Body Mass Index (BMI) Calculator

BMI is calculated using the Quetelet formula: weight in kilograms divided by height in meters squared. It's a quick, approximate measure of the relationship between weight and height, recommended by the World Health Organization (WHO).

BMI = Weight (kg) ÷ Height² (m)

WHO classification: below 18.5 — underweight, 18.5–24.9 — normal, 25.0–29.9 — overweight, 30.0+ — obesity. The figure is approximate and doesn't account for muscle mass, age, or body frame, so it can be inaccurate for athletes and older adults.

This calculation is for informational purposes only and is not medical advice or a diagnosis. For questions about weight and health, consult a doctor.

The calculator works the index out to one decimal place and immediately attaches a colour-coded category label. The boundaries are strict: a value of 24.9 and a value of 25.0 fall into different categories.

Underweight: below 18.5 · Normal: 18.5 – 24.9 · Overweight: 25.0 – 29.9 · Obesity: 30.0 and above

The normal band is wide: 18.5 and 24.9 differ by almost a third. At a height of 175 cm that is a weight of roughly 57 to 76 kg, and all of it counts as normal. The index therefore separates "conditionally healthy weight" from "conditionally overweight", but it says nothing about how you feel or about health risks.

Near the boundary a few hundred grams decide everything. At 175 cm, 76.3 kg gives an index of 24.9 while 76.6 kg already gives 25.0. The exact cut-off sits at 76.6 kg, so the gap between the two categories is under 300 grams.

The index does not tell fat mass from muscle mass, so on its own it is not a diagnosis. The WHO applies these thresholds to adults, and for adults they remain the generally accepted reference point.

The formula needs height in metres, so centimetres are divided by 100 while the weight stays in kilograms. Check the arithmetic by hand on the examples below.

Example 1 — 70 kg at 175 cm. Square of the height: 1.75 × 1.75 = 3.0625. BMI = 70 ÷ 3.0625 = 22.9, a normal weight. This is exactly the value the calculator shows by default.

Example 2 — 60 kg at 175 cm. 60 ÷ 3.0625 = 19.6. Still normal, but noticeably closer to the lower edge of the band.

Example 3 — 85 kg at 175 cm. 85 ÷ 3.0625 = 27.8, which is overweight. The threshold for obesity is 2.2 points away.

Example 4 — 50 kg at 175 cm. 50 ÷ 3.0625 = 16.3, which is underweight.

Example 5 — 65 kg at 180 cm. 1.8 × 1.8 = 3.24. 65 ÷ 3.24 = 20.1, normal. The same weight at 170 cm would give 65 ÷ 2.89 = 22.5: also normal, but already near the upper edge of the band.

Note the reverse dependence on height: the index contains height squared, so centimetres matter more than they look. At 70 kg, moving from 175 to 180 cm changes the index from 22.9 to 21.6 — the same body, different numbers.

Body Mass Index measures only the mass of the body, not what that mass is made of. Muscle is heavier than fat tissue, so the index comes out too high for an athlete with well-developed muscles. Example: 85 kg at 180 cm gives 85 ÷ 3.24 = 26.2 — formally overweight, even though it may be a professional athlete carrying very little fat.

The opposite case is older adults. Muscle mass shrinks with age while the share of fat can grow at the same weight. A normal BMI in a 70-year-old does not mean there is no excess fat.

For children BMI is used differently: you look up a percentile for age and sex. At 8 years a value of 18.5 means underweight, at 16 years it is the lower edge of normal, because the body is still growing. The calculator uses the adult WHO thresholds, so its colour label should not be applied to a child — you need an age-based percentile table, and the child's age comes from the age calculator.

Two more groups the index does not suit are pregnant and breastfeeding women, whose weight changes by plan and says nothing about body composition. In people with oedema or after surgery the figure is distorted for a time.

The calculator form takes whole numbers: height from 120 to 220 cm and weight from 30 to 180 kg, in steps of 1 cm and 1 kg. For fractional values you have to work the formula out by hand. The unit converter converts kilograms to pounds and back.

If you need a more precise reference than BMI, look at waist circumference: its thresholds are calculated separately and depend on muscle mass far less. BMI stays a baseline, not a diagnosis — the calculator says so itself.

The index is known and you need the weight. The formula reverses: at a given height the weight equals the index multiplied by the square of the height in metres.

Weight (kg) = BMI × (Height in m)²

Take a height of 175 cm: the square is 3.0625. At the lower edge of normal, 18.5, that gives 18.5 × 3.0625 = 56.7 kg; at the upper edge 24.9 it gives 24.9 × 3.0625 = 76.3 kg. Intermediate reference points: an index of 21.0 gives 64.3 kg, an index of 22.5 gives 68.9 kg.

Approximate limits of the normal band for common heights: 160 cm — 47.4 to 63.7 kg; 170 cm — 53.5 to 72.0 kg; 175 cm — 56.7 to 76.3 kg; 180 cm — 59.9 to 80.7 kg; 190 cm — 66.8 to 89.9 kg.

The calculator does not output those weights itself — it shows only the index and the category label. The reverse calculation has to be done by hand, or you can use the calculator as a check: feed the weight back in and see whether the index matches.

The opposite problem works the same way: given a weight and a target index, the height follows as the square root of the ratio. If 70 kg should correspond to an index of 22.5, the square of the height is 70 ÷ 22.5 = 3.111 and the height is √3.111 ≈ 1.764 m, that is 176 cm. Check: 70 ÷ 1.764² = 22.5.

Bear in mind that the "healthy weight" from this formula is the weight at a normal index in the statistical sense, not a doctor's recommendation. Body build, the ratio of muscle to fat and age are not in the formula. The date difference calculator helps you keep track of timelines and plans.