BMI calculator: work out your body mass index
Calculate your BMI for free by simply entering your weight and height.
BMI calculator
Enter your weight and height and your result appears straight away.
For adults aged 18 and over, excluding pregnancy. This result is for guidance only and does not replace medical advice. Your figures stay on your device: nothing is saved or sent.
Calculating your BMI is the simplest way to find out whether your weight is right for your height. Our calculator above gives you your body mass index in one click, and this article explains how to interpret it, what an ideal BMI is, and why this useful number doesn’t tell the whole story about your health.
What is BMI (body mass index)?
Body mass index, or BMI, is a measure that relates your weight to your height. It is the indicator of body size used by the World Health Organization (WHO), by GPs and by studies on excess weight all over the world.
It is older than you might think. The formula was proposed in the 19th century by the Belgian mathematician and statistician Adolphe Quetelet, who was trying to describe the “average man”. For a long time it was known as the “Quetelet index”. The name “body mass index” only appeared in 1972, coined by the American physiologist Ancel Keys, who considered it more reliable than the other formulas of the time for estimating the body size of a population.
Its great advantage: all you need is a pair of scales and a tape measure. Its great weakness, which we will come back to: it cannot tell the difference between fat and muscle.
The formula to calculate your BMI
Calculating BMI takes just one line. You divide your weight in kilograms by your height in metres, squared:
BMI = weight (kg) ÷ (height (m) × height (m))
Let’s take an example. Someone who weighs 70 kg and is 1.75 m tall first squares their height: 1.75 × 1.75 = 3.06. They then divide their weight by this number: 70 ÷ 3.06 = 22.9. Their BMI is therefore 22.9 kg/m², which corresponds to a healthy weight.
When working out BMI in your head, the most common mistake is forgetting to square your height, or leaving it in centimetres. The calculator at the top of this page does the conversion for you: just enter your height in centimetres. If you usually weigh yourself in stones and pounds, remember that 1 stone equals 6.35 kg and 1 pound equals 0.45 kg.
BMI chart: how to interpret your result
Once you have your BMI, simply find it in the WHO classification. These thresholds are the same for all adults aged 18 and over:
The so-called healthy range, between 18.5 and 24.9, statistically corresponds to the lowest risk of weight-related diseases. Above 25, and especially above 30, the risk of type 2 diabetes, high blood pressure, cardiovascular disease and certain cancers increases. Below 18.5, nutritional deficiencies, fragile bones and a weaker immune system become more likely.
To give you an idea, the WHO estimated that in 2022, 43% of adults worldwide were overweight and 16% were living with obesity. In England, the Health Survey for England 2022 found that 64% of adults were overweight or obese, including 29% living with obesity.
What is an ideal BMI?
There is no magic number, but a range: a healthy BMI is between 18.5 and 24.9. Rather than aiming for a precise figure, it is more useful to know the weight range that corresponds to this zone for your height, often called your “healthy weight”.
The table below gives these ranges for the most common heights. The calculator shows yours automatically, to the nearest centimetre.
A BMI in the middle of the range is not necessarily “better” than one close to its edges. Your doctor can take into account your age, physical activity, family history and how your weight has changed over time, which often matters more than a single measurement.
BMI for women and men: is there a difference?
This is one of the most frequently asked questions. For adults, the answer is simple: the BMI calculation and thresholds are the same for women and men. A BMI of 23 is read in the same way, whatever your sex.
Yet for the same BMI, the body is not made up in the same way. On average, women have a higher proportion of body fat than men, who have more muscle mass. Two people with the same BMI can therefore have different builds and different health profiles. This is one of the reasons why professionals always combine BMI with other measurements, such as waist circumference.
The limits of BMI
BMI is an excellent tool for monitoring the body size of a population. For an individual, it remains an estimate. Here are its main limitations.
It doesn’t distinguish muscle from fat
Muscle is denser than fat. A rugby player or a very muscular person can have a BMI of 27 or 28, which is “overweight” on paper, without carrying excess fat. Conversely, someone with little muscle can have a healthy BMI while having a high proportion of body fat.
It doesn’t show where fat is stored
For your health, where fat is stored matters as much as how much there is. Fat stored around the stomach, known as abdominal or visceral fat, is linked to a higher cardiovascular and metabolic risk than fat on the hips and thighs. BMI makes no distinction.
