How the BMI calculator works
Pick Metric (cm, kg) or Imperial (ft, lb), then type your height and weight. Your body mass index appears on the colour scale as you type, and switching units converts what you have already entered. BMI is weight in kilograms divided by height in metres squared: 70 kg at 1.75 m gives 70 ÷ 3.06 = 22.9. If you want to know how much of your weight is fat rather than just how heavy you are for your height, try our body fat calculator next.
The result shows more than one number. You get your WHO category, the healthy weight for your height (the weights that give a BMI of 18.5 to 24.9), and how far you are from that range. Add your waist, which is optional, and you also get your waist-to-height ratio, a simple check of belly fat that BMI cannot see.
This calculator is for adults aged 18 and over. Children and teenagers are still growing, so their BMI has to be compared with others of the same age and sex on a BMI-for-age percentile chart, such as the CDC growth charts for ages 2–19. This tool does not do that calculation.
How to read your BMI
The World Health Organization classifies adult BMI as follows. These are the cut-offs on the colour scale:
| BMI | WHO category |
|---|---|
| below 18.5 | Underweight |
| 18.5–24.9 | Normal (healthy) weight |
| 25–29.9 | Overweight (pre-obesity) |
| 30–34.9 | Obesity class I |
| 35–39.9 | Obesity class II |
| 40 and above | Obesity class III |
An example: someone 175 cm tall weighing 82 kg has a BMI of 26.8, in the overweight range. The healthy weight range for that height is about 56.7–76.3 kg, so the calculator shows 5.7 kg to reach BMI 24.9. With a 90 cm waist the waist-to-height ratio is 0.51, which falls in the "increased" band.
For the waist-to-height ratio, the UK's NICE obesity guideline (2022 update) uses 0.4–0.49 as healthy, 0.5–0.59 as increased risk and 0.6 or more as high risk. The short version, backed by a 2012 meta-analysis by Margaret Ashwell and colleagues: keep your waist to less than half your height.
Why BMI is only a starting point
BMI is cheap, quick and useful for comparing large groups, but it has well-known blind spots for individuals:
- It cannot tell muscle from fat. A muscular athlete can land in the overweight range with little body fat, while someone with low muscle mass can have a "normal" BMI and still carry a lot of fat.
- It ignores where fat sits. Fat around the waist is more closely linked to heart disease and type 2 diabetes than fat on the hips. That is why the waist field is there.
- Risk differs between ethnic groups. A WHO expert consultation in 2004 found that many Asian populations have higher body fat and health risk at the same BMI. It kept the international cut-offs but added public health action points at 23 and 27.5 for Asian populations. If you are of Asian heritage, keep in mind that risk may start below 25.
- Age and sex matter. Older adults tend to have more fat at the same BMI, and women generally have more body fat than men at the same BMI.
What you can do
- Look at trends, not single readings. Your weight can swing by a kilo or two from day to day. Weigh at the same time of day for a few weeks.
- Measure your waist. It takes a tape and a minute and adds information BMI misses.
- Stay active. WHO advises adults to get at least 150 minutes of moderate activity a week. Simple checks like the balance test can be part of tracking your fitness, and the heart rate monitor online reads your resting pulse with your phone camera.
- Aim for realistic change. If your BMI is 25 or higher, losing 5–10% of your weight is a meaningful first target that already improves health markers for many people.
When to see a doctor
BMI is a screening number, not a diagnosis. See a doctor if your weight changes a lot without you trying, if your BMI is under 18.5 or 30 and above and you would like help, or if weight changes come with symptoms such as tiredness, thirst, breathlessness or joint pain. A doctor can check blood pressure, blood sugar and cholesterol, which tell you far more about your health than BMI alone.
More body checks are collected on our fitness tests page.
Sources
- World Health Organization (2000). Obesity: Preventing and managing the global epidemic. Report of a WHO consultation. WHO Technical Report Series 894. Geneva: WHO.
- WHO Expert Consultation (2004). Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet, 363(9403), 157–163.
- Ashwell, M., Gunn, P., & Gibson, S. (2012). Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: Systematic review and meta-analysis. Obesity Reviews, 13(3), 275–286.
- National Institute for Health and Care Excellence (2022). Obesity: identification, assessment and management (CG189, September 2022 update; since replaced by NG246, 2025). London: NICE.
- Centers for Disease Control and Prevention. Losing Weight (a 5–10% weight loss is likely to improve blood pressure, cholesterol and blood sugar).
- Centers for Disease Control and Prevention. Child and Teen BMI Categories (BMI-for-age percentiles, ages 2–19).
- World Health Organization (2020). WHO guidelines on physical activity and sedentary behaviour. Geneva: WHO.
Frequently asked questions
How is BMI calculated?
BMI is your weight in kilograms divided by your height in metres squared. For example, 70 kg at 1.75 m gives 70 ÷ 3.06 = 22.9. In imperial units the calculator converts feet, inches and pounds for you.
What is a healthy BMI for adults?
WHO classifies 18.5 to 24.9 as normal (healthy) weight for adults. Below 18.5 is underweight, 25 to 29.9 overweight, and 30 or above obesity in three classes starting at 30, 35 and 40.
Can I use this BMI calculator for my child?
No. Under 18, BMI has to be compared with children of the same age and sex on a BMI-for-age percentile chart. This calculator uses the adult WHO categories only.
Why does my BMI say overweight when I am muscular?
BMI only uses height and weight, so it cannot tell muscle from fat. Muscular people often land in the overweight range with little body fat. A waist under half your height or a body fat estimate gives a better picture.
Are BMI cut-offs different for Asian people?
A WHO expert consultation in 2004 kept the international cut-offs but added public health action points at 23 and 27.5 for Asian populations, because health risk often rises at a lower BMI.
What is a good waist-to-height ratio?
Under 0.5, meaning your waist is less than half your height. NICE treats 0.5 to 0.59 as increased risk and 0.6 or more as high risk.
How do I find my ideal weight for my height?
The calculator shows the weights that give a BMI between 18.5 and 24.9 at your height. It is a range, not a single ideal number, and it applies to adults.