Inertial mass of a body is a measure of the ability of a body to oppose the production of acceleration in it by an external force. According to Newton’s second law of motion (F = ma), the mass of a body can be determined by measuring the acceleration produced in it by a constant force, m = F/a.
If a constant force acts on two masses mA and mB and produces accelerations aA and aB respectively, then, F = mAaA = mBaB
So, mA/ mB = aA/aB
The ratio of two masses is independent of the constant force. If the same force is applied on two different bodies, the inertial mass of the body is more in which the acceleration produced is less.
If one of the two masses is a standard kilogram, the unknown mass can be determined by comparing their accelerations.