It varies with age and ethnicity
With age, muscle mass naturally decreases, and a slightly higher BMI does not mean the same thing at 75 as at 30. Some national guidelines, such as the French recommendations on undernutrition, use a higher alert threshold after 70: a BMI below 22. Likewise, the WHO has recognised that people of Asian origin face weight-related risks at lower BMIs than people of European origin. In the UK, NICE recommends lower thresholds (23 for overweight and 27.5 for obesity) for people of South Asian, Chinese, Black African and African-Caribbean family background.
Who BMI is not suitable for
The calculator on this page is designed for adults. In several situations, the result should not be read using the WHO table:
- Children and teenagers: their BMI naturally changes as they grow. It is plotted on growth charts that take age and sex into account.
- Pregnant women: weight gain during pregnancy is normal and monitored by the midwife or doctor. The BMI from before pregnancy is used instead.
- Very muscular athletes: their BMI can overestimate their body fat.
- Older people: as we have seen, the usual thresholds are not always relevant.
Going further than BMI: waist circumference
To go beyond BMI, the simplest thing to do is to measure your waist circumference. Place a tape measure halfway between the bottom of your ribs and the top of your hip bone, roughly level with your belly button, and measure after breathing out, without pulling your stomach in.
According to the international benchmarks used by the WHO, the risk linked to abdominal fat increases above 80 cm for women and 94 cm for men. It becomes much higher above 88 cm for women and 102 cm for men.
Another simple benchmark is becoming more popular: the waist-to-height ratio. The idea is to keep your waist measurement below half your height. Someone who is 170 cm tall, for example, would aim for a waist of less than 85 cm. This benchmark works for women and men alike, and it is now recommended by NICE in the UK.
BMI and diet: a few simple tips
If your BMI is outside the healthy range, there is no need to start a strict diet: very restrictive diets often lead to regaining the weight lost, or even more. Public health advice instead focuses on lasting habits: more vegetables, pulses and wholegrains, fewer ultra-processed foods, sensible portions and regular physical activity, even moderate, such as walking.
Home cooking is a valuable ally, because it lets you know exactly what is on your plate. Among our recipes, several are light, filling and full of vegetables:
- red lentil dhal with coconut milk, rich in fibre and plant protein;
- red kuri squash and chestnut soup, comforting and low in calories;
- chickpea, feta and cucumber salad, ideal for a quick lunch;
- courgette gratin without cream, just as soft and creamy as the classic version.
And where does turmeric fit in? It adds plenty of flavour and colour without adding calories, which helps make simple dishes more enjoyable. However, no spice will make you lose weight: studies on turmeric and weight remain limited and do not show any slimming effect. To find out what it can really offer, read our article on the health benefits of turmeric.
Frequently asked questions about BMI
How can I work out my BMI without a calculator?
Divide your weight by your height in metres, then divide the result by your height again. For 70 kg and 1.75 m: 70 ÷ 1.75 = 40, then 40 ÷ 1.75 = 22.9. It is exactly the same calculation as the official formula, in two steps.
What is a good BMI for a woman?
As for men, a BMI between 18.5 and 24.9 corresponds to a healthy weight for adult women. During pregnancy, however, BMI should not be interpreted using this table.
At what BMI are you overweight?
Overweight starts at a BMI of 25, and obesity at 30. Between 30 and 34.9 is class 1 obesity, between 35 and 39.9 class 2, and 40 and over class 3, sometimes called severe or morbid obesity.
Is BMI reliable?
It is reliable for spotting excess or insufficient weight in most adults, which is why it is used everywhere. It is less reliable for very muscular people, older people, children and pregnant women. Combining it with waist measurement gives a more accurate picture.
Should I worry if my BMI is not in the healthy range?
A BMI outside the healthy range is not a diagnosis, but a signal. Talk to your GP: they can assess your situation as a whole and, if needed, refer you to a dietitian.
This article is provided for general information only. It never replaces a medical consultation. Sources: WHO classification, WHO 2022 data on overweight and obesity, Health Survey for England 2022, NICE guidance on overweight and obesity (2022–2025), French HAS recommendations on undernutrition (2021).