BMI (body mass index) is your weight in kilograms divided by your height in metres squared. For an adult, the NHS treats a BMI of 18.5 to 24.9 as the healthy range. It is a quick screening guide, not a diagnosis, and it cannot tell fat from muscle.
How BMI is calculated
The formula is BMI = kg ÷ m².
Worked example: someone who is 175 cm (1.75 m) tall and weighs 70 kg.
- Height squared: 1.75 × 1.75 = 3.0625
- BMI: 70 ÷ 3.0625 = 22.9
That falls in the healthy range. The BMI tool does this for you and accepts metric or imperial units. For this height it also shows a healthy weight range of 56.7 kg to 76.3 kg.
NHS BMI ranges for adults
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 or more | Obese |
The NHS notes that lower thresholds apply to people of Black, Asian and other minority ethnic backgrounds, because health risks can start at a lower BMI. Check the NHS website or ask a GP or practice nurse which thresholds suit you.
What BMI does well
BMI is quick, free and needs only height and weight. Across large groups of people it is a useful first check, and it gives a common language for health professionals. It is easy to track over time, which can matter more than a single reading.
What BMI cannot tell you
BMI is a blunt tool. It does not measure body fat directly, so it has several limits:
- It cannot tell muscle from fat. A muscular person can have a BMI in the overweight range with low body fat.
- It says nothing about where fat is stored. Fat around the waist carries different risks from fat elsewhere.
- It is designed for adults. Children and teenagers are assessed differently, using centiles.
- It may be less reliable in older adults, who can lose muscle without changing weight.
- Pregnancy changes weight, so BMI is not used in the same way.
If you want a closer look at body composition, see Body fat percentage: how it's measured and what's typical, which covers tape-measure methods and typical ranges.
Healthy weight range and “ideal weight”
Because BMI ranges are broad, many people prefer to look at a healthy weight range for their height. The Ideal weight tool shows the weight range that corresponds to a BMI of 18.5 to 24.9 and compares four common formulas. For someone 165 cm tall, that range is 50.4 kg to 67.8 kg. There is no single ideal number: bone structure, muscle and age all matter.
If your goal involves changing weight, working out how many calories you burn is a sensible next step, covered in Calories and TDEE: how many calories do you need?.
Note: This is general information, not medical advice. BMI is a screening guide for adults. Speak to a GP or practice nurse about your own health and any weight changes.
How to use your BMI result sensibly
A single reading is a snapshot. Measure at a similar time of day, with the same scales, and focus on the trend rather than small changes. If your result is near a boundary, such as 24.9 against 25, do not read much into it: normal daily weight variation can move you across the line.
Waist measurement is a useful companion, since it reflects where weight sits. The NHS gives guidance on waist size, so check its website. A GP or practice nurse can combine BMI, waist, blood pressure and family history into a fuller picture, which is far more informative than BMI alone.
BMI for children and teenagers
The adult ranges above do not apply to under-18s. For children, the NHS uses growth charts that compare BMI with other children of the same age and sex. If you are worried about a child’s weight, speak to a GP, health visitor or school nurse rather than using an adult calculator.
Try the calculator
Use BMI to find your BMI and healthy weight range, and Ideal weight to compare healthy ranges for your height. The results are estimates and a starting point for a conversation with a health professional.
Step-by-step: working out BMI by hand
You can do the sum with any calculator in three moves.
- Convert your height to metres (centimetres divided by 100).
- Multiply that figure by itself to get height squared.
- Divide your weight in kilograms by that number.
For imperial units, convert first: pounds divided by 2.205 gives kilograms, and inches multiplied by 0.0254 gives metres. Stones are 14 pounds each. The House of Calculator BMI calculator accepts either system so you do not have to convert.
Worked example 2: two people, two results
The same formula gives very different readings as height and weight change. Both results below come from the calculator engine.
| Person | Height | Weight | BMI | NHS category | Healthy weight for that height |
|---|---|---|---|---|---|
| A | 160 cm | 82 kg | 32.0 | Obese | 47.4 to 63.7 kg |
| B | 70 in | 176 lb | 25.3 | Overweight | 128.9 to 173.6 lb |
For Person A, 1.60 × 1.60 = 2.56, and 82 ÷ 2.56 = 32.0. For Person B, 70 inches is 1.778 m and 176 lb is 79.8 kg, so BMI is 79.8 ÷ 3.161 = 25.3.
