# BMI Calculator > Source: World Aid Network — https://worldaidnetwork.org/bmi-calculator > Last reviewed: 15 August 2026 ## In short 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. 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. Use the interactive calculator at https://worldaidnetwork.org/bmi-calculator — or apply the NHS formula below without JavaScript. World Aid Network publishes this as general health education. We fund cancer treatment for patients who cannot pay in Pakistan, Indonesia and Malaysia; we do not run NHS weight clinics. ## At a glance - 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 to calculate your BMI Work out adult body mass index with the NHS metric formula, then compare the result with NHS and NICE category tables. BMI = weight (kg) ÷ height (m)². Imperial: (lb ÷ in²) × 703. - 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). - 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). - Square your height: Multiply height in metres by itself. Example: 1.70 × 1.70 = 2.89. - Divide weight by height squared: BMI = kg ÷ m². Example used by the NHS: 70 ÷ 2.89 = 24.2, shown to one decimal place. - 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. ## Worked examples - NHS adult example — 70 kg and 1.70 m (about 5 ft 7 in). 1.70 × 1.70 = 2.89; 70 ÷ 2.89 = 24.22. BMI 24.2 — healthy weight for most adults - Metric — 80 kg and 165 cm (1.65 m). 1.65 × 1.65 = 2.7225; 80 ÷ 2.7225 = 29.39. BMI 29.4 — overweight on standard NHS thresholds - Imperial — 12 st 8 lb and 5 ft 10 in. Convert to kg and metres, then kg ÷ m² (or lb ÷ in² × 703). About BMI 25.2 — just into the overweight band for most adults ## What are the NHS BMI categories? Standard adult bands from NICE NG246, used by the NHS adult BMI calculator. - Underweight: Below 18.5. A BMI below 18.5 can reflect undernutrition, illness or another underlying condition. Unintentional weight loss should be checked by a GP. - Healthy weight: 18.5 – 24.9. For most UK adults this range is associated with the lowest weight-related health risk. Central fat can still raise risk, which is why NICE also uses waist-to-height ratio. - Overweight: 25 – 29.9. Overweight raises the chance of type 2 diabetes, heart disease and several cancers. A sustained 5–10% weight loss can improve metabolic markers. - Obesity class 1: 30 – 34.9. Obesity class 1 is linked to higher cardiometabolic risk and to the 13 cancers CRUK associates with excess weight. A GP can discuss NHS weight-management options. - Obesity class 2: 35 – 39.9. Class 2 obesity carries substantially higher health risk. Structured clinical support is usually recommended. - Obesity class 3: 40 or above. Class 3 obesity (sometimes called severe obesity) has the highest weight-related health risk. Specialist assessment may include medicines or surgery where appropriate. ## Which BMI thresholds apply to Asian, Black African and Middle Eastern adults? NICE NG246 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. - Underweight: Below 18.5. A BMI below 18.5 can reflect undernutrition, illness or another underlying condition. Unintentional weight loss should be checked by a GP. - Healthy weight: 18.5 – 22.9. NICE uses a lower overweight cut-off for these family backgrounds because cardiometabolic risk rises at a lower BMI. Healthy here means below 23. - Overweight: 23 – 27.4. NICE NG246: overweight is BMI 23–27.4 for adults with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean family background. - Obesity class 1: 27.5 – 32.4. NICE NG246: obesity starts at BMI 27.5 in these groups. Class 2 and 3 cut-offs are 2.5 BMI points lower than the general adult thresholds. - Obesity class 2: 32.5 – 37.4. Class 2 on the NICE ethnic scale. Structured clinical support is usually recommended. - Obesity class 3: 37.5 or above. Class 3 on the NICE ethnic scale. Specialist assessment may include medicines or surgery where appropriate. ## How do you measure waist-to-height ratio? NICE: keep your waist to less than half your height (ratio under 0.5). Use this mainly when BMI is under 35. - 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 - Men: lower risk Below 94 cm (37 in); increased 94–102 cm; high Above 102 cm (40 in) - Women: lower risk Below 80 cm (31.5 in); increased 80–88 cm; high Above 88 cm (34.6 in) ## Is BMI accurate for everyone? BMI cannot tell muscle from fat. Do not use this adult calculator for children, pregnancy, or as a substitute for eating-disorder care. Children’s BMI uses centiles: https://www.nhs.uk/health-assessment-tools/calculate-your-body-mass-index/calculate-bmi-for-children-teenagers ## Does a high BMI increase cancer risk? Cancer Research UK: overweight and obesity is the second biggest preventable cause of cancer in the UK after smoking, linked to 13 types and more than 1 in 20 UK cancer cases. 