BMI Calculator
Free NHS-aligned BMI calculator for adults aged 18+. Enter height and weight for your category, healthy weight range and optional waist-to-height ratio.
Quick answer
A BMI calculator works out body mass index from weight in kilograms divided by height in metres squared. For most UK adults the NHS healthy range is 18.5–24.9; 25–29.9 is overweight and 30 or above is obese. Enter height and weight below for an instant result. This is general health information, not personal medical advice.
66%
of adults in England are overweight or living with obesity (Health Survey for England 2024)
13
cancer types Cancer Research UK links to overweight and obesity
1 in 20
UK cancer cases CRUK attributes to excess body weight
How do you use this BMI calculator?
Enter height and weight. The result uses the NHS adult formula and either standard NICE bands or the lower ethnic-minority cut-offs. Add a waist measurement if you want the waist-to-height ratio. JavaScript is required for the live result; the formula below still works without it.
Enter height and weight to calculate BMI. Formula: weight (kg) ÷ height (m)². NHS example: 70 kg at 1.70 m = 24.2.
How is BMI calculated?
Adult BMI is weight in kilograms divided by height in metres squared (kg/m²). The NHS shows the result to one decimal place. In imperial units, divide weight in pounds by height in inches squared, then multiply by 703.
- 1
Measure height in metres
Stand straight without shoes. Convert centimetres to metres (170 cm = 1.70 m). In imperial, convert feet and inches to centimetres first (1 inch = 2.54 cm).
- 2
Measure weight in kilograms
Weigh yourself without shoes or heavy clothing. Convert stones and pounds if needed (1 stone = 14 pounds; 1 pound = 0.4536 kg).
- 3
Square your height
Multiply height in metres by itself. Example: 1.70 × 1.70 = 2.89.
- 4
Divide weight by height squared
BMI = kg ÷ m². Example used by the NHS: 70 ÷ 2.89 = 24.2, shown to one decimal place.
- 5
Read the NHS category
For most adults: below 18.5 underweight; 18.5–24.9 healthy; 25–29.9 overweight; 30+ obesity. Use the lower NICE cut-offs if they apply to your family background.
| Example | Measurements | Result |
|---|---|---|
| NHS adult example | 70 kg and 1.70 m (about 5 ft 7 in) | BMI 24.2 — healthy weight for most adults |
| Metric | 80 kg and 165 cm (1.65 m) | BMI 29.4 — overweight on standard NHS thresholds |
| Imperial | 12 st 8 lb and 5 ft 10 in | About BMI 25.2 — just into the overweight band for most adults |
What are the NHS BMI categories?
NICE NG246 classes most adults as follows. These bands match the NHS adult BMI calculator. They are for people aged 18 and over who are not pregnant.
| Category | BMI (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obesity class 1 | 30 – 34.9 |
| Obesity class 2 | 35 – 39.9 |
| Obesity class 3 | 40 or above |
Which BMI thresholds apply to Asian, Black African and Middle Eastern adults?
People with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean family background tend to carry more central fat, so cardiometabolic risk rises at a lower BMI. NICE therefore uses overweight from 23 and obesity from 27.5. Class 2 and 3 cut-offs are 2.5 BMI points lower than the general table.
| Category | BMI (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 22.9 |
| Overweight | 23 – 27.4 |
| Obesity class 1 | 27.5 – 32.4 |
| Obesity class 2 | 32.5 – 37.4 |
| Obesity class 3 | 37.5 or above |
How do you measure waist-to-height ratio?
NICE recommends waist-to-height ratio alongside BMI when BMI is under 35. Measure the waist midway between the lowest rib and the top of the hip bone, after breathing out. Divide waist by height in the same units. Keep your waist to less than half your height (ratio under 0.5).
| Band | Ratio | Meaning |
|---|---|---|
| Healthy central adiposity | 0.4 – 0.49 | No increased health risk from this measure |
| Increased central adiposity | 0.5 – 0.59 | Increased health risk — NICE: keep your waist to less than half your height |
| High central adiposity | 0.6 or more | Further increased health risk |
| Group | Lower risk | Increased | High |
|---|---|---|---|
| Men | Below 94 cm (37 in) | 94–102 cm | Above 102 cm (40 in) |
| Women | Below 80 cm (31.5 in) | 80–88 cm | Above 88 cm (34.6 in) |
Is BMI accurate for everyone?
BMI is a screening number, not a full picture of health. The NHS adult calculator cannot tell muscle from fat. A rugby player may sit in the overweight band with little body fat; an older adult may sit in the healthy band while carrying extra abdominal fat.
- Do not use this BMI calculator if you are under 18 — children’s BMI uses age- and sex-specific centiles. Use the NHS child BMI calculator.
- Do not use it in pregnancy, or as a substitute for eating-disorder care. Speak to a GP or specialist service.
