BMI Calculator

About this BMI Calculator

The BMI Calculator divides weight in kilograms by height in meters squared. BMI is a screening measure, not a diagnosis, and it does not directly measure body fat, fitness or individual health. The result is therefore best treated as a general reference value.

BMI became popular because it is quick to calculate from just two measurements, but it was originally developed for population-level statistics rather than individual health assessments, which is why doctors increasingly pair it with other measures like waist circumference or body composition for a fuller picture.

For related tasks, try Calorie Calculator, Weight Converter or Length Converter.

How to use it

1. Enter your weight in kilograms.
2. Enter your height in centimeters.
3. Select Calculate to view the BMI value and broad category.

Practical example

A weight of 70 kg and height of 175 cm gives a BMI of about 22.9 because 70 ÷ 1.75² is approximately 22.9.

Two people who are both 175 cm tall but weigh 70 kg and 90 kg will show BMI values of about 22.9 and 29.4 respectively; the second falls in the overweight range, but the number alone does not indicate whether that extra weight is muscle, fat or both.

Things to double-check

BMI does not distinguish between muscle and fat mass, so athletes and very muscular individuals can show a BMI in the ‘overweight’ category despite having low body fat, while older adults can have a ‘normal’ BMI while carrying a higher proportion of fat. Pregnant women, children and adolescents also need different reference standards than the adult categories used here.

How BMI is calculated and read

Body mass index is weight in kilograms divided by height in metres squared. A person who weighs 72 kg and is 1.75 m tall has a BMI of 72 divided by 3.0625, which is about 23.5. In pounds and inches the formula is 703 times weight divided by height squared. The World Health Organization groups adult results as under 18.5 for underweight, 18.5 to 24.9 for a healthy range, 25 to 29.9 for overweight and 30 or more for obesity. These bands are screening categories for populations, not a diagnosis for an individual.

A healthy weight range for your height

You can turn the categories into a range of weights. At 1.75 m the healthy range of 18.5 to 24.9 corresponds to about 56.7 kg to 76.3 kg, found by multiplying the squared height of 3.0625 by those two BMI values. This is often more useful than a single number because it shows how much room you have. Height should be measured without shoes and weight at a similar time of day to make repeated readings comparable.

Limits of BMI

BMI does not distinguish muscle from fat, so a muscular athlete can be labelled overweight. It also does not show where fat is stored, and it can overestimate body fat in older adults who have lost muscle. Risk levels differ between populations: for example, some guidelines for South Asian adults use lower cut-offs, such as 23 for increased risk, because health risks can appear at lower BMI values. Children and teenagers are assessed on age-specific growth charts, not these adult bands.

What else to look at

Doctors usually combine BMI with other measures. Waist circumference is a simple one, and a waist-to-height ratio below 0.5 is often used as a guide to lower health risk. Blood pressure, blood sugar, cholesterol, fitness and family history matter as much as the number on the scale. If your BMI is outside the healthy range, it is a reason to ask a health professional for advice, not a reason to start a drastic diet. Pregnant women and people with certain medical conditions should not rely on a general BMI calculator.

Understanding changes over time

For a person who is 1.75 m tall, each kilogram of weight changes BMI by about 0.33, so a 6 kg change moves BMI by about two points. Because BMI is only one number, watch trends instead of a single reading, and measure under the same conditions each time. Gradual changes are generally considered safer than rapid ones, and the right target depends on your health, age and goals, so discuss it with a doctor or dietitian. For children, growth charts compare a child with others of the same age and sex instead of using these adult ranges.

Worked example: three adults

Consider three adults. The first is 1.60 m tall and weighs 52 kg, so the BMI is 52 divided by 2.56, which is 20.3. The second is 1.75 m and 82 kg, with a BMI of 82 divided by 3.0625, which is 26.8. The third is 1.82 m and 95 kg, with a BMI of 95 divided by 3.3124, or 28.7. On the standard adult bands, the first is in the healthy range, and the other two are in the overweight range. But the second might be a rower with a lot of muscle and the third a person who carries weight around the waist, which carries more health risk. The number is a starting point, not a verdict.

BMI for different groups

BMI was designed for adults in general populations, and it needs care for specific groups. For children and teenagers, a BMI is compared with growth charts for the same age and sex and is read as a percentile. For older adults, a slightly higher BMI may not carry the same risks, and unplanned weight loss is often the bigger concern. Some health bodies suggest lower cut-offs for people of South Asian and East Asian background, because they may develop health problems at lower BMI values. Athletes and people with a lot of muscle can be classified as overweight without excess fat, and pregnant women should not use BMI at all.

Weight change in realistic terms

Changing weight is slow. A rough rule is that about 7,700 kilocalories correspond to 1 kg of body fat, so a daily deficit of 500 kilocalories leads to a loss of about 0.45 kg a week, although the exact figure varies between people and falls over time as the body adapts. Fast diets often cause the loss of water and muscle, and the weight often returns. Sustainable change comes from steady eating habits, regular activity, enough sleep and support. If you want to lose or gain a large amount of weight, or have a medical condition, talk to a doctor or a dietitian before you start a plan.

Beyond BMI: other useful measures

Because BMI ignores where fat is stored, doctors often add other measures. Waist circumference is simple to measure with a tape at the level of the navel. Many guidelines treat a waist above about 94 cm for men and 80 cm for women as a sign of increased risk, with lower thresholds suggested for some Asian populations. A waist-to-height ratio below 0.5 is another easy guide. Blood pressure, blood sugar and cholesterol tests give direct information about health that no formula can. Use BMI as one clue, track changes over months, and let a health professional interpret the whole picture.

Key terms explained

Body mass index is weight divided by height squared. The adult categories are underweight, healthy weight, overweight and obesity, based on thresholds that are the same for adults of all ages and for both sexes. Waist circumference measures fat around the middle, and body fat percentage measures fat directly. For children, a percentile compares BMI with others of the same age and sex. Basal metabolic rate is the energy the body uses at rest, and lean mass is everything that is not fat.

Rules of thumb and quick checks

BMI equals kilograms divided by metres squared, or 703 times pounds divided by inches squared. For a height of 1.70 m, the healthy range of 18.5 to 24.9 is about 53.5 to 72 kg. At that height, a change of 5 kg moves BMI by about 1.7 points. Use the figure as a screening tool, together with waist measurement and advice from a health professional, and do not use it for children, pregnant women or competitive athletes without guidance.

Frequently asked questions