BMI calculator body mass index
Enter your weight and height to see your body mass index, its WHO category and the healthy weight range for your height. Below I explain how it's worked out and when not to trust the number.
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Where the number comes from
The formula, a worked example and where it stops working
What BMI is and how it's calculated
Body mass index (BMI) is your weight in kilograms divided by your height in metres squared. That's the WHO definition. Take 70 kg and 175 cm. Height in metres is 1.75, and squared that's 3.06. Divide 70 by 3.06 and you get 22.9. For adults, WHO calls a BMI under 18.5 underweight, 25 and above overweight, and 30 and above obesity. Anything from 18.5 to 24.9 is the healthy range. The calculator uses the same cut-offs to give you a healthy weight for your height: at 175 cm that's 56.7 to 76.3 kg. BMI has no idea what your weight is made of: muscle, fat, water and bone all count the same. That's why WHO calls it a surrogate marker of fatness. It works well across large groups of people and only roughly for one person. Treat it as a pointer, not a diagnosis. It doesn't apply to children and teenagers, who have their own scale.
The formula and cut-offs come from the WHO fact sheet on obesity and overweight and the WHO page on body mass index.
WHO BMI categories
WHO uses one set of adult cut-offs, with no split by sex and no adjustment for age. The calculator shows four categories:
| BMI | Category |
|---|---|
| under 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25–29.9 | Overweight |
| 30 and above | Obesity |
Adults only. WHO cut-offs, checked 27 September 2026.
Healthy weight for your height
Flip the formula around and you get the weight range that keeps your BMI in the healthy band: from 18.5 × height² to 24.9 × height², with height in metres. At 175 cm that's 18.5 × 3.06 and 24.9 × 3.06, so roughly 56.7 to 76.3 kg. The calculator works this range out for you and shows it under the result.
For a quick look, here it is for a few heights:
| Height | Healthy weight (BMI 18.5–24.9) |
|---|---|
| 150 cm | 41.6–56.0 kg |
| 160 cm | 47.4–63.7 kg |
| 170 cm | 53.5–72.0 kg |
| 180 cm | 59.9–80.7 kg |
| 190 cm | 66.8–89.9 kg |
Adults only. WHO cut-offs: BMI 18.5 to 24.9. The same maths as in the BMI calculator.
Why BMI gets athletes wrong
Muscle is denser than fat. Someone who has lifted for years can be 180 cm and 90 kg, which is a BMI of about 28. On paper that's overweight, even if they carry little fat. It also works the other way: a normal weight with little muscle and more fat than you'd like. BMI can't see either case.
That's why WHO says a second measure helps: waist circumference. It takes five seconds with a tape. If your BMI says overweight but your waist hasn't changed in six months while your lifts keep going up, it's most likely muscle.
For older people and during pregnancy BMI is usually off too: body composition changes, and the same number means different things. That's a conversation for your doctor, not a calculator.
How to measure weight and height so the number doesn't jump
A couple of centimetres of height or a kilo of water shifts BMI noticeably, especially near a category boundary. So measure the same way every time.
Weight: in the morning, before eating, after the bathroom, with no clothes or the same clothes each time. It easily moves a kilo up or down over a day from water and food, and that isn't fat. If you weigh yourself often, use the weekly average. Height: no shoes, back against a wall, heels and the back of your head touching it, looking straight ahead. Mark the top of your head with a book or ruler and measure down to the floor. A height from an old ID card or a check-up ten years ago is worth rechecking.
There's no point recalculating BMI every day. Once a month, or after a noticeable change in weight, is enough.
What to do with the result
BMI on its own doesn't change anything. What you do next does.
- Healthy BMI and you just want to hold your weight — find your daily calorie target
- BMI of 25 or more and you want to lose weight — work out a gentle calorie deficit
- BMI under 18.5 — see a doctor first, and if they're happy for you to gain, use the calorie surplus calculator
- Want protein, fat and carbs as well as calories — there's a macro calculator
Children and teenagers use a different scale
The 18.5, 25 and 30 cut-offs are for adults only. Children's weight and height change with age, so WHO assesses them differently: it compares a child's BMI with the average for their age and sex and looks at how many standard deviations above or below it they are. That's done with WHO growth charts, not a single formula.
So this calculator is meant for adults 18 and over. If you want to check a child's weight, ask a paediatrician.
BMI only moves when your weight does, and your weight follows what you eat every day. Counting that by hand is slow, which is why there's a bot that counts calories from a photo: it remembers your target and adds up what you've eaten each day.
Frequently asked questions
How do I calculate my BMI?
Divide your weight in kilograms by your height in metres squared. For example, 70 kg and 175 cm: 70 / (1.75 × 1.75) = 22.9. The calculator above does exactly this and shows the category straight away.
What is a healthy BMI for women and for men?
WHO uses the same adult cut-offs for both: 18.5 to 24.9 is healthy. Under 18.5 is underweight, 25 and above is overweight, 30 and above is obesity.
Does BMI take age into account?
No. For adults the formula and cut-offs are the same at 20 and at 60. Age only comes in for children and teenagers, who have their own WHO scale.
Why can an athlete have an overweight BMI?
Because BMI only uses weight and height and can't see what the weight is made of. Lots of muscle gives a high BMI with little fat. In that case WHO suggests looking at waist circumference too.
Can I use this BMI calculator for a child?
No. For children and teenagers WHO compares BMI with the average for their age and sex, and the adult cut-offs don't apply. A child's weight is best discussed with a paediatrician.
What should I do if my BMI is over 25?
First check your waist: if you train a lot, it may be muscle. If you want to lose weight, start with a gentle deficit — the calorie deficit calculator will work it out. With a BMI of 30 or more, or any health issues, talk the plan through with a doctor.
Reminders
Water and weight