You calculate mass by relating it to density, volume, force, or weight , depending on what you know.

What is mass?

Mass is how much matter an object contains, not how heavy it feels in gravity.

Weight changes if you go to the Moon or another planet, but mass stays the same.

Main formulas to calculate mass

1. From density and volume

If you know how dense something is and how much space it takes up:

m=ρ×Vm=\rho \times Vm=ρ×V

  • mmm = mass
  • ρ\rho ρ = density
  • VVV = volume

Example:
An object has density 5 g/cm35,\text{g/cm}^35g/cm3 and volume 8 cm38,\text{cm}^38cm3.

m=5×8=40 gm=5\times 8=40,\text{g}m=5×8=40g

This method is common in school science problems: you measure volume (e.g., with a measuring cylinder) and look up or measure density.

2. From force and acceleration (Newton’s second law)

From physics, Newton’s second law says:

F=maF=maF=ma

So if you know the net force and the acceleration, you can rearrange:

m=Fam=\frac{F}{a}m=aF​

  • FFF = net force
  • aaa = acceleration

Example: If a net force of 10 N10,\text{N}10N causes an acceleration of 2 m/s22,\text{m/s}^22m/s2, then:

m=102=5 kgm=\frac{10}{2}=5,\text{kg}m=210​=5kg

This is essentially how inertial mass is defined: how much an object resists acceleration.

3. From weight (on Earth or another planet)

Weight is the force of gravity on an object:

W=mgW=mgW=mg

So:

m=Wgm=\frac{W}{g}m=gW​

  • WWW = weight (force due to gravity, in newtons)
  • ggg = gravitational field strength (on Earth, about 9.8 m/s29.8,\text{m/s}^29.8m/s2)

Example: If something weighs 98 N98,\text{N}98N on Earth:

m=989.8=10 kgm=\frac{98}{9.8}=10,\text{kg}m=9.898​=10kg

This is how gravitational mass is found from weight measurements.

4. Directly with a balance scale

In everyday life, you often measure mass rather than calculate it:

  • Place an object on a digital or spring scale set to kilograms or grams.
  • The device internally uses the local value of ggg to convert weight to mass.

In a lab, a balance compares an unknown mass to a known reference mass.

Quick table of situations

[1][3][5] [9] [3][10] [7]
What you know Formula / method Example use
Density and volume $$m = \rho V$$ Finding mass of a metal block from density tables.
Force and acceleration $$m = F / a$$ Physics experiment pushing a cart and measuring acceleration.
Weight (force) and gravity $$m = W / g$$ Converting a weight reading in newtons to kilograms.
Nothing but a scale Read mass directly Kitchen or bathroom scale giving grams or kilograms.

Mini story to fix the idea

Imagine you drop a metal cube into a measuring cylinder and the water level rises from 50 ml to 90 ml, so the cube’s volume is 40 ml (which is 40 cm³).
You look up the density of that metal as 7.8 g/cm37.8,\text{g/cm}^37.8g/cm3.

Using m=ρVm=\rho Vm=ρV, the mass is 7.8×40=312 g7.8\times 40=312,\text{g}7.8×40=312g.
Later you put the same cube on a scale at home and it reads about 0.312 kg, matching your calculation.

Tiny FAQ

  • Is mass the same as weight?
    No. Weight is the gravitational force, mass is the amount of matter.
  • Which unit do we usually use?
    The SI unit is kilograms, but grams and tonnes are also common.

TL;DR:
Most often, you calculate mass with m=ρVm=\rho Vm=ρV (from density and volume), m=F/am=F/am=F/a (from force and acceleration), or m=W/gm=W/gm=W/g (from weight and gravity).