The pregnancy weight gain calculator in most cases is based on the calculation of the BMI value. The calculation is very simple. It is enough to divide your weight in kilograms (e.g. 56 kg) before pregnancy by twice your body height in meters (e.g. 160 cm - 1.6 m). The calculation will be as follows: 56 : (1.60x1.60)= 21.9 BMI