BMI Calculator

The short answer

• BMI is your weight in kilograms divided by your height in metres squared.
• A BMI of 18.5 to 24.9 is a healthy weight for most adults.
• For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, overweight starts at 23 and obesity at 27.5.
• Keeping your waist to less than half your height is a useful second check.

How BMI is calculated

BMI = weight (kg) ÷ height (m) ÷ height (m).

The same formula is used for men and women. It was designed for adults, so children and teenagers are measured differently.

The adult BMI ranges

Doctors sometimes split obesity into classes: 30 to 34.9, 35 to 39.9 and 40 or more. These classes help decide which treatments may be offered.

Lower thresholds for some ethnic groups

People of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background have a higher risk of type 2 diabetes and heart disease at a lower BMI. NICE therefore recommends lower thresholds: overweight from 23 and obesity from 27.5.

Healthy weight by height

Using feet, inches, stones and pounds

The calculator converts US and UK units for you. One inch is 2.54 cm and one pound is about 0.454 kg. In the US weight is given in pounds; in the UK it is often given in stones and pounds, with 14 pounds in a stone. 5 ft 7 in and 159 lb is the same person as 5 ft 7 in and 11 st 5 lb, with a healthy range of about 118 lb to 159 lb.

Waist size and waist-to-height ratio

Fat around the middle is linked to type 2 diabetes, heart disease and stroke, even in people with a healthy BMI. NICE advises everyone to keep their waist to less than half their height.

A person 175 cm tall with an 88 cm waist has a ratio of 0.50, at the point where risk starts to rise. Measure your waist halfway between your lowest rib and the top of your hips, breathing out normally.

What BMI cannot tell you

BMI is a screening tool, not a diagnosis. Your GP will also consider your waist, blood pressure, blood sugar, cholesterol, family history and lifestyle.

Children and young people

Children’s bodies change as they grow, so their BMI is compared with other children of the same age and sex using centile charts. A child above the 91st centile is classed as overweight, and above the 98th as living with obesity. Use the NHS healthy weight calculator for children rather than an adult BMI calculator.

The National Child Measurement Programme measures children in Reception and Year 6 and tells parents the results.

Older adults

As people age, they tend to lose muscle and gain fat, so BMI can look healthy when body composition is not. Being slightly above the healthy range may not carry the same risks in later life, while unplanned weight loss can be a warning sign. Strength exercises twice a week help keep muscle.

Pregnancy

BMI is not useful during pregnancy because weight rises naturally. Your midwife will usually record your BMI at your first appointment to plan your care. Dieting during pregnancy is not recommended; talk to your midwife about healthy eating instead.

Why weight matters for health

Living with overweight or obesity raises the risk of:

• type 2 diabetes and high blood pressure;
• heart disease and stroke;
• some cancers, including breast and bowel cancer;
• joint pain, sleep apnoea and fertility problems;
• depression and low self-esteem.

Losing even 5% of body weight can improve blood pressure and blood sugar for many people.

Losing weight safely

• Aim for 0.5 kg to 1 kg a week, through small changes you can keep up.
• Eat more fruit, vegetables and fibre, and fewer sugary drinks and snacks.
• Check food labels and watch portion sizes.
• Aim for 150 minutes of moderate activity a week.
• Get enough sleep: tiredness makes healthy choices harder.

If you are underweight

A BMI under 18.5 can be linked to weak bones, a weaker immune system, tiredness and fertility problems. It can also be a sign of an eating disorder or another health condition. See your GP, especially if you have lost weight without trying.

Getting help in the UK and US

In the UK, your GP or practice nurse can refer you to a free local weight management service. Adults with a BMI of 30 or more, or 27.5 for the lower thresholds, who have diabetes or high blood pressure can be referred to the NHS Digital Weight Management Programme, and the free NHS Weight Loss Plan app offers a 12-week programme for anyone.

In the US, the US Preventive Services Task Force recommends that adults with a BMI of 30 or more be offered intensive behavioral programs, which most health insurance plans must cover without cost-sharing under the Affordable Care Act. Ask your doctor what your plan covers; Medicare covers obesity counseling with a primary care provider for people with a BMI of 30 or more.

Weight-loss medicines

Some weight-loss medicines are available on the NHS for people who meet NICE criteria, usually alongside a diet and exercise programme. Eligibility depends on BMI and weight-related health conditions. Buying them online without a proper consultation can be dangerous; only use registered pharmacies.

Where BMI came from

BMI was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was studying the “average man” rather than individual health. It was adopted widely in the 1970s because it is simple, cheap and works well across large groups of people. That history explains both its strength and its weakness: it is good for spotting trends and risks in a population, but less precise for any one person.

The NHS and NICE still use it as the first step because nothing else is as quick, needs no equipment beyond scales and a tape measure, and is so closely linked to the risk of type 2 diabetes and heart disease across the population.

BMI compared with body fat measures

For most people, BMI plus waist-to-height ratio gives a good enough picture. Home body-fat scales can track change over time, but the number on any one day can vary by several percentage points.

Athletes and muscular people

Muscle is denser than fat, so rugby players, rowers and people who do regular strength training can have a BMI in the overweight or even obese range while carrying little body fat. If that describes you, waist-to-height ratio is a better guide. A waist under half your height with a BMI of 27 is a very different picture from a large waist with the same BMI.

Very tall and very short people

The BMI formula squares height, but weight does not rise exactly with the square of height in real bodies. As a result, very tall people tend to have a slightly higher BMI than their body fat suggests, and very short people a slightly lower one. The effect is small for most adults, but worth knowing if you are well outside average height.

Calories and energy balance

Weight changes when the energy you take in from food and drink differs from the energy you use. The NHS uses a rough guide of around 2,000 calories a day for women and 2,500 for men to maintain weight, though needs vary with age, size and activity. Reducing intake by about 600 calories a day is often suggested for steady weight loss of around 0.5 kg a week.

Calories from sugary drinks and snacks add up quickly without making you feel full. Swapping them for water, fruit, vegetables and foods high in fibre and protein is one of the most effective changes for many people.

Activity and fitness at any size

Being active improves health at every BMI. The UK Chief Medical Officers recommend that adults do at least 150 minutes of moderate activity, such as brisk walking or cycling, or 75 minutes of vigorous activity each week, plus strength exercises on two days. Fitness lowers the risk of heart disease and diabetes even if your weight does not change much.

Breaking up long periods of sitting also helps. Short walks, standing while on the phone and using stairs all count.

Weight-related conditions to check

• Blood pressure: free checks at many pharmacies for adults over 40.
• Type 2 diabetes: a blood test (HbA1c) shows your average blood sugar; the NHS Diabetes Prevention Programme helps people at risk.
• Cholesterol: checked as part of the NHS Health Check.
• Sleep apnoea: loud snoring and daytime sleepiness are signs worth raising with your GP.
• Fatty liver disease: often has no symptoms and is linked to weight around the middle.

Free health checks

In England, adults aged 40 to 74 without certain existing conditions are invited for a free NHS Health Check every 5 years. It includes your BMI, blood pressure and cholesterol, and a conversation about your risk of heart disease, stroke, diabetes and kidney disease. If you have not been invited, ask your GP practice. In the US, most health plans cover a yearly preventive visit, including a BMI check, with no copay, and Medicare covers an annual wellness visit.

Common myths about weight

Tracking progress sensibly

Weigh yourself at the same time of day, in similar clothing, no more than once a week. Daily changes of a kilogram or more are usually water. Measure your waist every month or so. Celebrate changes in how you feel, sleep and move, not just the number on the scales.

Key numbers