UK weight-loss calculator
BMI calculator
Work out your body mass index and see the healthy weight range for your height. Enter your weight in stone and pounds, kilograms or pounds.
Updated
I weigh st lb and I’m cm tall. For the thresholds, my background is .
2blanks still to fill
The last part is optional. The NHS applies lower BMI thresholds for some family backgrounds, because the risk of conditions such as type 2 diabetes rises at a lower BMI, their own BMI tool asks the same question. Leave it as it is if you would rather not say.
Enter your height and weight to see your BMI, the category it falls in, and the healthy weight range for your height.
What does my BMI mean?
BMI compares your weight to your height. It is a screening measure, used to flag whether your weight might be worth a conversation with a health professional. It is not a diagnosis, and it says nothing on its own about how healthy you are.
For most adults, the NHS uses these categories:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 or above | Obese |
If you are from a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, the NHS uses lower thresholds, because the risk of conditions such as type 2 diabetes rises at a lower BMI[NHS]. Their own BMI tool asks about ethnic background for the same reason[NHS].
| BMI | Category |
|---|---|
| 23 to 27.4 | Overweight |
| 27.5 or above | Obese |
Why we do not split obesity into classes for these thresholds
The NHS publishes the 23 and 27.5 boundaries but does not publish adjusted class 2 and class 3 boundaries. Rather than invent them, this calculator reports a single obese category when the adjusted thresholds are selected.
How is BMI calculated?
BMI is your weight in kilograms divided by the square of your height in metres.
BMI = weight (kg) ÷ height (m)²If you enter stone and pounds, or feet and inches, the calculator converts to kilograms and centimetres first. Every calculation on this site works in those base units and converts only for display, so switching between stone and kilograms never changes the underlying number. You can read the full workings on our methodology page.
Worked example: someone who is 1.8 m tall and weighs 81 kg has a BMI of 81 ÷ (1.8 × 1.8) = 25.0.
Limitations of BMI
BMI is quick and useful across a population, but it has real limits for any individual:
- It does not measure body fat, muscle or where fat is stored. A very muscular person can have a high BMI without excess fat.
- It does not account for build or frame size.
- It is less reliable for older adults, whose body composition changes with age.
- It is not appropriate during pregnancy.
- It is not used the same way for children and teenagers, who are assessed against growth centiles instead.
The NHS suggests that if your BMI is under 35, your waist-to-height ratio is a useful additional check, because it reflects where fat is stored rather than weight alone[NHS].
Related calculators
- Weight loss calculatorSet a goal and see roughly how long it could take to reach it.
- Ideal weight calculatorThe healthy weight range for your height, in stone or kilograms.
- Waist-to-height ratioA useful additional check that BMI alone cannot give you.
- TDEE calculatorEstimate how many calories you burn in a day.
Frequently asked questions
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is described as a healthy weight. For people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, the healthy range tops out lower, at a BMI below 23.
Can I calculate my BMI in stone and pounds?
Yes. Choose st/lb and enter your weight the way you normally describe it, for example 13 stone 6. The calculator converts it for you, and you can switch between stone, kilograms and pounds at any time without re-entering anything.
Is BMI accurate?
It is accurate as a calculation, but it is a rough indicator of health. BMI cannot tell the difference between muscle and fat, and it does not know where fat is stored. Two people with the same BMI can have very different body compositions. Treat it as one signal among several rather than a verdict.
My BMI is above the healthy range. What should I do?
A BMI above the healthy range is a prompt to look at the wider picture, not a diagnosis. Losing 5 to 10% of your body weight is associated with meaningful health benefits and is a more realistic starting point than aiming straight for a "healthy" BMI.
If you would like to see what a goal might involve, our weight loss calculator estimates a timeframe and a calorie range. If you have any health concerns, speak to your GP or pharmacist.
Does this work for children?
No. This calculator is for adults aged 18 and over. Children and teenagers are assessed against age and sex growth centiles rather than fixed thresholds, so an adult BMI category would be misleading. The NHS has a separate tool for under-18s.
Sources
Important information
The calculators on this site provide general information and estimates. They are not medical advice and they do not diagnose any condition.
Everyone is different, and individual energy needs vary for reasons these equations cannot capture. If you have a health condition, take medication, are pregnant or breastfeeding, or have any concerns about your weight, speak to your GP, pharmacist or a registered dietitian.