📏 BMI Calculator

The Overweight Line Moved in 1998

The WHO cut-off of 25 can seem fixed, but the United States used a different line for more than a decade. After the 1985 NIH consensus conference on the health implications of obesity, American adults counted as overweight from a BMI of 27.8 for men and 27.3 for women, the 85th percentiles for 20- to 29-year-olds in NHANES II (1976–1980). On that definition, 33.4% of US adults were overweight in 1988–1991. In 1998, NIH clinical guidelines replaced that pair with a single threshold of 25 for both sexes, the line WHO uses internationally. Applied to NHANES III data (1988–1994), it took in 59.4% of men and 50.7% of women, or 54.9% of everyone aged 20 and over: an estimated 97 million adults. Tens of millions of Americans went from "healthy" to "overweight" without any change in their bodies. A result between 25 and about 28 falls in the band reclassified in 1998.

Your Result Is Less Precise Than the Decimal Suggests

Height is squared in the formula, so an error in height counts about twice as much as the same proportional error in weight. Take 75 kg: at 175 cm the BMI works out to 24.5, and at 173 cm to 25.1. Two centimetres crosses a category boundary, and that is well within normal daily variation. Spinal discs lose fluid under load. In a 1985 study in the journal Spine, young adults were on average 19 mm shorter by the end of the day, and 54% of that loss came in the first hour after rising. The same body can give both readings in one day. Weight also swings with meals, drinks and body fluid, and each kilogram is worth roughly 0.3 to 0.4 BMI points at adult heights. For a figure you can compare month to month, measure in the morning, without shoes. If your result is within half a point of 18.5, 25 or 30, treat it as sitting on the line, not clearly on one side.

The Measurement Worth Adding: Your Waist

BMI cannot show where fat is carried, so current guidance pairs it with a waist measurement. NICE, which writes clinical guidance for the NHS in England, advises working out waist-to-height ratio alongside BMI for adults with a BMI below 35 (guideline NG246, 2025). The same bands apply to both sexes, all ethnic groups and adults with high muscle mass, the group NICE says BMI reads least reliably. A ratio of 0.4 to 0.49 is healthy. From 0.5 to 0.59, central fat and health risk are increased, and at 0.6 or more they are high. In plain terms, as the NHS puts it: keep your waist to less than half your height. Under the WHO protocol, the waist is measured midway between the lowest palpable rib and the top of the hip bone, which is often higher than where a waistband sits. Take the reading at the end of a normal breath out, with the tape snug but not pulled tight. Divide by height in the same units: a 96.5 cm waist at 170 cm gives 0.57, in the increased band.

Under 20, BMI Is Read by Age and Sex

The adult cut-offs of 18.5, 25 and 30 are meant for people aged 20 and over. For children and teens, BMI shifts with growth: it falls through early childhood, bottoms out between ages five and six on CDC's charts, then rises through adolescence. A BMI of 19 sits just under the 85th-percentile overweight line at age 10 (19.4 for boys, 20.0 for girls) but between the 10th and 25th percentiles at 17. For ages 2 through 19, CDC reads BMI as a percentile for age and sex: below the 5th percentile is underweight, the 5th to below the 85th is healthy weight, the 85th to below the 95th is overweight, and the 95th and above is obesity. The 2000 charts draw no percentile line above the 95th and stop at a BMI of 37, so in 2022 CDC added extended charts reaching a BMI of 60, with curves at the 98th, 99th, 99.9th and 99.99th percentiles. Under 20, use the same formula, then plot the result on one of these charts.