Body Mass Index, or BMI, is a quick screening number that relates your weight to your height. It is used everywhere — by doctors, insurers, and fitness apps — because it is simple to calculate and easy to compare across populations. But it is best understood as a rough first look, not a verdict on your health, and knowing its limits is as important as knowing the number itself.
This guide covers the formula, how to work it out in both metric and imperial units, what the standard categories mean, and — crucially — where BMI falls short and what to look at alongside it.
BMI is your weight in kilograms divided by your height in metres squared: BMI = kg ÷ m². If you use pounds and inches, the formula is weight in pounds divided by height in inches squared, multiplied by 703.
Rather than doing this by hand — and risking a slip in the arithmetic — enter your figures into the BMI Calculator, which accepts both metric and imperial units and returns your BMI along with the category it falls in. It removes the unit-conversion step that causes most manual errors.
The World Health Organization defines standard BMI ranges for adults:
These bands are population-level guidelines. They describe statistical associations with health outcomes across large groups, which is not the same as a precise assessment of any one individual.
BMI cannot distinguish muscle from fat, and this is its most significant flaw. Muscle is denser than fat, so a very fit, muscular person can register as "overweight" or even "obese" despite carrying little fat. Conversely, someone with low muscle mass may sit comfortably in the healthy range while carrying excess fat.
This is why BMI is a screening tool, not a diagnosis. A high reading in a strength athlete means something completely different from the same reading in a sedentary person, and the number alone cannot tell them apart.
Beyond the muscle-fat problem, BMI ignores several factors that matter for health:
Where BMI is genuinely useful is in tracking change over time. A single reading tells you little, but watching the number move as you adjust your habits gives a simple, consistent measure of direction. Pair it with a waist measurement and how you feel, and you have a far more complete picture.
If your scale reads in one unit system and your target is set in another, convert before you calculate so your input is accurate. The Unit Converter switches weight and height between metric and imperial quickly.
And if you are tracking progress as a proportion — for example, the percentage change in your weight over a few months — the Percentage Calculator works it out cleanly, which is often a more motivating figure than the raw BMI number on any given day.
Because BMI is such a blunt instrument, it is most useful when combined with measures that capture what it misses. Together they give a far more honest picture of health than any single number.
None of these requires special equipment beyond a tape measure, and together they compensate for BMI's biggest weaknesses. If your BMI reading concerns you, checking your waist-to-height ratio is a quick, informative next step before drawing any conclusions from BMI alone.
The standard adult BMI categories do not apply to everyone. For children and teenagers, BMI is interpreted against age- and sex-specific percentile charts rather than the fixed adult bands, because healthy body composition changes rapidly through growth. A child's BMI number is compared with others of the same age and sex, not against the adult thresholds.
Older adults are another exception. Some research suggests that a slightly higher BMI can be protective in later life, providing reserves during illness, so the standard "healthy" range may be conservative for seniors. Muscle loss with age also skews the number. In both cases, the fixed adult categories can mislead, which is another reminder that BMI is a starting point for a conversation with a healthcare professional rather than a definitive judgement on its own.
If you do decide to act on a BMI reading, the way you set goals matters as much as the target itself. Health improvements are best pursued gradually and sustainably, not through drastic measures aimed at a number.
A sensible approach focuses on habits rather than the scale: regular movement you enjoy and will keep doing, balanced meals, adequate sleep, and manageable stress. These improve health markers — energy, fitness, blood pressure — regardless of what the BMI does in the short term, and they are far more sustainable than restrictive plans built around hitting a specific figure quickly. Where weight change is a goal, a slow, steady rate is both healthier and more likely to last than rapid loss.
It is also worth remembering that the healthiest choice for some people is to focus on fitness and wellbeing rather than on moving between BMI categories at all. BMI is a screening number, not a life goal, and fixating on it can do more harm than good, particularly for anyone prone to an unhealthy relationship with food or exercise. If weight or body image is a source of real distress, the most useful step is not another calculation but a conversation with a doctor or qualified professional who can offer personalised, supportive guidance.