Weight Calculator,
across every planet and unit.

Field guide

Mass, weight, and gravity explained.

In everyday language, “mass” and “weight” are used interchangeably, but in physics and engineering they are fundamentally different quantities. Understanding the distinction is essential for everything from basic mechanics to spacecraft trajectory calculations.

Mass: the constant quantity

Mass is the measure of the amount of matter in an object. It is an intrinsic property; it does not change regardless of where the object is located. A 70 kg astronaut has a mass of exactly 70 kg on Earth, on the Moon, on Mars, and in empty space. Mass is measured in kilograms (SI), grams, pounds-mass, or slugs.

Weight: the gravitational force

Weight is the force exerted on a mass by a gravitational field. It varies with location because gravitational acceleration (g) varies. The relationship is Newton’s second law applied to gravity:

Where W is weight in Newtons, m is mass in kilograms, and g is gravitational acceleration in m/s². Weight is a force; it is measured in Newtons (SI), pound-force (lbf), kilogram-force (kgf), or dynes.

Standard gravity and why it matters

The standard acceleration of gravity (g₀) is defined exactly as 9.80665 m/s² by the International Bureau of Weights and Measures (BIPM). This is a conventional reference value, not a measured one, and it forms the basis for unit conversion between mass and force in everyday contexts. It is used to define the kilogram-force (1 kgf = 9.80665 N) and the pound-force (1 lbf = 0.453592… kg × g₀).

Gravity across the solar system

Gravitational acceleration depends on a body’s mass and radius, specifically, g = G·M / r², where G is the gravitational constant, M is the body’s mass, and r is its radius. The result spans orders of magnitude across the solar system:

Why everyday scales measure mass, not weight

A bathroom scale is calibrated to display kilograms or pounds-mass . It shows mass, not force. It does this by measuring the downward force (weight) and dividing by the local gravitational constant built into its calibration. On the Moon, the same scale would read about one-sixth of your Earth value, not because your mass changed but because the gravitational force is weaker. For most practical purposes on Earth’s surface this distinction doesn’t matter, but for physics, engineering, and off-Earth contexts it is critical.

Converting between weight units

The five weight/force units in this calculator relate as follows:

• 1 Newton (N) = 1 kg·m/s² (SI base unit of force)
• 1 kilonewton (kN) = 1,000 N
• 1 pound-force (lbf) = 4.44822 N
• 1 kilogram-force (kgf) = 9.80665 N (by definition)
• 1 dyne = 10⁻⁵ N (CGS unit, used in surface-tension and colloidal physics)

Worked examples

Example 1: Find weight on Mars: A rover chassis has a mass of 180 kg. Mars gravity is 3.721 m/s².

Example 2: Find mass from weight: A spring scale on the Moon reads 98.1 N. Moon gravity is 1.62 m/s².

Disclaimer

Gravity values used for planetary bodies are standard reference values from NASA and IAU. Surface gravity varies with altitude and latitude; these figures represent mean surface values. This calculator is intended for educational and engineering estimation purposes.