BMI Calculator
Calculate your Body Mass Index (BMI) to check whether you are at a healthy weight for your height, with NHS categories and ethnicity-adjusted thresholds.
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How Body Mass Index Is Measured
A BMI calculator divides your weight in kilograms by the square of your height in metres. The number it produces is the single most common screening figure used across NHS general practice, hospital outpatient clinics, occupational health checks and community pharmacy weight services in the United Kingdom. It is not a diagnosis. It is a starting point that tells your GP or practice nurse whether your weight, in proportion to your height, sits inside a band associated with low long-term health risk, or outside it.
The NHS uses six bands for adults aged 18 and over. A BMI under 18.5 is classed as underweight. A BMI between 18.5 and 24.9 is the healthy weight band, and is the range associated with the lowest population-level risk of cardiovascular disease, type 2 diabetes and several common cancers. A BMI of 25.0 to 29.9 is overweight. From 30.0 upwards the NHS uses three obesity sub-classes: 30.0 to 34.9 is obese class 1, 35.0 to 39.9 is obese class 2, and 40.0 or higher is obese class 3, often described in NHS letters and referrals as severely obese.
These bands matter because they determine the pathway your GP follows. A BMI in the healthy band typically means no further action beyond standard preventive advice. A BMI in the overweight band usually triggers a brief intervention, lifestyle conversation and signposting to the NHS Better Health resources or the NHS Weight Loss Plan app. A BMI of 30 or more opens the door to Tier 2 weight management programmes, structured 12-week behavioural support delivered locally by NHS-commissioned providers. A BMI of 35 with significant comorbidity, or a BMI of 40 or more, brings bariatric surgery into scope under NICE referral criteria.
The headline you should keep in mind: BMI is a population screening tool, not an individual verdict. Two people with the same BMI can have very different bodies, very different risks, and very different next steps. The number on this calculator is the start of the conversation, not the end of it.
How to Calculate Your BMI Using Height and Weight
Enter your height in centimetres and weight in kilograms. Click Calculate to see your BMI value, the NHS weight category it falls into, and whether ethnicity-adjusted thresholds apply.
- Remove your shoes and any heavy outer clothing before measuring.
- Measure your height in centimetres standing flat against a wall, heels back, looking straight ahead.
- Weigh yourself in kilograms first thing in the morning after using the toilet, before eating or drinking.
- Enter your height in centimetres in the height field of the calculator.
- Enter your weight in kilograms in the weight field.
- Click the Calculate button to generate your BMI to one decimal place.
- Read the NHS category displayed beside your number.
- If you are of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean heritage, also check your result against the lower NICE thresholds explained below.
The BMI Formula (kg/m² and Imperial Conversion)
The formula used: BMI = weight (kg) / height (m)²
The BMI formula is simple but unforgiving of input errors: BMI = weight (kg) / height (m)².
Convert centimetres to metres before squaring. For 175 cm, divide by 100 to get 1.75 m. Square it: 1.75 × 1.75 = 3.0625. Then divide weight by the squared height: 80 / 3.0625 = 26.1.
BMI is a unitless number, but the inputs must be in the right units. Mixing imperial and metric is the most common cause of impossible results. If you weigh in stones and pounds at home, convert to kilograms first: 1 stone = 6.35 kg, 1 lb = 0.4536 kg. If you measure in feet and inches, convert to centimetres: 1 inch = 2.54 cm, 1 foot = 30.48 cm. The NHS BMI healthy weight calculator on nhs.uk accepts both unit systems, but most clinical records and this tool work in metric.
For accuracy, weigh yourself on the same scales, on a hard floor, at the same time of day, ideally first thing in the morning. Body weight can vary by 1 to 2 kg across a single day because of food, fluid intake and bowel content. Height should be measured without shoes, with heels and shoulders against a wall. Most adults lose 1 to 2 cm of standing height through the day as the spinal discs compress, so morning measurements are more representative.
BMI Results for Different Heights and Weights
For a person 175 cm tall weighing 80 kg: BMI = 80 / (1.75)² = 26.1, which falls in the Overweight category under standard NHS thresholds.
Healthy-weight adult — office worker in Manchester
Female, 32 years, height 170 cm, weight 65 kg. Convert height to metres: 1.70 m. Square it: 2.89. BMI = 65 / 2.89 = 22.5.
BMI 22.5 — Healthy weight band (18.5 to 24.9). No further screening action under standard NHS pathways.
Overweight adult — applies under standard but not adjusted thresholds
Male, 45 years, height 180 cm, weight 90 kg. Square the height: 1.80 × 1.80 = 3.24. BMI = 90 / 3.24 = 27.8.