- Athletes and people with high muscle mass often need waist-to-height ratio or a clinician’s judgement as well as BMI.
- Some physical disabilities and conditions that affect height make BMI a poor fit. Use clinical judgement, not this tool alone.
Does a high BMI increase cancer risk?
Yes. Cancer Research UK says overweight and obesity is the second biggest preventable cause of cancer in the UK after smoking. It is linked to 13 types and causes more than 1 in 20 UK cancer cases (about 22,800 a year). Risk rises the more excess weight you carry and the longer it is carried.
Being overweight does not mean you will definitely develop cancer. Keeping a healthy weight lowers risk of those 13 types. World Aid Network funds treatment for patients who cannot pay overseas; we do not run UK weight-loss clinics.
Oestrogen from fat tissue
Fat cells produce oestrogen. After the menopause this is a main source of the hormone, which can drive womb-lining and breast-cell growth.
Insulin and IGF-1
Excess weight promotes insulin resistance. Higher insulin and insulin-like growth factor 1 can speed cell division.
Chronic inflammation
Excess fat is linked to low-grade inflammation, which can damage DNA and support tumour growth.
Growth signals from fat
Fat tissue changes levels of adipokines such as leptin and adiponectin, shifting the balance toward cell growth.
Which 13 cancers are linked to overweight and obesity?
Womb (endometrial) cancer
Highest relative risk of the 13Obesity is the main modifiable risk factor. Women with BMI 30+ have around twice the risk versus a healthy weight; at BMI 40+ the risk is around four times higher.
Breast cancer (after menopause)
High case numbersAfter the menopause, body fat becomes a main source of oestrogen. CRUK lists post-menopausal breast cancer among the 13 weight-linked types.
Bowel (colorectal) cancer
High case numbersAbdominal fat, insulin resistance and inflammation all contribute. Waist size is a useful extra signal alongside BMI.
Kidney cancer
Established linkCRUK lists kidney cancer among the 13 types caused by overweight and obesity.
Liver cancer
Established linkExcess weight drives fatty liver disease, which can progress to cirrhosis and hepatocellular carcinoma.
Pancreatic cancer
Established linkCRUK groups pancreatic cancer with the harder-to-treat weight-linked types.
Ovarian cancer
Established linkListed by CRUK among the 13. The association is clearest for some epithelial subtypes.
Thyroid cancer
Established linkA consistent but more modest association with excess body weight.
Meningioma (a type of brain tumour)
Established linkCRUK includes meningioma in the 13. It is usually a tumour of the membranes around the brain, not a typical brain cancer.
Oesophageal adenocarcinoma
Strong linkExcess weight promotes reflux and Barrett’s oesophagus, a pathway to adenocarcinoma of the oesophagus.
Upper stomach (gastric cardia) cancer
Established linkCancer at the join of the stomach and oesophagus is the stomach subtype linked to excess weight.
Gallbladder cancer
Established linkObesity increases gallstone risk, a precursor pathway, and has independent metabolic effects.
Myeloma (a type of blood cancer)
Established linkCRUK includes myeloma in the 13 weight-linked cancers.
How do you reach a healthy BMI?
CRUK’s advice is practical: if you are overweight, first avoid further gain; losing weight helps. A 5–10% reduction can improve insulin sensitivity. Combine movement, food, alcohol limits and — if you need it — NHS support.
Move more
UK Chief Medical Officers advise at least 150 minutes of moderate activity a week, plus strength work on two days. Walking counts.
Eat a plant-rich diet
Base meals on vegetables, fruit, wholegrains and pulses. Limit ultra-processed food, processed meat and sugary drinks.
Keep alcohol within 14 units
Alcohol adds calories and independently raises several cancer risks. Have alcohol-free days.
Do not smoke
Smoking is still the largest preventable cause of cancer in the UK. Stopping helps weight-related and cardiovascular risk too.
Ask for structured support
A GP can discuss NHS weight-management programmes, dietetic support or medicines where they are clinically appropriate.
Attend screening
Go to bowel, breast and cervical screening invitations regardless of BMI. Early detection still saves lives.
Frequently asked questions
NHS- and NICE-aligned answers, including how BMI links to cancer and why World Aid Network publishes this tool.
What is BMI?
BMI (body mass index) is weight in kilograms divided by height in metres squared (kg/m²). The NHS uses it as a screening tool to class adults aged 18+ as underweight, healthy weight, overweight or living with obesity. It is not a diagnosis and it cannot tell muscle from fat.
How is BMI calculated?
BMI = weight (kg) ÷ height (m)². The NHS example is 70 kg at 1.70 m: 1.70 × 1.70 = 2.89; 70 ÷ 2.89 = 24.2. In imperial, divide pounds by height in inches squared and multiply by 703. This BMI calculator does both conversions for you.
What is a healthy BMI for adults in the UK?
