BMI Calculator
Use this BMI calculator in metric or imperial units to see your body mass index, weight category and healthy weight range for adults.
Formula & worked example
Formula: BMI = weight (kg) ÷ height (m)². Healthy weight range = 18.5 to 24.9 × height².
Example: 70 kg at 1.70 m: BMI 24.2 (healthy weight). Healthy range for that height is about 53.5–72.0 kg.
Method & source: Adult BMI categories used by the NHS and World Health Organization. See how we test our calculators.
Next step: try exact age or price per unit.
Body Mass Index, Explained
A BMI calculator works out your body mass index – your weight in kilograms divided by your height in metres squared – and places you in a standard adult weight category. It is a quick screening number used by doctors, nurses and health services.
This BMI calculator works in metric or imperial units, including stones and pounds, and also shows the healthy weight range for your height. It is intended for adults aged 18 and over.
Calculating BMI From Height and Weight
Choose your units, enter your height and weight and confirm you are an adult.
- Formula: BMI = weight (kg) ÷ height (m) × height (m).
- Categories: under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30 or more obese.
- Obesity classes: class 1 (30–34.9), class 2 (35–39.9) and class 3 (40+).
- Healthy range: the weights that would give a BMI of 18.5 to 24.9 at your height.
- Visual scale: a colour bar shows where your result sits.
Height and weight are checked for sensible ranges so a typing slip does not give a misleading result.
Why BMI Is Useful – and Its Limits
BMI is popular because it is simple, but it is only a starting point.
- It gives a quick, consistent measure to compare with standard categories.
- Health professionals use it alongside other checks such as waist size and blood pressure.
- It helps you set a realistic healthy weight range for your height.
- It can flag when a conversation with a doctor or nurse may be worthwhile.
- It tracks change over time using the same method.
BMI does not measure body fat directly and does not separate muscle from fat, so it can mislead for athletes, older adults and some ethnic groups. It is not a diagnosis.
Worked walkthrough: a BMI example
Someone who is 1.70 m tall and weighs 70 kg has a BMI of 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 ≈ 24.2, which falls in the healthy weight range of 18.5 to 24.9. For that height, weights from about 53.5 kg to 72.0 kg give a BMI in the healthy range. In imperial units, 5 ft 7 in and 11 stone convert to about 1.70 m and 69.9 kg and give a BMI of around 24.1. A weight of 90 kg at 1.75 m would give a BMI of about 29.4, in the overweight range.
Measuring height and weight accurately
- Measure your height without shoes and your weight at a consistent time of day for comparable results.
- Treat BMI as one clue, not a verdict. Waist size, fitness, diet and medical history all matter for health.
- Muscular people can have a high BMI without excess body fat; older adults can have a normal BMI with low muscle mass.
- If you are pregnant, under 18, or have a medical condition that affects weight, speak to a health professional instead of relying on this result.
- If your result is outside the healthy range, a doctor or nurse can advise on what, if anything, to do – the calculator cannot give medical advice.
BMI in metric and imperial units
BMI relates weight to height. In metric units it is weight in kilograms divided by height in metres, squared. In imperial units the same idea is written as 703 × weight in pounds ÷ height in inches squared. The number 703 simply converts pounds and inches into the metric scale, so both routes give the same answer.
A person who is 5 ft 9 in tall and weighs 11 stone is 69 inches and 154 pounds. The imperial formula gives 703 × 154 ÷ 4,761 ≈ 22.7. Entering the same person as 1.753 m and 69.85 kg gives the same result. Height matters more than most people expect, because it is squared: at 70 kg, going from 1.75 m to 1.80 m lowers BMI from 22.9 to 21.6.
BMI examples in metric and imperial units
Each row is the result the calculator gives for an adult with these measurements. In imperial mode, height goes in feet and inches and weight in stone and pounds. If you weigh yourself in pounds only, leave stone at 0 and type all the pounds into the pounds box.
| Height | Weight | BMI | Band |
|---|---|---|---|
| 165 cm | 80 kg | 29.4 | Overweight |
| 170 cm | 70 kg | 24.2 | Healthy weight |
| 5 ft 4 in | 9 st 0 lb | 21.6 | Healthy weight |
| 6 ft 0 in | 200 lb (0 st) | 27.1 | Overweight |
| 5 ft 11 in | 185 lb (0 st) | 25.8 | Overweight |
The 6 ft example by hand: 6 ft is 72 inches, so BMI = 703 × 200 ÷ 72² = 703 × 200 ÷ 5,184 ≈ 27.1, the same answer as the metric route. The result also shows the healthy weight range for that height, here about 9 st 10 lb to 13 st 2 lb. A BMI is a screening number, not a diagnosis.
