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 for athletes and gym-goers
Strength athletes, sprinters and many team-sport players routinely score 25–30 on BMI because of muscle. In that case, body-fat percentage (for example from a DEXA scan or skinfold test) and waist-to-height ratio describe health better. Endurance athletes are the opposite case: a BMI under 18.5 combined with fatigue or frequent injuries can point to under-fuelling.
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.