BMI Calculator
Free BMI calculator. Enter weight and height (imperial or metric) for instant BMI and WHO category with a visual gauge. No signup.
What BMI is
Body mass index (BMI) is a single number that compares your weight to your height. You get it by dividing weight in kilograms by the square of your height in metres. It is a screening tool, not a diagnosis: it sorts adults into broad weight categories so a clinician knows whether to look closer at waist measurement, body composition, and history. For most people, BMI is the fastest first read on whether weight is in a healthy range.
How this BMI calculator works
The tool accepts either imperial or metric input and converts behind the scenes:
- Weight: pounds × 0.45359 = kilograms (skipped when you enter kilograms directly).
- Height: inches × 0.0254 = metres; centimetres are divided by 100.
- BMI: weight in kilograms ÷ the square of height in metres.
- Category: the result is labelled with the World Health Organization adult bands and plotted on a gauge from BMI 15 to 40.
- Unit switching converts the value you have already typed, so nothing is lost when you change systems.
A worked example
Someone weighs 170 lb and stands 67 inches (5 ft 7 in) tall — the calculator’s defaults:
| Step | Calculation | Result |
|---|---|---|
| Weight in kg | 170 × 0.45359 | 77.1 kg |
| Height in m | 67 × 0.0254 | 1.70 m |
| BMI | 77.1 ÷ 1.70² | 26.6 |
| Category | 25 ≤ BMI < 30 | Overweight |
WHO adult BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese class I |
| 35.0 – 39.9 | Obese class II |
| 40.0 and above | Obese class III |
What BMI can and cannot tell you
BMI is deliberately blunt. It cannot separate muscle from fat, so a lean, muscular athlete can read as “overweight” while carrying very little fat. Equally, someone with low muscle mass can sit inside the normal range while carrying excess fat. Frame size, age, sex, and ethnicity all shift what a given BMI means. That is why BMI works as a first screen alongside waist circumference and other measures, never on its own — the tool prints that caveat on the result. It is intended for adults aged 20 and over; children are assessed on age- and sex-specific growth charts instead.
Common mistakes
- Treating one reading as a verdict. BMI is a screening category, not a diagnosis. A single number should prompt a conversation, not a conclusion.
- Using it for athletes or children. High muscle mass inflates BMI, and childhood BMI uses percentile charts, not the adult table.
- Mixing units. Entering pounds together with centimetres — or kilograms with inches — produces a meaningless result. Pick one system and stay in it.
- Ignoring the waist. Two people with the same BMI can store fat very differently, and where fat sits changes the health picture.
Frequently asked questions
Is BMI accurate for muscular people?
No — it systematically overstates body fat in people with above-average muscle. A body fat estimate describes composition more usefully, and a waist-to-hip measurement adds the distribution that BMI misses.
What counts as a healthy BMI?
The WHO defines 18.5 to 24.9 as the normal-weight band for adults. That is a population range, though, not a personal target: the healthiest range for you also depends on body composition, waist, and medical history.
Should I aim for a specific BMI number?
Treat it as a directional check rather than a score to hit. If your BMI drifts outside the healthy band and your waist is rising, that is worth acting on. If your BMI reads high but you are lean and muscular, the number is simply misleading.
Does BMI change with age?
The formula does not, but its interpretation does. Muscle mass tends to fall and fat tends to rise with age, so the same BMI can represent different body composition at 25 and at 65. Some clinicians apply a slightly higher normal range for older adults for exactly that reason.
If your BMI lands outside the healthy band, the useful next step is not another BMI reading — it is checking composition and where you carry weight. A waist-to-hip ratio takes a minute and adds information BMI cannot provide.