BMI Calculator

What is BMI?#

BMI (Body Mass Index) is a single number that relates your weight to your height: weight in kilograms ÷ (height in metres)². It is the simplest widely used screen for whether body mass is in a healthy band for a given stature — quick to compute, needing nothing but a scale and a tape measure, which is exactly why clinics and fitness apps lean on it.

The number is then sorted into four WHO adult bands: below 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, and 30 or above is obese. BMI is a screen, not a diagnosis: it cannot tell muscle from fat, so a heavily muscled athlete can land “overweight” while carrying little fat. It is most useful as a trend over time and as a starting check for the average adult.

How to use it#

1. Pick your unit system at the top: metric (kilograms and centimetres) or imperial (pounds, feet and inches).
2. Enter your weight. In metric mode that is kilograms; in imperial mode it is pounds.
3. Enter your height — a single centimetres field in metric, or separate feet and inches fields in imperial (e.g. 5 ft 10 in).
4. Click Calculate.
5. The result card shows the BMI to one decimal, a coloured category badge (underweight / normal / overweight / obese), and a healthy weight range line giving the kilograms (or pounds) that correspond to BMI 18.5–24.9 at your height.

Key features#

• Both unit systems, converted through exact SI factors (1 lb = 0.45359237 kg, 1 in = 0.0254 m), so metric and imperial return the same BMI for the same body.
• Category computed on the raw value, not the rounded display, so a true 24.96 is correctly read as “normal” rather than being bumped up by rounding.
• One-decimal display — the precision a home scale can actually justify.
• Healthy weight range for your exact height, so the number lands in context rather than as a bare figure.
• 100% local. Weight and height stay in the browser.

Worked example#

A 70 kg adult who is 175 cm tall. With metric selected, enter 70 for weight and 175 for height, then Calculate:

• BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.9
• Category badge: Normal (it sits between 18.5 and 24.9)
• Healthy weight range: roughly 56.7 kg – 76.3 kg for someone 1.75 m tall (18.5 × 3.0625 and 24.9 × 3.0625)

Now switch the dropdown to imperial and enter the same body — 155 lb, 5 ft 9 in. The tool converts 155 lb → 70.31 kg and 69 in → 1.753 m, computes 70.31 ÷ (1.753)², and lands at 22.9 again, normal. BMI is a unit-independent ratio, so the system you measure in must not change the answer — and here it does not.

FAQ#

BMI says I’m overweight but I’m muscular. Is it wrong?#

BMI cannot tell fat mass from lean mass — both count as “weight”. A rugby player or a serious lifter with low body fat can easily come out “overweight” because muscle is dense. For that population a body-fat measurement (calipers, bioimpedance, DEXA) is a better check than BMI. For the general sedentary adult the band is still meaningful.

Is BMI valid for children, the elderly, or pregnant women?#

The standard adult cut-offs (18.5 / 25 / 30) are for ages 20–65. Children use age- and sex-specific percentile charts, not fixed thresholds. In older adults a slightly higher BMI (toward 25–27) is sometimes associated with better outcomes. Pregnancy is outside BMI’s scope entirely. Treat the number as an adult screen only.

Why do metric and imperial give the same BMI?#

Because BMI is defined in SI units (kg/m²). The imperial path converts pounds and inches into kilograms and metres through exact constants, computes the SI ratio, and reports it — so the two paths are mathematically the same formula, not two different approximations. If you ever see them disagree, a tool is rounding the conversion.

What is the healthy weight range based on?#

It is simply your height squared multiplied by the WHO normal band (18.5 and 24.9). The lower number is the lightest weight at which you stay “normal” at your height, and the upper number is the heaviest. It is a sanity range, not a target — body composition matters more than hitting any single kilogram.