Free tool
BMI calculator
Work out your body mass index from your height and weight, plus the healthy weight range for your height and an estimate of body fat — free, no sign-up, and nothing stored.
From 2 to 120 years
From 80 to 250 cm
From 20 to 400 kg
Everything is calculated on your device. Your height, weight and age are never stored or sent anywhere.
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We need sex, age, height and weight.
How is body mass index calculated?
The formula is simple: weight in kilograms ÷ (height in metres × height in metres). If you weigh 80 kg and stand 175 cm, convert the height to metres (1.75), square it (3.0625), then divide: 80 ÷ 3.0625 = 26.1, which falls in the 'above range' band. The calculator above does all of that as you type, and adds the healthy weight range for your height plus a body fat estimate that uses your age and sex.
Step by step
- 1Convert your height from centimetres to metres (divide by 100): 175 cm → 1.75 m
- 2Square it: 1.75 × 1.75 = 3.0625
- 3Divide your weight by the result: 80 ÷ 3.0625 = 26.1
- 4Compare the number with the category table below
Body mass index categories for adults
| Index value | Category |
|---|---|
| < 18.5 | Below the reference range |
| 18.5 – 25 | Within the reference range |
| 25 – 30 | Above the reference range |
| 30 – 35 | Obesity — class I |
| 35 – 40 | Obesity — class II |
| 40+ | Obesity — class III |
These are the World Health Organization categories, and they apply to adults from 18 years.
Ideal weight for your height — how is it worked out?
There is no single 'ideal weight' — there is a range. Because the reference band runs from 18.5 to 25, the healthy weight for any height is those two numbers multiplied by height in metres squared. For 170 cm: 18.5 × 2.89 = 53.5 kg at the lower end and 25 × 2.89 = 72.3 kg at the upper. Any weight between them sits in the reference range, and the calculator works that out for your own height and tells you how far above or below you are.
The range is wide on purpose: people differ naturally in frame and muscle mass, so two people of the same height can both be in good health with fifteen kilos between them.
BMI or body fat percentage — what is the difference?
The index looks at total weight without knowing whether it is muscle, fat or water. Body fat percentage measures only the fat, and it sits closer to actual health. That is why the calculator shows a body fat estimate next to the index — derived from the index, your age and your sex. It is an estimate with a margin of roughly ±4%; precise measurement needs equipment (InBody or DEXA) or skinfold calipers in trained hands.
When is the number worth acting on?
The index is a starting signal, not a verdict. Situations where reviewing things with a specialist is advisable:
- An index of 25 or above together with a large waist (over 94 cm for men or 80 cm for women)
- An index under 18.5 without an obvious reason, or unintended weight loss
- A family history of diabetes, hypertension or heart disease
- Blood work suggesting insulin resistance or fatty liver
- Polycystic ovaries or a thyroid disorder
- Weight that will not move despite genuinely eating less
What does BMI actually measure?
Body mass index is your weight divided by the square of your height in metres. The World Health Organization uses it as a quick screening tool at population level — it flags that weight may be worth reviewing, it does not diagnose anything. It does not measure body fat directly, nor where that fat sits.
Limits of the index — know them before acting
- It cannot separate muscle from fat: a muscular athlete can land in the 'above range' band while carrying very little fat.
- Older adults lose muscle mass, so the index can underestimate actual body fat.
- It does not apply during pregnancy or breastfeeding — weight in those periods has an entirely different reference.
- It says nothing about where fat sits, and fat around the abdomen carries a different meaning from fat on the limbs.
Common questions
It is accurate as a screening indicator, not as a measure of body fat. It gives a quick picture of whether your weight is worth reviewing, but real assessment needs other measurements, a health history and blood work your physician requests.
Because the formula takes total weight without knowing whether it is muscle or fat. Muscle is denser than fat, so many athletes land in the 'above range' band while in excellent shape. Body fat percentage and waist circumference are more useful there.
Not with these categories. Children and adolescents are assessed against growth percentiles by age and sex, and applying adult bands to them is misleading. This calculator says so explicitly when the age is under 18.
It is the weight range that produces an index between 18.5 and 25 for your height. It is a range, not a single number — people differ naturally in frame and muscle mass, and the aim is to sit inside the range rather than at one figure.
Once every couple of weeks is enough if you are tracking a change. Daily weighing misleads more than it helps: body weight swings one to two kilos within a single day from water, food, salt and the menstrual cycle — so the difference between yesterday and today is not fat. Measure at the same time of day (morning, after the bathroom, before eating) on the same scale.
They complement each other. The index tells you the size of your weight relative to height; waist circumference tells you where the fat sits — and abdominal fat carries higher risk than fat on the hips and limbs. Measure with a tape midway between the lowest rib and the hip bone, after a normal breath out. Values worth reviewing: over 94 cm for men and over 80 cm for women.
The formula itself is standard with no interpretation involved — the same one the World Health Organization uses. What differs is the reading: the same number means different things for an athlete, a pregnant woman and an older adult. That is why the page sets out the limits as clearly as the result, and stops entirely when the age is under 18.