Person B is only just over the line. A weight of 173.6 lb at that height would be a BMI of 24.9, the top of the healthy range. That shows how a few pounds can move a result across a boundary, and why a number near 25 should not be read as a verdict. If Person B were very muscular, the label would also be misleading.
BMI at a glance: weight by height
This table shows the weight range that corresponds to the NHS healthy BMI range of 18.5 to 24.9 at several heights. These are hand calculations from the formula, rounded to the nearest 0.5 kg.
| Height | Healthy range (kg) |
|---|---|
| 150 cm | 41.5 to 56.0 |
| 160 cm | 47.5 to 63.5 |
| 170 cm | 53.5 to 72.0 |
| 180 cm | 60.0 to 80.5 |
| 190 cm | 67.0 to 90.0 |
Notice that height has a big effect, because it is squared. Ten extra centimetres at 180 cm adds around 10 kg to the top of the range.
Common mistakes with BMI
- Using the wrong units. Entering centimetres where metres are expected, or pounds as kilograms, produces nonsense results.
- Treating the number as a diagnosis. BMI is a screening measure for adults, not a judgement about health.
- Ignoring ethnicity thresholds. The NHS notes that lower thresholds apply to people of Black, Asian and other minority ethnic backgrounds.
- Using an adult calculator for a child. Children are assessed with growth charts, as covered above.
- Chasing a single reading. Weight changes through the day and week, so watch the trend.
- Forgetting muscle. If you lift weights or play sport, BMI can overstate how much fat you carry.
Other measures that add context
BMI is one piece of information. Others a health professional may consider include waist size, blood pressure, blood tests, family history, activity level and diet. For a closer look at body composition, see Body fat percentage: how it's measured and what's typical. If you are thinking about changing your weight, Calories and TDEE: how many calories do you need? explains how energy needs are estimated, and How much water should you drink each day? covers everyday hydration.
Glossary of terms
- BMI: body mass index, weight in kilograms divided by height in metres squared.
- Healthy weight range: the weights for your height that give a BMI of 18.5 to 24.9.
- Centile: a ranking against children of the same age and sex, used instead of adult BMI categories.
- Waist circumference: a measure of fat around the middle, often used alongside BMI.
- Body composition: the mix of fat, muscle, bone and water in the body.
When to speak to a health professional
Speak to a GP or practice nurse if your BMI is outside the healthy range and you are concerned, if your weight has changed without explanation, if you are pregnant, or if you are thinking about a major change to your diet or exercise. They can look at your whole health, which a single formula cannot.
Frequently asked questions
What is a healthy BMI?
The NHS defines 18.5 to 24.9 as the healthy range for adults. Lower thresholds may apply to some ethnic groups, so check with a health professional.
Is BMI accurate for athletes?
Often not. Muscle is heavier than fat for the same volume, so a muscular person can have a high BMI without excess body fat.
How do I calculate BMI in stones and pounds?
Convert to kilograms and metres first (1 stone is about 6.35 kg and 1 inch is 0.0254 m), or use a calculator that accepts imperial units.
Should I worry if my BMI is outside the healthy range?
It is a prompt to look further, not a diagnosis. A GP or practice nurse can consider your waist size, health history and lifestyle.
What BMI is classed as underweight?
The NHS classes a BMI under 18.5 as underweight for adults. If you are concerned, speak to a GP, especially if the change was unplanned.
Does BMI change with age?
The formula does not, but its usefulness does. Older adults can lose muscle without a change in weight, so BMI may hide changes in body composition.
Is BMI different for men and women?
The NHS adult ranges are the same for both. Women and men tend to carry fat differently, which is one reason body fat percentage and waist size are also used.
How often should I check my BMI?
Every few weeks or months is plenty for most people, using the same scales and a similar time of day.