What Is a Weight Calculator?

A Weight Calculator is a physics tool that calculates the gravitational force acting on an object. In physics, weight is not the same thing as mass. Weight is a force, and it depends on both mass and gravitational field strength. The standard near-surface formula is \(W=mg\), where \(W\) is weight force, \(m\) is mass, and \(g\) is gravitational field strength or acceleration due to gravity.

This calculator helps you calculate weight in newtons, pounds-force, kilogram-force, kilonewtons, and dynes. It also solves for mass when weight and gravity are known, and solves for gravitational field strength when weight and mass are known. For example, if a 70 kg person stands on Earth, their weight is about \(70\times9.80665\), or about 686 N. The same person standing on the Moon has the same mass but a much smaller weight because lunar gravity is weaker.

The calculator includes multiple modes. The basic mode handles \(W=mg\). The planet weight mode compares weight on Earth, Moon, Mars, Jupiter, and other bodies. The apparent weight mode models what a scale reads in an accelerating elevator. The gravity-from-planet mode estimates surface gravity from planetary mass and radius using \(g=\frac{GM}{r^2}\). The comparison table shows how one mass weighs across several planets and bodies. The unit converter converts between force, mass, and gravity units.

Weight is important in mechanics, engineering, astronomy, sports science, aviation, space science, construction, and everyday measurement. It affects normal force, friction, lift, load, structural support, and apparent heaviness. A scale on Earth usually estimates mass from weight by assuming a local value of gravity, but a physics calculation treats weight as a force.

This tool is designed for students, teachers, tutors, science learners, and content creators who need clear explanations, not just a final number. It includes formula steps, SI conversions, a visual force diagram, and a copyable calculation summary that can be used in homework, study notes, lab reports, or educational content.

How to Use This Weight Calculator

Use the W = mg tab for the main weight formula. Choose whether you want to solve for weight, mass, or gravity. To solve weight, enter mass and gravity. To solve mass, enter weight and gravity. To solve gravity, enter weight and mass. Choose the input and output units before calculating.

Use the Planet Weight tab to calculate how much an object weighs on Earth, Moon, Mars, Venus, Jupiter, and other bodies. Use Apparent Weight when the support force changes because of acceleration, such as in an elevator. Use Gravity from Planet to estimate gravitational field strength from planet mass and radius. Use Multi-Planet Table for a side-by-side comparison. Use Unit Converter for force, mass, and gravity conversions.

Keep signs and context in mind. Weight force is directed downward near a planet’s surface, but this calculator reports magnitude unless apparent weight becomes zero or negative in a strong downward acceleration case. A negative apparent normal force means contact would be lost in the ideal model, so the realistic scale reading becomes zero.

Weight Formula: W = mg

The standard weight formula near a planetary surface is:

Where \(W\) is weight force, \(m\) is mass, and \(g\) is gravitational field strength. In SI units, mass is in kilograms and gravity is in meters per second squared, so weight is measured in newtons:

On Earth, a commonly used standard value is \(g_0=9.80665\,m/s^2\). Local gravity varies slightly with altitude, latitude, and geology, so the exact value is not identical everywhere on Earth. For most classroom problems, \(9.8\,m/s^2\) or \(9.81\,m/s^2\) is used.

Mass vs Weight

Mass is a measure of how much matter an object has and how strongly it resists acceleration. Weight is the gravitational force acting on that mass. The distinction is critical:

A person with a mass of 70 kg has a mass of 70 kg on Earth, the Moon, Mars, or in orbit. But their weight changes because gravity changes. On Earth, their weight is about 686 N. On the Moon, using about \(1.62\,m/s^2\), their weight is about 113 N. The person has not lost mass; the gravitational force is smaller.

Everyday language often uses “weight” to mean body mass, especially when scales report kilograms or pounds. Physics is stricter. A bathroom scale measures support force and converts it into an estimated mass using assumed Earth gravity. In this calculator, weight means force.

Solving for Weight, Mass, or Gravity

To solve for weight, use:

To solve for mass from weight:

To solve for gravitational field strength:

These rearrangements are useful in laboratory work and mechanics problems. For example, if a spring scale reads 98 N for a 10 kg object, then \(g=\frac{98}{10}=9.8\,m/s^2\).

Weight on Planets and the Moon

Weight changes from one celestial body to another because gravitational field strength changes. For the same mass:

If the body’s gravity is lower than Earth’s, the weight force is lower. If gravity is higher, the weight force is higher. Jupiter’s cloud-top gravity is much higher than Earth’s, so the same mass would have a much larger weight there. The Moon’s gravity is much lower, so the same mass would feel lighter.

This does not mean the object’s mass changes. A 70 kg object remains 70 kg in every location. Only the gravitational force changes. This is why astronauts can move more easily on the Moon while still having the same mass and inertia.

Apparent Weight and Elevators

Apparent weight is the support force a scale reads. In a simple vertical elevator model:

If the elevator accelerates downward:

When the elevator accelerates upward, the scale reading increases. When it accelerates downward, the scale reading decreases. If downward acceleration equals \(g\), apparent weight becomes zero. This is the ideal free-fall condition, often described as weightlessness, even though gravity is still acting.

Gravity from Planet Mass and Radius

Surface gravity can be estimated using Newton’s law of gravitation:

Where \(G\) is the gravitational constant, \(M\) is the mass of the planet or body, and \(r\) is the distance from the center of the body to the surface or location. The formula shows that gravity increases with planetary mass and decreases with the square of distance from the center.

This explains why a large planet does not always have surface gravity proportional only to mass. Radius matters strongly. If two planets have the same mass but one has a larger radius, the larger-radius planet has weaker surface gravity at its surface.

Weight Units and Conversions

The SI unit of weight is the newton. Common force units include newton, kilonewton, pound-force, kilogram-force, and dyne. The calculator also shows related values in lbf and kgf because many learners encounter “weight” in everyday imperial or gravitational units.

Mass units include kilograms, grams, tonnes, ounces, pounds mass, and slugs. Gravity units include \(m/s^2\), \(ft/s^2\), and standard g. A common source of confusion is mixing pound-mass and pound-force. Pound-mass is mass. Pound-force is force. The calculator treats them as different physical quantities.

Common Mistakes

The first common mistake is treating mass and weight as identical. They are related, but not the same. The second mistake is reporting weight in kilograms. Kilogram is a unit of mass, not force, although everyday scales often display kilogram-equivalent mass. The third mistake is using Earth gravity for every planet.

The fourth mistake is ignoring apparent weight. A scale reading can change in an accelerating elevator even though the person’s true gravitational weight \(mg\) has not changed much. The fifth mistake is mixing pound-mass and pound-force. The sixth mistake is using the planet gravity formula \(g=GM/r^2\) with radius in kilometers without converting to meters. This calculator converts units internally to reduce those errors.

Worked Examples

Example 1: Weight on Earth. A 70 kg person near Earth’s surface has:

Example 2: Mass from weight. If an object weighs 98 N where \(g=9.8\,m/s^2\):

Example 3: Moon weight. A 70 kg person on the Moon where \(g\approx1.62\,m/s^2\):

Example 4: Apparent weight in an upward accelerating elevator. A 70 kg person accelerates upward at \(2\,m/s^2\):