Body Mass Index (BMI) is the most widely used screening tool for body weight, printed on health charts and used by doctors worldwide. It is quick, free and needs only your height and weight. But BMI is often misunderstood and frequently over-interpreted. This guide explains what it really measures, how to read your result, where it falls short, and what to look at alongside it.
What is BMI?
BMI is a single number that relates your weight to your height. It was devised in the 19th century as a simple way to classify body weight across large populations, and it remains a useful first screen for whether someone may be underweight, a healthy weight, overweight or obese. Crucially, it does not measure body fat directly — it is an estimate based on the assumption that, for a given height, higher weight usually means more body fat. That assumption holds for most people but not all, which is the source of both its usefulness and its limitations.
How to calculate your BMI
BMI is your weight in kilograms divided by the square of your height in metres: BMI = weight ÷ height². For example, a person weighing 70 kg who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which sits comfortably in the healthy range. If you prefer imperial units, the formula is weight in pounds divided by height in inches squared, multiplied by 703. You do not need to do the arithmetic by hand — the BMI calculator works it out instantly and tells you which category you fall into.
BMI categories and what they mean
For most adults, the standard ranges are: below 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or above is classed as obese, which is further split into classes. These thresholds are associated with average health risks across populations, but they are guidance, not a diagnosis. A BMI just over 25 in a fit, active person means something very different from the same number in someone sedentary, so always read the category in the context of your overall health.
The limitations of BMI
BMI's biggest weakness is that it cannot tell muscle from fat. A muscular athlete may have a BMI in the overweight range while carrying very little fat, and an older person who has lost muscle may have a normal BMI while carrying too much. BMI also ignores where fat is stored — fat around the waist and organs carries more health risk than fat on the hips and thighs. Finally, the standard cut-offs were derived mainly from European populations, so they do not fit everyone equally.
How BMI differs across groups
Because of these factors, BMI is interpreted differently for different people. Children and teenagers use age- and sex-specific BMI percentiles rather than the fixed adult categories, because healthy body composition changes as they grow. Many health bodies also use lower thresholds for South Asian and other Asian populations, who tend to carry more visceral fat at a given BMI and face higher metabolic risk. Pregnant women, very tall or short people, and highly trained athletes are all groups for whom BMI alone can mislead.
Beyond BMI: a fuller picture of health
Because BMI is only a screen, it is best read alongside other measures. Your body fat percentage shows how much of your weight is actually fat, while waist circumference and the waist-to-hip ratio reveal fat distribution. To understand your energy needs, your basal metabolic rate and total daily energy expenditure show how many calories your body uses at rest and in total. Together these give a far richer view than BMI alone, and they are better progress markers because you can lose fat and gain muscle with little change in BMI.
Improving your BMI healthily
If your BMI suggests you would benefit from losing or gaining weight, aim for slow, sustainable change rather than crash diets. A modest calorie adjustment combined with regular activity protects muscle and lasts far longer than extreme measures. Track the trend over months, not single readings, and focus on habits — movement, sleep, whole foods — rather than a single number on a chart.
When to see a doctor
BMI is a starting point for a conversation, not a verdict. If your BMI falls outside the healthy range, or if you have concerns about your weight regardless of BMI, speak to a healthcare professional who can assess your full picture — body composition, waist measurement, blood markers and family history — and give personalised advice.