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.
Decimals and rounding
Enter height and weight with decimals if you have them — 172.5 cm and 68.4 kg are accepted. The calculator works with the exact numbers and rounds the BMI to one decimal place at the end, then places that rounded value in its category, so the result you see always matches the category shown.
How to measure for an accurate BMI
- Height: stand barefoot with heels, back and head against a wall; rest a flat object on your head and mark the wall.
- Weight: weigh first thing in the morning, after using the toilet, in light clothing, on a hard floor.
- Consistency: use the same scale at the same time of day when you recheck — daily weight can move by 1–2 kg with food and fluid.