Weight Calculator

Free Online Weight Calculator (W = mg)

This weight calculator instantly solves for weight, mass, or gravitational acceleration using the formula W = m × g — Weight equals Mass times the local gravitational acceleration. Enter any two known values — mass and gravity, weight and gravity, or weight and mass — in any common unit, and the calculator converts everything automatically, works through the full step-by-step solution, and plots your numbers on a labeled Weight-Mass-Gravity triangle diagram along with a bar chart comparing your weight across six different worlds.

Whether you're a physics student solving a weight-versus-mass word problem, curious how much you'd weigh on the Moon or Mars, or an engineer working with gravitational force, this tool handles kilograms, grams, pounds, and slugs for mass, and newtons, kilonewtons, pounds-force, dynes, or kilogram-force for weight — and always shows its complete working, not just the final number.

What Is Weight? (Definition and Formula)

Weight is the force of gravity acting on an object's mass. It is not the same thing as mass — mass measures how much matter an object contains and never changes, while weight measures the gravitational pull on that mass and depends entirely on location. A 70 kg person has the same mass on Earth, the Moon, or in deep space, but their weight is very different in each place.

The formula for weight comes directly from Newton's second law, F = m × a, with acceleration replaced by gravitational acceleration g:

• Weight (W) = Mass (m) × Gravitational Acceleration (g)
• On Earth's surface, standard gravity is g = 9.80665 m/s² (often rounded to 9.8 m/s²).
• Example: a 70 kg person on Earth has weight = 70 × 9.80665 ≈ 686.5 N

Weight vs. Mass: What's the Difference?

Mass and weight are two of the most commonly confused concepts in physics. Mass is a scalar measure of the amount of matter in an object, measured in kilograms, and stays constant no matter where the object is located — on Earth, on the Moon, or floating in orbit. Weight is a force — specifically, the gravitational force pulling that mass toward a planet or other body — and it changes depending on the local gravitational acceleration.

This is why astronauts in orbit are 'weightless' (they experience zero apparent weight due to free-fall) while their mass remains completely unchanged. It's also why the same person weighs roughly six times less on the Moon than on Earth, since the Moon's gravity is only about 1.62 m/s² compared to Earth's 9.80665 m/s².

How to Find Mass from Weight and Gravity

If you know an object's weight and the local gravitational acceleration, you can rearrange the weight formula to solve for mass instead.

• Mass (m) = Weight (W) ÷ Gravitational Acceleration (g)
• Example: an object weighs 686 N on Earth → Mass = 686 ÷ 9.80665 ≈ 69.96 kg
• This rearrangement comes directly from dividing both sides of W = m × g by g.

How to Find Local Gravity from Weight and Mass

If you know an object's mass and how much it weighs at a particular location, you can calculate the local gravitational acceleration by rearranging the formula once more.

• Gravitational Acceleration (g) = Weight (W) ÷ Mass (m)
• Example: a 68.04 kg mass (150 lb) weighs 667 N at some location → g = 667 ÷ 68.04 ≈ 9.8 m/s², confirming it's on Earth
• This rearrangement comes from dividing both sides of W = m × g by m.

The Weight–Mass–Gravity Triangle

Just like the Speed-Distance-Time and Force-Mass-Acceleration triangles, the weight formula has its own 'magic triangle.' Weight sits at the top, with Mass and Gravity on the bottom row. Cover the quantity you're solving for with your finger, and what's left shows you the formula: covering W leaves m next to g (m × g); covering m leaves W over g (W ÷ g); covering g leaves W over m (W ÷ m). This calculator's diagram builds exactly this triangle using your real numbers, plus a comparison chart showing your exact mass weighed on six different worlds.

Gravitational Acceleration on Different Planets

Gravitational acceleration varies enormously across the solar system, which is why the same mass produces very different weights depending on where it's measured.

