Weight Calculator (W = mg)

This calculator works out weight, mass or gravitational field strength using the physics formula W equals mg, where W is weight in newtons, m is mass in kilograms and g is gravity in metres per second squared. Weight and mass are not the same thing: mass is a fixed amount of matter, while weight is the force gravity exerts on it, so the same object weighs differently depending on where it is. You choose which of the three values to solve for from the dropdown, then enter the other two into the Weight, Mass and Gravity fields. The calculator instantly returns the solved value with a labelled result showing which variable it worked out, updating live as you change any input. The gravity field defaults to Earth's standard 9.81 metres per second squared, but you can overwrite it with about 1.62 for the Moon or 3.71 for Mars, which makes the tool handy for comparing how the same mass behaves in different gravitational environments. It suits physics students checking homework, teachers preparing worked examples, and engineers who need a quick, reliable conversion between mass and weight. Results are rounded for display, so treat them as an exact calculation of the formula entered rather than a physical measurement, and always check that your inputs match the kilograms, newtons and metres per second squared units as labelled.

Weight (N)

Mass (kg)

Gravity (m/s²)

Solve for

Weight = 98.1 N

Value98.1

Resultsolved for Weight

The formula

Weight is W = mg, where m is mass in kilograms and g is the gravitational field strength in metres per second squared (or newtons per kilogram). Rearranged: m = W / g and g = W / m.

Worked example

A 10 kg mass on Earth (g = 9.81) weighs W = 10 times 9.81 = 98.1 N. On the Moon, with g about 1.62, the same mass weighs about 16.2 N. Solve for W with m = 10, g = 9.81 to confirm.

Frequently asked questions

What is the difference between mass and weight?

Mass is the amount of matter, in kilograms, and never changes. Weight is the gravitational force on that mass, in newtons, and depends on g.

What value of g should I use?

9.81 metres per second squared on Earth. Use about 1.62 for the Moon or 3.71 for Mars.

Why is weight measured in newtons?

Because weight is a force, and the SI unit of force is the newton.

Who this calculator is for

This calculator is for physics students, teachers and engineers.

What this calculator assumes

You enter SI units as labelled.
One variable is solved from the others.
Results are rounded for display.

Formula and sources

Related calculators

Kinetic Energy Calculator: energy of motion.
Momentum Calculator: mass times velocity.
Projectile Motion Calculator: range and height.

If you've found a bug, or would like to contact us, or learn more about James Graham and Calculate.co.nz.

Calculate.co.nz is partnered with Interest.co.nz for New Zealand's highest quality calculators and financial analysis.

Calculate.co.nz is the sister site of CalculatorHub.com, the world's largest calculator website by tool count.

All calculators and tools are provided for educational and indicative purposes only and do not constitute financial advice.

Calculate.co.nz is proudly part of the Realtor.co.nz group, New Zealand's leading property transaction literacy platform, helping Kiwis understand the home buying and selling process from start to finish. Whether you're a first home buyer navigating your first property purchase, an investor evaluating your next acquisition, or a homeowner planning to sell, Realtor.co.nz provides clear, independent, and trustworthy guidance on every step of the New Zealand property transaction journey.

Calculate.co.nz is also partnered with Health Based Building and Premium Homes to promote informed choices that lead to better long-term outcomes for Kiwi households.

Calculate.co.nz is hosted in Auckland by SiteHost New Zealand.

All content on this website, including calculators, tools, source code, and design, is protected under the Copyright Act 1994 (New Zealand). No part of this site may be reproduced, copied, distributed, stored, or used in any form without prior written permission from the owner.

About & trust: Why Calculate is NZ's most comprehensive · By the Numbers · How we compare · Editorial standards · How we keep data current · NZ finance glossary · Research & data · Financial literacy NZ · About · Privacy policy · Terms of use

Reviewed and maintained. Last reviewed 2026-08-15 and checked on a twice-monthly cycle against IRD, RBNZ and Stats NZ. How we keep data current.