BMI 27.8 — Overweight (25.0 to 29.9). Already in the obese band under NICE adjusted thresholds for South Asian or Black adults (≥27.5).
Underweight adult — flagged for further investigation
Female, 28 years, height 165 cm, weight 48 kg. Squared height: 2.7225. BMI = 48 / 2.7225 = 17.6.
BMI 17.6 — Underweight (under 18.5). NHS guidance suggests a GP review to rule out underlying causes.
Obese class 1 adult — meets Tier 2 referral threshold
Male, 52 years, height 175 cm, weight 95 kg. Squared height: 3.0625. BMI = 95 / 3.0625 = 31.0.
BMI 31.0 — Obese class 1. Eligible for NHS Tier 2 weight management referral; bariatric surgery if BMI ≥35 with significant comorbidity such as type 2 diabetes.
South Asian adult — adjusted thresholds change the category
Female, 38 years, British Indian heritage, height 160 cm, weight 62 kg. Squared height: 2.56. BMI = 62 / 2.56 = 24.2.
BMI 24.2 — Healthy under standard thresholds, but Overweight under NICE adjusted thresholds for South Asian adults (≥23). GP would discuss cardiometabolic risk.
When BMI Is Useful — and When It Is Not
Use this BMI calculator before a routine GP appointment, NHS Health Check (offered to adults aged 40 to 74 in England every five years), or occupational health assessment. Having your number ready saves time in the consultation and lets the clinician focus on context — your blood pressure, family history, waist circumference and lifestyle — rather than the basics of measurement.
Use it to prepare for entry into an NHS Tier 2 behavioural weight management service. Most local Tier 2 programmes commissioned by integrated care boards in England, and equivalent services delivered through Public Health Wales, Public Health Scotland and the Public Health Agency in Northern Ireland, set a starting BMI threshold of 30 (or 27.5 for adults from higher-risk ethnic backgrounds). Knowing your BMI clarifies whether you meet the entry criteria.
Use it before applying for NHS bariatric surgery assessment. NICE guidance NG7 sets the thresholds at BMI ≥40, or BMI ≥35 with a significant obesity-related comorbidity such as type 2 diabetes, hypertension, obstructive sleep apnoea or severe joint disease. Adults from South Asian, Chinese, Black African and African-Caribbean backgrounds are assessed at lower BMI thresholds.
Do not use this calculator if you are pregnant, under 18, an elite athlete with high muscle mass, frail and over 65, or living with a condition that affects body composition such as oedema, ascites or significant muscle wasting. In each of those cases the BMI number is misleading. Children and teenagers should be assessed using BMI centile charts published by the Royal College of Paediatrics and Child Health (RCPCH), not adult cutoffs.
BMI Categories: Underweight to Obese
BMI vs Body Fat Percentage vs Waist-to-Height Ratio
| BMI Range | Standard NHS Category | NICE Adjusted Category (South Asian, Chinese, Black African, African-Caribbean, Middle Eastern adults) | Typical NHS Pathway |
|---|---|---|---|
| Below 18.5 | Underweight | Underweight | GP review to investigate cause |
| 18.5 – 22.9 | Healthy weight | Healthy weight | Standard preventive advice |
| 23.0 – 24.9 | Healthy weight | Overweight (increased risk) | Lifestyle conversation if adjusted thresholds apply |
| 25.0 – 27.4 | Overweight | Overweight (increased risk) | NHS Better Health, Weight Loss Plan app |
| 27.5 – 29.9 | Overweight | Obese (high risk) | Tier 2 referral if adjusted thresholds apply |
| 30.0 – 34.9 | Obese class 1 | Obese (high risk) | Tier 2 weight management referral |
| 35.0 – 39.9 | Obese class 2 | Obese (high risk) | Bariatric surgery if significant comorbidity |
| 40.0+ | Obese class 3 (severely obese) | Obese (very high risk) | Direct bariatric surgery assessment under NICE |
Tips for Interpreting Your BMI Result
- Measure first thing in the morning, after using the toilet, before eating or drinking. Body weight can vary by 1 to 2 kg across a single day.
- Use the same scales every time. Bathroom scales differ by up to 1 kg between models even when calibrated.
- Pair your BMI with a waist circumference reading. NICE recommends both for adults with BMI under 35; central fat carries higher cardiometabolic risk than fat distributed elsewhere.
- Calculate your waist-to-height ratio (WHtR) as a complementary check: keep your waist measurement to less than half your height (ratio under 0.5).
- Track BMI monthly, not daily. Daily fluctuation hides genuine trend.
- If you are of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean heritage, apply the lower NICE thresholds: 23 for overweight and 27.5 for obese.
- Ask your GP to record your BMI in your patient record so it appears in your NHS App.
