Body Mass Index and Healthy Weight for Your Height

Because BMI doesn’t know where fat sits, and the risk comes from the fat inside the belly, around the organs. Two people with the same BMI can differ in that risk several times over. Since 2022, the UK NICE guideline has called for measuring waist-to-height alongside BMI in adults with a BMI below 35. It takes one tape measurement, and the rule is easy to remember: keep your waist under half your height.

BMI comes from height and weight and can’t tell muscle from fat, and muscle is denser. For someone who lifts regularly, BMI predictably overstates things. That is a known limit of the formula, not something about you. In that case, look at waist-to-height: it measures the belly, not total weight.

Adult cutoffs don’t fit children: a child’s BMI changes with age on its own. The calculator compares it with WHO tables for the same sex and age to the month — the child growth standards under five, and the 2007 growth reference from five to nineteen. Under five, overweight starts above +2 standard deviations; after five, above +1. For children under two, BMI isn’t used: weight is judged against body length instead.

It depends on your BMI before pregnancy. The US Institute of Medicine (2009) recommends 28–40 lb if underweight, 25–35 lb at a healthy weight, 15–25 lb if overweight and 11–20 lb with obesity. For twins the ranges are higher and provisional. The calculator shows your range; the week-by-week pace is something to discuss with your doctor.

Because at the same BMI, people in the Asia-Pacific region carry more body fat on average, and high blood sugar, insulin resistance and high blood pressure show up at lower values. The WHO recognized this in 2004 and named extra points at 23 and 27.5. Korea, Japan, Singapore and India use them. The switch stays in view: a country doesn’t determine a body.

After 65, the link between BMI and health shifts. In a meta-analysis of nearly 200,000 older adults (Winter et al., 2014), risk was not raised at a BMI of roughly 24 to 31 and started rising below 23. Low weight at this age is riskier than moderately high weight, and losing muscle becomes a concern of its own. BMI still works as a number, but reading it strictly by adult cutoffs is a mistake.

Standing, after a relaxed breath out, without pulling your stomach in. Run the tape around the narrowest point — usually halfway between the lowest rib and the top of the hip bone, just above the belly button. Keep it level with the floor, snug but not tight. Measure on bare skin or thin clothing, ideally in the morning: waist size changes noticeably by evening.