BMI Calculator

What BMI is

Body mass index is your weight divided by the square of your height. It was devised in the 1830s by the Belgian mathematician Adolphe Quetelet as a way to describe body size across a population for statistical purposes — not as a personal health diagnosis, though it's widely used as one today. The WHO later adopted it as a simple, inexpensive population-level screening tool, which is the role it's best suited to.

Reading the categories

The bands used here are the standard WHO thresholds: under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obese. The healthy weight range shown alongside your number is simply the weight span that would put you inside the 18.5–24.9 band at your current height — useful context, not a target you're obliged to aim for.

What BMI can't see

BMI is calculated from height and weight alone, so it cannot distinguish muscle from fat, or where on the body weight is carried. A very muscular person can register as overweight despite low body fat; an older adult can register as "normal" while carrying a higher fat percentage than the number suggests, since muscle mass tends to decline with age even if weight doesn't. Frame size, ethnicity-related differences in health risk at a given BMI, and pregnancy are other well-documented factors it doesn't account for.

Waist-to-height ratio, as a companion measurement

Because BMI can't see where weight sits on the body, some clinicians and researchers use waist-to-height ratio alongside it — your waist circumference divided by your height, with a commonly cited threshold of 0.5 as a rough boundary for increased health risk. It's a useful companion precisely because it captures something BMI structurally cannot: two people with the same BMI can carry weight very differently, and abdominal fat specifically is more consistently linked to metabolic risk than fat carried elsewhere. Neither number replaces the other — used together, they tell a fuller story than either alone.

Who this suits

Anyone who wants a fast, standard reference number — for a check-up, for tracking a long-term trend, or simply out of curiosity. It's a reasonable first screening step, especially when tracked over time rather than read as a single verdict. It also suits anyone who's seen their BMI quoted at a doctor's visit and wants to understand what the category actually means and where its limits are, rather than take the single word "overweight" or "obese" at face value without context.

Have a website? Embed this calculator

This BMI calculator is available as a free, lightweight widget you can add to your own site with one line of code — no account, no ads, no tracking. See the embed page for the copy-paste code and a live preview.

Limitations

Precisely because of what it can't see, BMI works best alongside other information rather than alone: waist-to-height ratio, a body fat estimate, or simply how your weight and how you feel have changed over recent months. If your BMI falls outside the normal range, that's a reasonable prompt to talk to a doctor — not a number to act on by itself, and never the whole picture on its own, whichever direction it happens to point in your particular case.

Waist-to-Hip Ratio vs. BMI: Which to Trust? — where BMI's blind spot is real, and which alternative measurements fill it.

Frequently asked questions

What BMI counts as a healthy weight?

The WHO categories used here are: under 18.5 is classed underweight, 18.5 to 24.9 is classed normal weight, 25 to 29.9 is overweight, and 30 or above is obese. These are population-level screening bands, not individual diagnoses — your doctor may weigh other factors before drawing conclusions from your number alone.

Is BMI accurate for athletes or very muscular people?

Often not. BMI can't distinguish muscle from fat — it only uses height and weight — so a muscular person can score as 'overweight' or even 'obese' despite low body fat. This is one of BMI's most cited limitations; if it applies to you, a body fat percentage or waist measurement generally tells you more.

Does a healthy BMI range differ by ethnicity?

Research suggests health risk at a given BMI can vary somewhat between population groups, and some health bodies use adjusted thresholds for certain ethnicities as a result. This calculator uses the standard WHO bands rather than an ethnicity-adjusted scale — worth keeping in mind when interpreting your category.

Why is BMI still used if it has these limitations?

Because it's cheap, fast, and needs no equipment beyond a scale and a tape measure, which makes it useful for screening large populations even though it says little about an individual's body composition. It was never intended as a precise personal health diagnosis.

What should I use instead of, or alongside, BMI?

Waist-to-height ratio, body fat percentage (the Navy circumference method is one accessible estimate), and simply tracking how your body composition changes over time all add context BMI can't provide on its own. The full calorie calculator on this site includes a healthy weight range and an ideal-weight range alongside BMI for exactly that reason.