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. - Womb (endometrial) cancer (Highest relative risk of the 13): Obesity 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. https://worldaidnetwork.org/cancer/womb-cancer - Breast cancer (after menopause) (High case numbers): After the menopause, body fat becomes a main source of oestrogen. CRUK lists post-menopausal breast cancer among the 13 weight-linked types. https://worldaidnetwork.org/cancer/breast-cancer - Bowel (colorectal) cancer (High case numbers): Abdominal fat, insulin resistance and inflammation all contribute. Waist size is a useful extra signal alongside BMI. https://worldaidnetwork.org/cancer/bowel-cancer - Kidney cancer (Established link): CRUK lists kidney cancer among the 13 types caused by overweight and obesity. https://worldaidnetwork.org/cancer/kidney-cancer - Liver cancer (Established link): Excess weight drives fatty liver disease, which can progress to cirrhosis and hepatocellular carcinoma. https://worldaidnetwork.org/cancer/liver-cancer - Pancreatic cancer (Established link): CRUK groups pancreatic cancer with the harder-to-treat weight-linked types. https://worldaidnetwork.org/cancer/pancreatic-cancer - Ovarian cancer (Established link): Listed by CRUK among the 13. The association is clearest for some epithelial subtypes. https://worldaidnetwork.org/cancer/ovarian-cancer - Thyroid cancer (Established link): A consistent but more modest association with excess body weight. https://worldaidnetwork.org/cancer/thyroid-cancer - Meningioma (a type of brain tumour) (Established link): CRUK includes meningioma in the 13. It is usually a tumour of the membranes around the brain, not a typical brain cancer. https://worldaidnetwork.org/cancer/brain-tumour - Oesophageal adenocarcinoma (Strong link): Excess weight promotes reflux and Barrett’s oesophagus, a pathway to adenocarcinoma of the oesophagus. https://worldaidnetwork.org/cancer - Upper stomach (gastric cardia) cancer (Established link): Cancer at the join of the stomach and oesophagus is the stomach subtype linked to excess weight. https://worldaidnetwork.org/cancer - Gallbladder cancer (Established link): Obesity increases gallstone risk, a precursor pathway, and has independent metabolic effects. https://worldaidnetwork.org/cancer - Myeloma (a type of blood cancer) (Established link): CRUK includes myeloma in the 13 weight-linked cancers. https://worldaidnetwork.org/cancer ## How do you reach a healthy BMI? - 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. ## Related pages - Cancer guides — Nineteen UK guides — symptoms, tests, treatment and survival. https://worldaidnetwork.org/cancer - Womb cancer — The weight-linked cancer with the highest relative risk. https://worldaidnetwork.org/cancer/womb-cancer - Breast cancer — Post-menopausal breast cancer is one of the 13 CRUK types. https://worldaidnetwork.org/cancer/breast-cancer - Bowel cancer — Weight, diet, FIT screening and treatment we fund. https://worldaidnetwork.org/cancer/bowel-cancer - Liver cancer — Fatty liver disease is one pathway from excess weight. https://worldaidnetwork.org/cancer/liver-cancer - Kidney cancer — Another of the 13 cancers linked to overweight and obesity. https://worldaidnetwork.org/cancer/kidney-cancer - Cancer Emergency Appeal — Fund treatment for patients who cannot pay — not research. https://worldaidnetwork.org/appeal/cancer-emergency - How to help cancer patients — Practical ways to fund care rather than only research. https://worldaidnetwork.org/cancer/how-to-help-cancer-patients ## Frequently asked questions ### 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. ## How you can help Donate to the Cancer Emergency Appeal at https://worldaidnetwork.org/appeal/cancer-emergency or https://worldaidnetwork.org/donate?cause=cancer-treatment&appeal=cancer-emergency ## Sources - NHS — Calculate your BMI for adults: https://www.nhs.uk/health-assessment-tools/calculate-your-body-mass-index/calculate-bmi-for-adults - NHS — Overweight and obesity in adults: https://www.nhs.uk/conditions/overweight-and-obesity/ - NICE NG246 — Overweight and obesity management (2025): https://www.nice.org.uk/guidance/ng246 - Cancer Research UK — How does obesity cause cancer?: https://www.cancerresearchuk.org/about-cancer/causes-of-cancer/obesity-weight-and-cancer/how-does-obesity-cause-cancer - Health Survey for England 2024 — Adults’ overweight and obesity: https://digital.nhs.uk/data-and-information/publications/statistical/health-survey-for-england/2024/adults-overweight-and-obesity - NHS — Child and teenager BMI calculator: https://www.nhs.uk/health-assessment-tools/calculate-your-body-mass-index/calculate-bmi-for-children-teenagers --- This is general information, not medical advice, published by World Aid Network. Always consult a qualified clinician about your own health.