• Mercury: g ≈ 3.7 m/s²
• Venus: g ≈ 8.87 m/s²
• Earth: g = 9.80665 m/s² (standard gravity)
• Moon: g ≈ 1.62 m/s² (about 1/6 of Earth's gravity)
• Mars: g ≈ 3.71 m/s² (about 38% of Earth's gravity)
• Jupiter: g ≈ 24.79 m/s² (roughly 2.5 times Earth's gravity)

How to Use This Weight Calculator

Choose what you want to find — Weight, Mass, or Gravity — from the dropdown. Then fill in the two values you already know: pick a gravity preset (Earth, Moon, Mars, Jupiter, the Sun, or a custom value) or type your own g in m/s², and enter mass in kilograms, grams, pounds, or slugs. The calculator instantly converts everything to a common base unit, applies W = m × g, and shows the result in your chosen unit — plus automatically converted into newtons, pounds-force, and kilogram-force for quick reference.

Scroll down for the complete step-by-step solution, which mirrors exactly how you'd solve it by hand: the formula first, then your real values substituted in with unit conversion shown explicitly, then the arithmetic worked through to the final answer.

Worked Examples

Finding weight on Earth: a 70 kg person → Weight = 70 × 9.80665 ≈ 686.5 N.

Finding weight on the Moon: the same 70 kg person → Weight = 70 × 1.62 ≈ 113.4 N, roughly one-sixth of their Earth weight.

Finding weight on Mars: the same 70 kg person → Weight = 70 × 3.71 ≈ 259.7 N, roughly 38% of their Earth weight.

Finding mass: an object weighs 686 N on Earth → Mass = 686 ÷ 9.80665 ≈ 69.96 kg.

Unit conversion example: a weight of 686.5 N equals 686.5 × 0.224809 ≈ 154.3 lbf, or 686.5 × 0.101972 ≈ 70.0 kgf.

Real-World Applications of the Weight Formula

The W = m × g relationship shows up throughout science, engineering, and everyday life:

• Space exploration — calculating how much lighter equipment and astronauts will be on the Moon or Mars for mission planning.
• Structural engineering — determining the gravitational load a building, bridge, or crane structure must support.
• Aerospace engineering — computing the weight of aircraft and payloads for lift and thrust calculations.
• Everyday scales — the number shown on a bathroom scale is actually measuring your weight (a force), even though it's usually displayed as an equivalent mass in kg or lb, assuming standard Earth gravity.
• Physics education — one of the most common introductory applications of Newton's second law.

Why Scales Show 'Mass' When They're Measuring Weight

Bathroom and kitchen scales technically measure the force (weight) pressing down on them, but they're calibrated to display that force as an equivalent mass in kilograms or pounds, assuming standard Earth gravity of 9.80665 m/s². This works perfectly fine on Earth's surface, but the same scale would show a completely different (much smaller) number on the Moon, even though your actual mass hasn't changed at all — because the scale is really responding to force, not mass directly.

Tips for Solving Weight, Mass, and Gravity Word Problems

Most textbook and exam word problems involving weight follow a predictable pattern, and a few habits make them far easier to solve correctly:

• Identify which two quantities are given and which one is being asked for — this tells you whether to use W = m × g, m = W ÷ g, or g = W ÷ m.
• Remember that mass never changes with location, but weight always does — don't confuse the two, even though everyday language often uses them interchangeably.
• Use g = 9.80665 m/s² for Earth unless the problem specifies a different planet or a rounded value like 9.8 m/s².
• Sanity-check your final answer: a typical adult human weighs roughly 500–900 N on Earth; on the Moon that same person would weigh only about 1/6 as much.

Frequently Asked Questions

What is the formula for weight? Weight = Mass × Gravitational Acceleration (W = m × g). Weight is a force, measured in newtons, that depends on both an object's mass and the local gravity.

What is the difference between weight and mass? Mass is the amount of matter in an object and never changes; weight is the force of gravity on that mass and varies by location.

How much would I weigh on the Moon? Roughly 1/6 of your Earth weight, since the Moon's gravity (1.62 m/s²) is about 1/6 of Earth's (9.80665 m/s²).

What is standard gravity on Earth? Standard gravity is defined as g = 9.80665 m/s², often rounded to 9.8 m/s² for everyday calculations.

Can this calculator solve for mass or gravity, not just weight? Yes — switch the 'What do you want to find?' dropdown to Mass or Gravity, and the calculator rearranges the formula automatically and solves for whichever value you need.

How do I convert newtons of weight to pounds? Multiply newtons by 0.224809 to get pounds-force (lbf). For example, 686.5 N × 0.224809 ≈ 154.3 lbf.