For most adults a healthy BMI is 18.5 to 24.9. Below 18.5 is underweight. 25–29.9 is overweight. 30 or above is obesity (class 1: 30–34.9, class 2: 35–39.9, class 3: 40+), following NICE NG246.
What BMI is overweight in the UK?
For most adults, overweight is a BMI of 25.0 to 29.9. For adults with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean family background, NICE treats 23.0 to 27.4 as overweight.
What BMI is obese in the UK?
For most adults, obesity starts at BMI 30. Class 1 is 30–34.9, class 2 is 35–39.9 and class 3 is 40 or above. For the NICE ethnic groups above, obesity starts at 27.5.
What is a healthy BMI for a woman in the UK?
The adult BMI bands are the same for women and men. After the menopause, even a BMI in the higher healthy range can matter for breast cancer risk because fat tissue becomes a main source of oestrogen. The lower NICE cut-offs still apply where the family-background rule applies.
How do I calculate BMI in stones and pounds?
Convert stones and pounds to kilograms, or use the imperial formula: BMI = (weight in pounds ÷ height in inches²) × 703. Example path: 12 st 8 lb is 176 lb; at 70 inches, 176 ÷ 4,900 × 703 ≈ 25.2. The calculator’s imperial mode does this conversion.
What BMI thresholds apply to South Asian, Black African and Middle Eastern adults?
NICE NG246 (2025) uses lower practical thresholds for adults with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean family background: overweight 23–27.4; obesity 27.5 or above. Select that option in the calculator to apply those bands.
Is BMI accurate?
BMI is a useful population screening tool, not a complete health measure. It cannot distinguish muscle from fat, does not show where fat is stored, and is a poor fit for athletes, pregnant people, people with some disabilities, and children. NICE recommends waist-to-height ratio alongside BMI when BMI is under 35.
Should I measure my waist as well as BMI?
Yes, if you can. NICE classifies central adiposity with waist-to-height ratio: 0.4–0.49 healthy, 0.5–0.59 increased, 0.6+ high. The practical message is to keep your waist to less than half your height. Older NHS waist cut-offs (men 94/102 cm, women 80/88 cm) are still widely used.
What is a healthy waist-to-height ratio?
NICE NG246: 0.4 to 0.49 is healthy central adiposity for adults with BMI under 35, including people with high muscle mass and all ethnicities. 0.5 or above indicates increased risk. Measure the waist midway between the lowest rib and the top of the hip bone.
Does BMI affect cancer risk?
Yes. Cancer Research UK says overweight and obesity is the second biggest preventable cause of cancer in the UK after smoking, and is linked to 13 types, causing more than 1 in 20 UK cancer cases. Risk rises the more excess weight you carry and the longer it is carried.
Can children use this BMI calculator?
No. This tool is for adults aged 18 and over. Children’s BMI is read against age- and sex-specific centiles, not adult cut-offs. Use the NHS child and teenager BMI calculator, or speak to a GP, health visitor or school nurse.
Can losing weight reduce cancer risk?
Keeping a healthy weight reduces the risk of the 13 weight-linked cancers. CRUK advises that if you are overweight, avoiding further gain is a starting point, and losing weight helps. Even a 5–10% reduction can improve insulin sensitivity and inflammatory markers. This is not a substitute for screening or GP advice.
Why does World Aid Network publish a BMI calculator?
Excess weight is a major, modifiable cancer risk. World Aid Network publishes this BMI calculator as general health education, then funds cancer treatment for patients who cannot pay in Pakistan, Indonesia and Malaysia. We do not run NHS weight clinics, and this tool is not personal medical advice.
Medical disclaimer
This BMI calculator is general health information for adults, not personal medical advice and not a diagnosis. It does not replace a GP, pharmacist or specialist. World Aid Network does not provide NHS care or UK weight-management clinics. Last reviewed 15 August 2026 against NHS, NICE NG246, Cancer Research UK and Health Survey for England 2024 sources.
Sources
Which related guides should you read?
Cancer guides
Nineteen UK guides — symptoms, tests, treatment and survival.
Womb cancer
The weight-linked cancer with the highest relative risk.
Breast cancer
Post-menopausal breast cancer is one of the 13 CRUK types.
Bowel cancer
Weight, diet, FIT screening and treatment we fund.
Liver cancer
Fatty liver disease is one pathway from excess weight.
Kidney cancer
Another of the 13 cancers linked to overweight and obesity.
Cancer Emergency Appeal
Fund treatment for patients who cannot pay — not research.
How to help cancer patients
Practical ways to fund care rather than only research.
How do we fund cancer treatment for patients who cannot pay?
World Aid Network funds diagnosis and treatment — imaging, surgery, chemotherapy and related care — for patients in Pakistan, Indonesia and Malaysia who cannot afford the hospital bill. We fund treatment access, not laboratory research and not UK NHS care.
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