BMI categories for adults
Adult BMI is grouped into bands that the NHS and the World Health Organization use as a first screening step. The band tells you where a result sits on the scale; it does not diagnose anything on its own.
| BMI | Band | What it usually prompts |
|---|---|---|
| Below 18.5 | Underweight | A GP or dietitian can check for any underlying cause |
| 18.5 to 24.9 | Healthy weight | Nothing specific from BMI alone |
| 25 to 29.9 | Overweight | Look at waist size, activity and diet |
| 30 to 34.9 | Obesity class 1 | Worth a conversation with a GP or nurse |
| 35 to 39.9 | Obesity class 2 | A GP or nurse can discuss support options |
| 40 or more | Obesity class 3 | A GP or nurse can discuss support options |
The adult cut-offs match the WHO fact sheet on obesity and overweight (checked on 4 October 2026): overweight from a BMI of 25 and obesity from 30, with underweight below 18.5.
Healthy weight range by height
The healthy weight range is found by turning the formula around: multiply the squared height by 18.5 for the lower limit and by 24.9 for the upper limit. That is why the range widens as people get taller.
| Height | Lower limit (BMI 18.5) | Upper limit (BMI 24.9) |
|---|---|---|
| 1.60 m (5 ft 3 in) | 47.4 kg | 63.7 kg |
| 1.70 m (5 ft 7 in) | 53.5 kg | 72.0 kg |
| 1.80 m (5 ft 11 in) | 59.9 kg | 80.7 kg |
| 1.90 m (6 ft 3 in) | 66.8 kg | 89.9 kg |
What BMI cannot tell you
BMI is a ratio of two measurements, so it is quick and repeatable, but it leaves out several things that matter for health.
Muscle, fat and body shape
The formula cannot tell muscle from fat. A strength athlete can land in the overweight band with little body fat, while an older person who has lost muscle can look “healthy” on the scale. Where fat is stored matters too: a larger waist is linked with higher health risk even at the same BMI, which is why clinicians often measure waist circumference as well.
Age, ethnicity, pregnancy and children
The bands above are for adults. Children and teenagers are assessed against age- and sex-specific growth charts, and BMI is not a reliable guide during pregnancy. Some health bodies also use lower thresholds for people of South Asian and East Asian heritage, because health risks can start at a lower BMI.
Using a BMI result sensibly
A single reading is a snapshot. If you are tracking change, weigh yourself under the same conditions and compare results over weeks rather than days. The percentage change calculator shows how far your weight has moved from a starting point.
This page gives an informational estimate, not medical advice. If your result is outside the healthy band, or you are worried about your weight, speak to a doctor, nurse or dietitian who can look at the whole picture.
Frequently Asked Questions (FAQs)
Divide your weight in kilograms by your height in metres squared. A person who is 1.70 m and 70 kg has a BMI of 70 ÷ 2.89 ≈ 24.2.
For adults, 18.5 to 24.9 is usually described as a healthy weight. Below 18.5 is underweight, 25 to 29.9 overweight and 30 or above obese.
Multiply your weight in pounds by 703 and divide by your height in inches squared. At 6 ft (72 inches) and 200 lb: 703 × 200 ÷ 5,184 ≈ 27.1. In the calculator, choose Imperial, enter 6 ft 0 in, leave stone at 0 and type 200 in the pounds box. You can also enter stone and pounds together if you weigh yourself that way.
Often not. BMI only compares weight with height, so it cannot tell muscle from fat, and a muscular athlete can land in the overweight band while carrying little body fat. A waist measurement or a body composition test gives a better picture for very muscular people. BMI is a screening number rather than a diagnosis, and it is also less reliable in pregnancy.
The result shows the weight range that gives a BMI of 18.5 to 24.9 at your height, in kilograms or stones and pounds.
No. Metric and imperial inputs are converted to the same underlying formula, so a person entered as 70 kg/175 cm or as 11 stone/5 ft 9 in gets the identical BMI result.
Weight tends to scale with the square of height for people of similar build, so dividing by height squared (rather than height alone) gives a fairer comparison between people of different heights.
Not for children. People under 18 are assessed with age- and sex-specific growth charts, so this calculator only gives categories for adults. The adult bands stay the same from age 18 onwards, but some guidance is more relaxed about a slightly higher BMI in older adults, because losing muscle and weight unintentionally can carry its own risks. A doctor can advise on an individual result.
BMI only compares weight with height. Body fat percentage measures how much of your weight is fat, which needs tools such as skinfold callipers or body composition scales. Two people with the same BMI can have quite different body fat.
Not medical advice. BMI is a general screening measure for adults and does not diagnose any condition. It does not account for muscle, age, ethnicity or pregnancy. Speak to a doctor, nurse or other health professional about your own health. See our full disclaimer.