NHS BMI Calculator (UK)
The same Body Mass Index formula and healthy-weight thresholds (18.5–24.9) used by the NHS Healthy Weight tool — free, instant, with no sign-up.
Independent UK site. Not affiliated with the NHS. Uses identical WHO/NHS-standard methodology.
Select your gender
NHS BMI categories for adults
The NHS uses four main BMI bands for adults aged 20 and over.
BMI Below 18.5
Underweight
May indicate undernutrition. Ask your GP for advice on healthy weight gain.
BMI 18.5 – 24.9
Healthy weight
You are at the lowest weight-related health risk. Aim to stay in this range.
BMI 25 – 29.9
Overweight
Increased risk of type 2 diabetes, heart disease, and high blood pressure. Small daily changes work.
BMI 30 – 39.9
Obese
High risk. Speak to your GP about an NHS tier 2 weight-management referral.
BMI 40 or above
Severely obese
Very high risk. NHS tier 3 and tier 4 services (including bariatric surgery) may be appropriate.
NICE / NHS thresholds for South Asian, Chinese, and Black adults
NICE (the body that sets NHS guidelines) recommends lower BMI cut-offs for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean family background, because cardiometabolic risk rises at lower BMIs in these groups.
Increased risk (overweight)
BMI 23.0+
vs standard 25.0
High risk (obese)
BMI 27.5+
vs standard 30.0
Source: NICE Public Health Guideline PH46 (2013, revised). NHS clinicians use these thresholds in routine practice.
How to check your BMI the NHS way
Choose your units — metric or imperial
Toggle between kilograms/centimetres and stones-pounds/feet-inches. The NHS Healthy Weight tool offers both — so does this calculator.
Enter your height and weight
Type your real height and weight. Even a 1 cm error noticeably shifts BMI because the formula uses height squared.
Read your NHS BMI category
You'll see your BMI to one decimal place and the matching NHS category — underweight, healthy weight, overweight, or obese — with NHS-style guidance.
NHS weight-management pathway
If your BMI is above 25, the NHS offers a four-tier weight-management pathway through your GP:
Universal prevention — Healthier Families, school programmes, NHS Better Health app, GP-led lifestyle advice.
Community lifestyle interventions — 12-week behavioural programmes for BMI ≥25 (or ≥23 for Asian groups) with comorbidities.
Specialist multidisciplinary teams — dietitian, psychologist, physiotherapist, and consultant input for BMI ≥35 or ≥30 with comorbidities.
Bariatric surgery — gastric band, sleeve, or bypass for BMI ≥40 (or ≥35 with comorbidities) where tier 3 has not achieved the goal.
NHS BMI Calculator — UK FAQs
Quick answers to the most common questions
This calculator uses the exact same WHO/NHS-standard formula (weight in kilograms divided by height in metres squared) and the same healthy BMI range of 18.5 to 24.9. The result you see here will match the NHS Healthy Weight tool at nhs.uk. We are an independent UK site and not affiliated with the NHS — our tool simply uses the same evidence-based formula.
The NHS defines a healthy adult BMI as 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, 30 to 39.9 is obese, and 40 or above is severely obese. These cut-points are taken from the World Health Organization classification and apply to most adults aged 20 and over of any ethnicity, with some adjustments for South Asian populations.
The NHS uses the standard BMI formula: BMI = weight in kilograms ÷ (height in metres × height in metres). For imperial units it is BMI = (weight in pounds ÷ height in inches²) × 703. The NHS Healthy Weight calculator accepts both metric and stones/pounds, and so does this tool.
Yes. NICE (the body that advises the NHS) recommends lower BMI thresholds for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean family background, because they develop type 2 diabetes and heart disease at lower BMIs. The adjusted thresholds are: overweight at BMI 23, and obese at BMI 27.5. This calculator shows both the standard and ethnicity-adjusted bands where relevant.
BMI is still used for older adults but interpreted with care. After 65, slight increases in BMI (up to around 27) may not raise health risks and can actually be protective during illness. The NHS focuses more on waist circumference, muscle mass, and functional fitness in this age group rather than BMI alone.
BMI is not appropriate during pregnancy because pregnancy weight gain is healthy and expected. The NHS instead uses your pre-pregnancy BMI to set a target weight-gain range for the pregnancy. Use our dedicated pregnancy BMI calculator for evidence-based weight-gain ranges based on Institute of Medicine and NICE guidance.
BMI does not distinguish between muscle and fat, which is a known limitation that the NHS acknowledges. Athletes, weightlifters, and very muscular people may register as overweight on BMI but have low body fat. The NHS recommends pairing BMI with waist circumference for these cases — a waist measurement below half your height (in cm) is a good rule of thumb.
For a BMI of 25 to 29.9 (overweight), the NHS recommends gradual lifestyle changes: a balanced calorie deficit of 500 to 600 kcal per day, 150 minutes of moderate activity per week, and improved sleep. For a BMI of 30 or above, the NHS funds tier 2 community weight-management programmes through your GP, with tier 3 multidisciplinary services and tier 4 bariatric surgery available for higher BMIs and BMI-with-comorbidity scenarios.
The BMI formula and NHS healthy range (18.5–24.9) is the same for women and men. However, women naturally carry more body fat than men at the same BMI, and women over 50 lose muscle and gain fat more readily. The NHS recommends women also check waist circumference (under 80 cm is healthy, 80–88 is increased risk, over 88 cm is high risk) alongside BMI.
The NHS Healthy Weight calculator accepts metric (kilograms and centimetres) or imperial (stones, pounds, feet and inches). This calculator does the same and converts between all four units automatically, so you can enter your height in feet and weight in kilograms if you prefer.
Have another question? Browse our full article library or try a free calculator.
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