- If your BMI is over 30, ask your GP about Tier 2 weight management referral or self-refer where local commissioning permits.
Why BMI Alone Can Be Misleading
- Entering height in metres instead of centimetres, producing a BMI in the thousands.
- Using imperial weight (stones and pounds) without converting to kilograms first.
- Forgetting to remove shoes before measuring height — adds 2 to 3 cm and shifts the BMI by 0.5 to 1.0 unit.
- Treating the standard 25 and 30 thresholds as universal when NICE has set lower ones (23 and 27.5) for several ethnic groups since 2013.
- Reading a single BMI number as a diagnosis. BMI is a screening tool. The categories signal increased statistical risk, not certainty.
- Applying adult BMI cutoffs to children or teenagers. Under-18s use age- and sex-specific centile charts from the RCPCH UK-WHO growth references.
- Ignoring high muscle mass. Rugby players, weightlifters and many manual workers carry BMIs in the overweight or obese range while having low body fat.
- Underestimating risk in older adults with sarcopenia, where a 'healthy' BMI can mask muscle loss and excess body fat.
BMI Calculator: Common Questions Answered
What is a healthy BMI range under NHS guidance?
A healthy BMI for adults is 18.5 to 24.9 under standard NHS categories. For adults of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean heritage, NICE recommends a healthy band of 18.5 to 22.9, with overweight starting at 23 and obese at 27.5.
Why does the NHS use lower BMI thresholds for some ethnic groups?
Population research shows adults of South Asian, Chinese, Black African and African-Caribbean heritage develop type 2 diabetes, hypertension and cardiovascular disease at lower BMI levels than white European adults. NICE guideline NG7 has recommended adjusted thresholds (23 overweight, 27.5 obese) since 2013 to flag increased cardiometabolic risk earlier.
Is BMI accurate for muscular adults or athletes?
No. BMI cannot tell muscle from fat. A rugby player or competitive weightlifter often has a BMI in the overweight or obese range while carrying low body fat. For these individuals BMI overestimates health risk. A waist circumference measurement, body fat percentage from DEXA, or waist-to-height ratio gives a more accurate picture.
How does BMI work for children and teenagers in the UK?
Adult BMI cutoffs do not apply to under-18s. The NHS and the Royal College of Paediatrics and Child Health (RCPCH) use age- and sex-specific BMI centile charts based on the UK-WHO growth references. A child between the 91st and 97th centile is overweight, and at or above the 98th centile is obese for clinical purposes.
What waist measurement does the NHS recommend?
NICE recommends keeping your waist circumference to less than half your height — a waist-to-height ratio (WHtR) under 0.5. For example, an adult 170 cm tall should aim for a waist under 85 cm. WHtR is now used alongside BMI in primary care to assess central adiposity, which carries higher cardiometabolic risk than fat stored on the hips or limbs.
Does my BMI determine if I qualify for NHS bariatric surgery?
BMI is the main eligibility criterion. NICE guidance sets thresholds of BMI ≥40, or BMI ≥35 with a significant obesity-related comorbidity such as type 2 diabetes, hypertension, severe sleep apnoea or significant joint disease. For adults from South Asian, Chinese, Black African and African-Caribbean backgrounds, lower BMI thresholds apply because of higher cardiometabolic risk.
What should I do if my BMI is in the obese category?
Book a GP appointment to discuss your overall health rather than the BMI number alone. The GP will measure blood pressure, may take blood for HbA1c and cholesterol, and ask about family history. Depending on the picture, they may refer you to an NHS Tier 2 weight management programme — a free 12-week behavioural service — or to a Tier 3 specialist multidisciplinary clinic if the situation is more complex.
Can I check my BMI on the NHS App?
Yes. If your GP practice has recorded a recent height and weight, your BMI may appear in the NHS App under your health record. You can also use the NHS BMI healthy weight calculator on nhs.uk for a quick check that follows the same six-band system used in this calculator.
Should pregnant women use a BMI calculator?
Pregnancy changes body weight and composition rapidly. Standard adult BMI categories do not apply during pregnancy. Midwives use pre-pregnancy or first-trimester BMI to plan antenatal care. If you are pregnant, ask your midwife to review your booking BMI rather than using a self-check tool during pregnancy.
Is BMI being phased out in the UK?
No, but it is increasingly used alongside other measures. NICE now recommends pairing BMI with waist-to-height ratio for adults under 35 BMI, and the Office for Health Improvement and Disparities (OHID) has emphasised that BMI is one input into a wider obesity assessment, not a stand-alone diagnosis. UK Health Security Agency (UKHSA) population surveillance still uses BMI bands to track obesity prevalence.
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