The BMI index formula
Body mass index is weight divided by height squared. In metric units that is BMI = kg ÷ m²; in US units it is BMI = 703 × lb ÷ in². A person who is 1.68 m tall and weighs 64 kg has a BMI of 64 ÷ (1.68 × 1.68) = 64 ÷ 2.8224 = 22.7. The same person measured as 5 ft 6 in and 141 lb gets 703 × 141 ÷ 66² = 22.8 — the tiny difference is only rounding in the unit conversion. Because height is squared, a small error in height changes the result more than the same error in weight, so measure height carefully.
BMI before and during pregnancy
BMI is not used to judge weight while pregnant, because weight rises naturally with the baby, placenta and fluid. Instead, your pre-pregnancy BMI sets the recommended weight gain: 12.5–18 kg if underweight, 11.5–16 kg in the healthy range, 7–11.5 kg if overweight and 5–9 kg with obesity. The Pregnancy Weight Gain Calculator works this out week by week.
BMI Prime and ponderal index
Alongside BMI, the calculator shows BMI Prime — your BMI divided by 25, the top of the healthy range. A BMI Prime under 0.74 is underweight, 0.74–0.99 is healthy, and 1.00 or more is above the healthy range; 1.20 means 20% above it. It also shows the ponderal index (kg ÷ m³), which divides by height cubed and is less distorted for very tall or very short people.