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.
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.