BMI Calculator: Body Mass Index and Healthy Weight for Your Height
Body mass index (BMI) is the most widely used screening tool for checking whether your weight suits your height. Enter your weight and height to get your BMI, its WHO category and the healthy weight range for your height, then read the guide below to understand what the number means and what it does not.
Calculate your BMI
WHO · NICE 2022What is body mass index (BMI)?
Body mass index is a simple number that relates your weight to your height. It is used to estimate whether your weight is in a healthy range, below it or above it. The idea dates back to the Belgian mathematician Adolphe Quetelet in the nineteenth century, and the World Health Organization adopted it as the international standard for classifying underweight, overweight and obesity in adults.
BMI is quick, free and needs only a scale and a tape measure. At population level it tracks body fat and the risk of chronic conditions such as type 2 diabetes, high blood pressure, cardiovascular disease and some cancers. But it is a screening tool, not a diagnosis: it does not measure fat directly and cannot tell where that fat is stored.
How is BMI calculated?
The calculator divides your weight in kilograms by the square of your height in metres:
BMI = weight (kg) ÷ [height (m) × height (m)]
Worked example: a person who weighs 78 kg and is 175 cm tall. Height squared = 1.75 × 1.75 = 3.0625, and 78 ÷ 3.0625 = 25.5, which is just into the overweight range. The healthy weight range for this height is found by multiplying height squared by the limits of the normal range: 18.5 × 3.0625 = 56.7 kg and 24.9 × 3.0625 = 76.3 kg.
The calculator also shows BMI Prime: your BMI divided by 25, the upper limit of the healthy range. A value of 1.00 means you are exactly at the limit; 1.10 means you are about 10% above it.
BMI chart for adults
The classification below follows the WHO report and is the same for men and women aged 18 and over:
| Category | BMI (kg/m²) | Risk of co-morbidities |
|---|---|---|
| Underweight | below 18.5 | Low for obesity-related disease, but higher risk of undernutrition, osteoporosis and weaker immunity |
| Healthy weight | 18.5 – 24.9 | Average (reference level) |
| Overweight (pre-obese) | 25.0 – 29.9 | Increased |
| Obesity class I | 30.0 – 34.9 | Moderate |
| Obesity class II | 35.0 – 39.9 | Severe |
| Obesity class III | 40 and above | Very severe |
Lower cut-offs for Middle Eastern, Asian and African backgrounds
The link between BMI and body fat is not the same in every population. WHO observed that diabetes and heart-disease risk starts to rise at lower values in Asian populations and proposed action points at 23 and 27.5. In its 2022 update, the UK National Institute for Health and Care Excellence (NICE) recommended these lower thresholds for people with a Middle Eastern, South Asian, Chinese, other Asian, Black African or African-Caribbean family background:
- 23 to 27.4: treated as overweight.
- 27.5 and above: treated as obesity.
If you have such a background and your BMI is between 23 and 25, do not stop at “healthy weight”: measure your waist with the waist-to-height ratio calculator and check your fasting glucose or HbA1c at your next check-up.
When is BMI less accurate?
- Athletes and very muscular people: muscle is denser than fat, so an athlete can land in the “overweight” range with little body fat.
- Older adults: muscle mass falls and fat rises with age, so BMI can look normal while body fat is actually high.
- Children and teenagers (under 18): they are assessed with BMI-for-age growth percentiles, not with this chart.
- Pregnancy and breastfeeding: weight changes for natural reasons; weight gain is monitored with your doctor or midwife.
- Fluid retention, oedema or amputation: weight changes for reasons unrelated to fat.
- Fat distribution: BMI cannot tell hip fat from the metabolically riskier visceral fat around the belly, so always pair it with a waist measurement.
What to do with your result: practical medical advice
If you are underweight
Unintended thinness deserves a medical check, especially with rapid weight loss, fatigue or digestive symptoms. Add calories with nutrient-dense foods such as nuts, legumes, yoghurt and full-fat dairy, and include resistance training so the extra weight is muscle rather than fat.
If your weight is healthy
The goal is to stay there: 150 to 300 minutes of moderate activity a week as WHO recommends, two days of strength training, enough sleep and a plate built on vegetables, good protein and whole grains.
If you are overweight or living with obesity
You do not need to reach an “ideal weight” for your health to improve. In the US Diabetes Prevention Program, losing about 7% of body weight with 150 minutes of activity a week cut the risk of developing type 2 diabetes by 58% in people with prediabetes. Start with these steps:
- Work out your daily calorie needs with the TDEE calorie calculator and start with a moderate deficit of about 20% rather than a crash diet.
- Fill half your plate with vegetables, a quarter with protein (chicken, fish, eggs, legumes) and a quarter with low-GI whole-grain starches.
- Swap sugary drinks for water; see how much you need with the daily water calculator.
- Measure your waist every month: a shrinking waist shows visceral fat loss even when the scale slows down.
- If you take medication for diabetes or blood pressure, tell your doctor before cutting calories noticeably, as doses may need adjusting.
Foods that support weight management
These foods are rich in fibre or protein and keep you full for longer. Open each one to see its calories, glycaemic index and full nutrition facts:
Browse the full food guide to compare more than 700 foods by glycaemic index and calories.
Frequently asked questions
What is a normal BMI for adults?
A healthy BMI is 18.5 to 24.9 for men and women aged 18 and over. For people with a Middle Eastern, Asian or African family background, NICE guidance treats 23 as the start of overweight.
How do I work out my ideal weight for my height?
Multiply your height in metres squared by 18.5 and by 24.9. For a height of 170 cm (2.89 squared), the healthy range is roughly 53.5 to 72 kg. The calculator above shows this range automatically.
Is BMI calculated differently for men and women?
The formula and cut-offs are the same for adult men and women, but at the same BMI women carry on average more body fat than men, so a waist measurement helps complete the picture.
Can I use this BMI calculator for children?
No. Under-18s are assessed with BMI-for-age growth percentiles because body composition changes as they grow. Ask your paediatrician to interpret the values.
Why does an athlete show up as overweight?
BMI uses total body weight and cannot separate muscle from fat. For athletes, waist size or body-fat percentage is more informative than BMI.
What is the difference between BMI and waist-to-height ratio?
BMI reflects overall body size, while waist-to-height ratio reflects fat stored around the abdomen, which is more strongly linked to insulin resistance and heart disease. Use both for a better assessment.
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Scientific sources
- World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO Consultation. WHO Technical Report Series 894. Geneva: WHO; 2000.
- WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157-163.
- National Institute for Health and Care Excellence (NICE). Obesity: identification, assessment and management (CG189), update on BMI thresholds and waist-to-height ratio. London: NICE; 2022.
- Knowler WC, Barrett-Connor E, Fowler SE, et al. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med. 2002;346(6):393-403.
- World Health Organization. WHO guidelines on physical activity and sedentary behaviour. Geneva: WHO; 2020.
This calculator is an educational screening tool for adults (18 and over). It is not a medical diagnosis and is not suitable during pregnancy or for children. Interpret your result with your doctor or dietitian in the context of your overall health.