BMI Explained: What It Measures and What It Doesn't
Health & Fitness - 3 min read - Published 2026-08-27 - Updated 2026-10-19
Body mass index (BMI) is a simple number calculated from height and weight. Doctors and public health agencies use it as a quick screening measure for whether someone's weight might be affecting their health. It's useful because it's free, fast and consistent - and limited, because it can't tell muscle from fat or where fat is stored. Knowing both sides helps you read your own number sensibly.
How BMI is calculated
In metric units, BMI is weight in kilograms divided by height in meters squared. For someone who weighs 70 kg and is 1.75 m tall: 1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 = 22.9.
In imperial units, multiply weight in pounds by 703 and divide by height in inches squared. For 180 lb at 5 feet 10 inches (70 inches): 180 × 703 = 126,540, and 70 × 70 = 4,900, so BMI = 126,540 ÷ 4,900 = 25.8. The 703 converts the units so both formulas give the same result.
The standard adult ranges
The World Health Organization's categories for adults are: below 18.5, underweight; 18.5 to 24.9, healthy weight; 25 to 29.9, overweight; and 30 or above, obesity, which is often split further into classes starting at 30, 35 and 40. On this scale, the first example (22.9) falls in the healthy range and the second (25.8) is just into overweight.
Some health organizations use lower thresholds for people of Asian descent, because health risks such as type 2 diabetes tend to rise at lower BMI values in those populations. For example, guidance has suggested treating 23 as a point for increased risk rather than 25. Local health services can say which thresholds apply where you live.
What BMI misses
Muscle: muscle is denser than fat, so muscular people often register as overweight or even obese while having little body fat. Many athletes and people who do heavy strength training fall into this group.
Fat distribution: fat around the abdomen (visceral fat) is linked to higher risks of heart disease and diabetes than fat on the hips and thighs, but two people with the same BMI can carry fat very differently.
Age and sex: older adults tend to have less muscle and more fat at the same BMI, and women on average have more body fat than men at the same BMI. The single scale doesn't adjust for either.
Children and teenagers: BMI is interpreted differently for anyone under 20, using percentile charts by age and sex, because healthy body composition changes as children grow. The adult categories don't apply.
Measures to look at alongside BMI
Waist circumference is the simplest addition, because it reflects abdominal fat. Measure around your middle, just above the hip bones, after breathing out. Many guidelines treat a waist above about 102 cm (40 in) for men or 88 cm (35 in) for women as a sign of increased risk, with lower thresholds for some ethnic groups.
Waist-to-height ratio (waist divided by height, in the same units) is another quick check; keeping your waist to less than half your height is a commonly cited guide. Body fat percentage, blood pressure, blood sugar and cholesterol give a much fuller picture, and those are the measures a doctor will look at.
Using your number sensibly
Treat BMI as a starting point for a conversation, not a diagnosis. A healthy-range BMI doesn't guarantee good health, and an overweight-range BMI in a fit, muscular person may be nothing to worry about. The trend over time, together with how you feel, your activity and your other health measures, says more than a single reading.
This article is general information, not medical advice. If you're concerned about your weight or health, talk to a doctor or other qualified health professional.