BMI and healthy weight: NHS categories, stones and pounds, and the limits of the number

How does the NHS work out your BMI?

Body mass index (BMI) is a single number that compares your weight with your height. The NHS calculates it by dividing an adult’s weight in kilograms by their height in metres squared.² The result is shown to one decimal place, for example 24.2.²

The NHS gives its own worked example. A person who weighs 70 kg (around 11 stone) and is 1.70 m (around 5 foot 7 inches) tall squares the height first, 1.70 x 1.70 = 2.89, then divides the weight, 70 ÷ 2.89 = 24.22.² Nothing else goes into the sum. Age, sex, body shape and fitness are not part of the formula.

The NHS calculator returns one of four weight categories: underweight, a healthy weight, overweight or obese.² It is meant for adults aged 18 and over. The NHS says not to use it if you are under 18, are pregnant, have been diagnosed with an eating disorder or think you may have one, or have a condition that affects your height.² Children and teenagers aged 2 to 17 have a separate calculator.²

The NHS also asks for your ethnic background, because people from an Asian, Black African, African-Caribbean or Middle Eastern background have a higher chance of health problems at a lower BMI.² That is why the same number can fall in different categories for different people, as the next sections show.

What do the BMI categories mean?

For most adults, the NHS says a BMI of 25 to 29.9 is overweight and a BMI of 30 or more is obese.¹ For adults from a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, the same page gives lower lines: 23 to 27.4 is overweight and 27.5 or higher is obese.¹

The NHS pages read for this guide give the numbers for overweight and obesity. The underweight category is shown by the calculator, and the NHS says being underweight could mean you are missing out on vitamins and minerals, with a weak immune system and bone fractures as possible consequences.⁵ If you are underweight, or have lost weight without meaning to, that page advises speaking to a GP.⁵

The NHS does not publish separate BMI charts for men and for women on the pages read. The lines above apply to adults whatever their sex.¹² If you searched for a BMI chart for women or a BMI chart for men, the same table below serves both.

BMI in stones and pounds: the weights that cross each line

Most British scales and most conversations use stones and pounds, while the NHS formula uses kilograms and metres. The table below does the conversion for you. Each cell is the body weight at which BMI reaches the line at the top of the column, worked out with the NHS formula: BMI x height in metres x height in metres. One stone is 14 lb and one kilogram is about 2.2 lb. Weights are rounded to the nearest pound.

How to read the table

Find your height in the first column. If your weight is below the figure under 25, your BMI is under 25. If you come from one of the backgrounds listed in the NHS guidance, use the 23 column for the same purpose.¹ A weight between the 25 and 30 columns falls in the overweight range for most adults, and a weight at or above the 30 column falls in the obese range.¹

Take a person who is 5 ft 6 in tall. The table shows 11 st 1 lb at BMI 25 and 13 st 4 lb at BMI 30. So the overweight range for most adults of that height runs from about 11 st 1 lb up to just under 13 st 4 lb. The gap between the two lines is just over 2 stone, which shows how wide a single NHS category is.

Worked examples with the NHS formula

Three examples show the arithmetic from stones and pounds to a BMI. Each one converts to kilograms and metres first, using 1 lb = 0.4536 kg and 1 in = 0.0254 m, then applies the NHS formula.²

The second row is worth a second look. A BMI of 25.8 is only 0.8 above the line, which is about 5 lb for a person of that height. The categories have sharp edges, but your body does not. A person at 24.9 and a person at 25.1 are not meaningfully different, and the NHS describes BMI as one measure of health, not a verdict.²

If you want to check your own number, use the NHS calculator, which handles the conversion and asks for your ethnic background so the advice fits.² The point of the worked rows above is to show what the calculator is doing, so the result is not a mystery.

