | 1.00× |
| Moon | 1.63 | 114 | 0.17× |
| Mars | 3.72 | 260 | 0.38× |
| Jupiter | 24.79 | 1,735 | 2.53× |
| Saturn | 10.44 | 731 | 1.06× |
| Uranus | 8.87 | 621 | 0.90× |
| Neptune | 11.15 | 781 | 1.14× |
| Pluto | 0.62 | 43 | 0.06× |
Source: NASA Planetary Fact Sheet. Values are equatorial surface gravity.
Apparent Weight in Elevators
When an elevator accelerates upward (a > 0), you feel heavier because the floor pushes harder against you. When it accelerates downward (a < 0), you feel lighter. In free fall (a = −g), apparent weight is zero — this is weightlessness.
| Scenario | a | Scale reads |
|---|---|---|
| At rest / constant velocity | 0 | W = mg |
| Accelerating up | +a | m(g + a) > mg |
| Accelerating down | −a | m(g − a) < mg |
| Free fall | −g | 0 (weightless) |
How to Use the Calculator
- Choose what to calculate: weight, mass, gravity, planetary comparison, or elevator physics.
- Enter mass, weight, gravity, and/or elevator acceleration.
- Click Calculate.
- Review the result, formula steps, and planetary extras.
Example Calculations
70 kg person on Earth
W = 70 × 9.807 = 686.5 N ≈ 687 N
70 kg person on Moon (g = 1.625 m/s²)
W = 70 × 1.625 = 113.8 N — about ⅙ of Earth weight
75 kg in elevator accelerating up at 2 m/s²
W’ = 75 × (9.807 + 2) = 75 × 11.807 = 885.5 N (vs 735.5 N at rest)
Common Mistakes
- Saying “I weigh 70 kg” — 70 kg is your mass; your weight is ≈ 687 N.
- Using g = 10 m/s² in calculations that require precision (g = 9.80665 m/s²).
- Confusing apparent weight with true weight in accelerating systems.
- Thinking astronauts on the ISS have zero gravity — they are in free fall, not beyond gravity’s reach.
- Forgetting that weight is a vector (directed toward the centre of the gravitating body).
Accuracy and Limitations
This calculator uses W = mg with constant g. In reality, g varies with altitude (g = GM/r²), latitude, and local geology. The planetary gravity values are equatorial surface values from NASA. For high-altitude or orbital calculations, use the Gravitational Force Calculator. This tool is for education and estimation.
FAQ
Is weight measured in kg or N?›
Weight is a force and should be measured in newtons (N). The everyday use of “kg” for weight is technically mass. When you say “I weigh 70 kg,” you mean your mass is 70 kg and your weight is about 686 N.
Why is weight different on the Moon?›
The Moon’s surface gravity is about 1.625 m/s² (vs Earth’s 9.81 m/s²). Since W = mg, the same mass weighs about ⅙ as much on the Moon.
What is the difference between mass and weight?›
Mass (kg) is the amount of matter in an object — it’s the same everywhere. Weight (N) is the gravitational force on that mass. An astronaut has the same mass on Earth and in orbit, but different weight.
How do astronauts become weightless?›
In orbit, astronauts and their spacecraft are in continuous free fall toward Earth. The apparent weight is zero because everything accelerates at the same rate (a = −g). Their mass is unchanged.
What is apparent weight?›
Apparent weight is what a scale reads. In an accelerating elevator, W’ = m(g + a). Accelerating upward increases apparent weight; accelerating downward decreases it.
Does weight change with altitude?›
Yes, slightly. Gravity decreases with altitude: g = GM/r². At 10 km altitude, g is about 0.3% less than at sea level. For most practical purposes the change is negligible.
What is 1 kilogram-force?›
1 kgf (or 1 kilopond) is the weight of a 1 kg mass under standard gravity (9.80665 m/s²). So 1 kgf = 9.80665 N.
How much would I weigh on Jupiter?›
Jupiter’s surface gravity is about 24.79 m/s² (2.53× Earth’s). A 70 kg person would weigh about 1,735 N (vs 686 N on Earth) — roughly 2.5 times heavier.
Sources
Author & technical reviewer
Manish Kumar
PhysicsCalcs tools are reviewed with an educational focus: clear formulas, transparent assumptions, and practical context for students and science learners.
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