What BMI means — and what it does not

BMI = weight (kg) ÷ height (m)². A person who is 70 kg and 1.75 m has BMI = 70 ÷ (1.75 × 1.75) ≈ 22.9. Imperial inputs are converted to metric first so the same formula applies.

For most adults, under 18.5 is classed as underweight, 18.5–24.9 as a healthy weight, 25–29.9 as overweight, and 30 or above as obese, with further subclasses above 30. Children’s charts are different — this tool is aimed at adults.

Some UK guidance uses lower BMI thresholds for increased risk in people from certain ethnic backgrounds. The on-screen WHO bands are a starting map, not a personalised clinical cut-off.

BMI does not measure body fat. Muscle is denser than fat, so many athletes show a high BMI without excess fat. Older adults can lose muscle and still sit in a “healthy” BMI band while health risks rise.

Where you carry fat (waist vs hips) and fitness, blood pressure, glucose, and family history often matter more than a single index. Waist measurement and the Body Fat Calculator (tape-measure estimate) add context; they are still estimates.

Pregnancy, oedema, amputation, and some medical conditions also make BMI misleading. Do not use a website number in those situations without clinical advice.

Pick UK (kg/cm) or US (lb/ft-in) so the fields match how you measure. Enter height and weight as you would on a clinic scale — morning weight after the bathroom is more stable than after a large meal.

If you are tracking change, use the same scale, similar clothing, and similar time of day. BMI will move when weight moves; it will not tell you whether that weight was fat, muscle, or water.

For calorie planning, BMI is only a rough size signal. The Calorie Calculator estimates energy needs from age, sex, height, weight, and activity — still an estimate, still not a meal plan.

Sudden weight change, eating-disorder concerns, or a BMI well outside the healthy range are reasons to talk to a clinician, not to chase a website category.

Utiio does not store your measurements to produce the result. Clear the page if you are on a shared device.

This article is educational. It is not medical advice. Confirm health decisions with a qualified professional.