Why the lines are lower for some ethnic backgrounds

The NHS says that if you are from a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, you are at risk of health problems with a slightly lower BMI.¹ That is the reason for the 23 and 27.5 lines.¹ The NHS calculator asks for ethnic background for the same reason, so the advice it gives is more accurate.²

The same adjustment appears in the NHS description of weight loss surgery. The usual thresholds are a BMI of 40 or over, or 35 or over with a weight-related condition such as type 2 diabetes or high blood pressure. For the backgrounds listed above, the NHS gives 37.5 or over, or 32.5 to 37.4 with such a condition.¹

NICE, which writes clinical guidance for the NHS, published a guideline on overweight and obesity management in January 2025. It covers the prevention and management of overweight, obesity and central adiposity in children, young people and adults, and it replaces several older NICE guidelines.⁴ If you are unsure which line applies to you, ask your GP or practice nurse.

The limits of BMI: muscle, waist and body shape

The NHS is plain about the main weakness. The BMI calculator cannot tell the difference between muscle and fat.¹ If you have a lot of muscle, it may place you in the overweight or obese category even if you do not have a lot of body fat.¹² A rugby player or someone who lifts weights several times a week can be an example.

The reverse also holds in principle. BMI knows nothing about where fat sits. The NHS says that after you get your BMI result you may be asked to measure your waist, because that helps you understand whether you are carrying too much weight around your tummy.² The British Heart Foundation explains that fat around the organs, called visceral fat, is linked to high cholesterol, high blood pressure and type 2 diabetes.³

So the NHS suggests using the waist alongside BMI. If your BMI is under 35, it points to the waist to height ratio calculator and says to try to keep your waist size below half your height.¹ The BHF says the ratio is recommended for adults with a BMI below 35 and that NICE recommends it as a way to find out how much fat is stored around your belly.³ Our guide to measuring your waist walks through the method.

• BMI is a screening number. It does not measure body fat directly.¹
• A high BMI in a muscular adult may not reflect high body fat.¹
• A waist above half your height is a reason to speak to a GP if your BMI is in the overweight or obese range.¹
• A body composition scale gives an estimate of fat and muscle, but it remains an estimate and varies with hydration and time of day.⁶

What a high BMI means for health, and what helps

A higher BMI raises the chance of long-term conditions such as type 2 diabetes and heart disease, according to the NHS.² The NHS lists a wider set of risks of living with overweight and obesity, including stroke, high blood pressure, some cancers, osteoarthritis and fatty liver disease, and it says obesity is likely to lower life expectancy by between 2 and 10 years.¹

There is encouraging news too. The NHS says just losing a small amount of weight, 5 to 10% of your body weight, can improve your health.¹ For a person who weighs 14 st (196 lb), 5 to 10% is about 10 to 20 lb. The NHS suggests aiming to lose 1 to 2 lb, or 0.5 to 1 kg, a week, and getting active for 45 to 60 minutes a day, which you can split into shorter sessions.¹

The NHS also lists what it calls day-to-day problems linked to obesity, among them breathlessness, difficulty moving around, joint and back pain, snoring and low mood.¹ If back pain is part of your picture, our page on back pain from sitting covers what the NHS and Arthritis UK say about staying active.

Nothing here is a plan for you personally. A GP can advise on losing weight safely and can refer you to weight management programmes, medicines or surgery where that is appropriate.¹

Measuring your weight at home so the number is comparable

BMI only changes when your weight changes, so the useful part of home tracking is a consistent weigh-in. Weight moves by a pound or two over a day with food, drink and the bathroom. If you compare a morning reading with an evening reading, you are comparing the day, not your body.

Once you have a stable weight, convert it to stones and pounds if you prefer, find your height in the table above and see which column you are nearest. Re-do it monthly, not daily. A weekly average tells you more than a single morning, and the NHS suggests setting realistic goals and checking how you are getting on by weighing yourself regularly.¹

If thinking about weight is stressful, or you notice your relationship with food or weighing is becoming difficult, speak to a GP. The NHS lists a possible eating disorder as a reason to see a GP, and says its BMI tool is not for people diagnosed with one.¹²

Frequently asked questions