Calculate mass from density and volume, or from force and acceleration.
Mass can be derived two ways: from its definition as density × volume (useful for objects of known material), or from Newton's second law F = ma (useful when you know the net force and resulting acceleration). Note that mass and weight are different quantities — mass is a measure of inertia, weight is the gravitational force on that mass.
Mass from density and volume
Mass = Density × Volume
Mass from Newton's second law
Mass = Net Force ÷ Acceleration
Mass is the amount of matter in an object (kilograms) and does not change with location. Weight is the force exerted by gravity on that mass (Newtons): W = mg. On the Moon, your mass is the same but your weight is 1/6 that on Earth.
Use F/a when you measure the force applied to an object and its resulting acceleration — for example, when calibrating a force sensor or solving a physics problem. Use ρV when you know the material and can measure the object's volume.
A slug is the US customary unit of mass: the mass that accelerates at 1 ft/s² under 1 lbf of force. 1 slug = 14.594 kg. It's rarely used outside engineering dynamics.
Water's density is 1 kg/L at standard conditions, so 1 litre of water has a mass of 1 kg. Enter density=1 kg/L, volume=1 L to verify.