Why adult BMI ranges don’t work for children
Body fat in children rises in the first year, falls until about age 6 and rises again through puberty, at different times for boys and girls. A fixed cut-off like 25 would misclassify most children. That is why the child BMI calculator compares a child only with others of exactly the same age and sex, using the CDC BMI-for-age growth reference.
Teenagers and puberty
During puberty, BMI can jump as children gain weight before a growth spurt and then settle as they grow taller. Girls typically start earlier than boys. A single high reading in early puberty is therefore not unusual; doctors look at whether a teenager keeps tracking along the same percentile line over time.
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.