Weight
Calculate weight from mass and gravity, or solve for mass and gravitational acceleration. Includes formula-based steps and unit-friendly inputs.
Key formula
W = mg
How do you calculate weight?
Weight is W = m × g. Multiply mass in kilograms by gravitational field strength g (usually 9.8 m/s² near Earth). The result is in newtons. Rearrange to m = W/g when weight is given and you need mass.
About this tool
Weight is the gravitational force on a mass: W = mg. This page keeps that distinction visible so you never put newtons into a mass slot or treat kilograms as a force.
How to use this calculator
- Choose the variable you want to solve for and confirm which values are already known.
- Enter inputs in consistent units before calculating so the formula is applied correctly.
- Check whether the result is physically reasonable by comparing it with the size of the inputs.
- Use the step-by-step breakdown to see how the equation was rearranged and evaluated.
Worked examples
Example 1: 5.0 kg on Earth
g = 9.8 m/s². W = m g = (5.0)(9.8) = 49 N downward.
Example 2: find mass from weight
W = 98 N, g = 9.8. m = W/g = 98/9.8 = 10 kg.
Example 3: g = 10 estimate
m = 2.5 kg, g = 10. W = 25 N — common exam estimate when the paper allows g = 10.
Frequently asked questions
What is the weight formula?
W = m g, where m is mass in kilograms and g is gravitational field strength in m/s² (or N/kg). Weight is a force in newtons.
What is the difference between mass and weight?
Mass measures how much matter (kg) and stays the same. Weight is the gravitational force on that mass (N) and changes if g changes.
Should I use g = 9.8 or g = 10?
Use the value given on the exam or worksheet. Many courses accept 9.8 or 10; some want 9.81. Do not mix two g values in one answer.
Is weight the same as normal force?
Not always. On a flat floor with no other vertical forces, N = mg = weight magnitude. On a ramp, N = mg cosθ, which is smaller than weight.
How does weight relate to buoyancy?
Compare object weight W = mg to buoyant force Fb = ρVg. If W is larger than maximum Fb, the object sinks; if smaller, it floats.