BMI Calculator
Work out your body mass index from height and weight — in metric (cm/kg) or imperial (ft+in/lb), with the WHO category band.
Healthy weight
24.5 BMI
BMI 18.5 – 24.9
A BMI in this range is associated with the lowest population-level health risk for most adults — but it is a single rough indicator, not a diagnosis.
Results update as you type. BMI is a population-level screening tool, not a diagnosis. Athletes, pregnant people, the very tall or very short, and people with very high muscle mass should treat the result with caution.
Formula
BMI is one division:
BMI = weight_kg / (height_m × height_m).
The imperial form folds in the lb→kg and inches→metres conversions:
BMI = (weight_lb × 703) / (height_in × height_in).
Both give the same number for the same person — only the input units change.
When this calculator helps
BMI is the quickest way to place your weight on a recognised scale, and it is the same screening figure your GP surgery starts with. Reach for this calculator if you want a baseline before a health check, you are tracking progress on a weight-loss or fitness plan, or a leaflet or app has quoted a number and you want to see where you actually sit. It works in either system, so you can enter your height in metres or centimetres and your weight in kilograms, or stick with feet, inches, stones and pounds if that is how you think.
It is aimed at adults aged 18 and over. For most people in the UK it gives a sensible first read on whether weight is in a range linked to lower long-term health risk — but it is a starting point for a conversation, not the conversation itself. If the number surprises you, that is a prompt to look at waist measurement, activity and diet, and to raise it with a GP or practice nurse rather than to act on the figure alone.
How to read your result
The NHS uses the WHO bands, so your number falls into one of four broad groups. Under 18.5 is underweight, which can signal under-nutrition or an underlying condition. From 18.5 to 24.9 is the healthy-weight range, linked in population studies with the lowest all-cause risk. From 25 to 29.9 is overweight, and 30 or above is obese, which clinicians split further into Class I, II and III as risk rises.
Think of the band as a flag, not a verdict. Sitting at 26 does not mean you are unwell, and sitting at 23 does not guarantee good health — it depends on your muscle, your waist, your blood pressure and your blood-sugar numbers. The bands describe statistical risk across large groups, so use yours to decide whether a closer look is worthwhile.
A worked example
Take an adult who is 1.75 m tall (about 5 ft 9 in) and weighs 75 kg, which is roughly 11 stone 11 lb. The sum is 75 divided by 1.75 squared, which is 75 divided by 3.0625, giving a BMI of about 24.5 — just inside the healthy-weight band. If the same person weighed 12 stone 8 lb (around 80 kg), their BMI would climb to about 26.1, nudging them into the overweight band even though the change is well under a stone. Small weight shifts move the number more than people expect, which is why a single reading matters less than the trend over time.
Common mistakes to avoid
BMI is simple, but a few habits make it misleading. Keep these in mind before you read too much into the figure.
- Treating it as a body-fat measurement — BMI cannot tell muscle from fat, so a well-trained rugby player and a sedentary person of the same height and weight get the same score.
- Ignoring fat distribution — a waist over 94 cm for men or 80 cm for women flags risk even when BMI looks fine, because visceral fat is the harmful kind.
- Using adult bands for under-18s — children are assessed on age-and-sex centile charts, not these thresholds.
- Overlooking ethnicity — for adults of South Asian, Chinese, and Black African heritage the NHS lowers the cut-offs, so the standard bands understate risk.
UK health guidance and thresholds
In the UK the NHS treats BMI as a screening tool that sits alongside a waist measurement and your wider health picture, not as a diagnosis on its own. NHS guidance lowers the obesity thresholds for people of South Asian, Chinese, Black African, and African-Caribbean family background, where cardiometabolic risk rises at a lower BMI: the overweight flag effectively starts nearer 23 and the obese band from about 27.5, with diabetes screening recommended from a BMI of 23. If that applies to you, read your result against the lower bands.
Whatever your number, the useful next steps are the same — check your waist, look honestly at activity and diet, and book a free NHS Health Check if you are eligible (offered to adults aged 40 to 74 in England). For any weight that is causing concern, or a figure at either extreme of the scale, speak to a GP or practice nurse who can interpret it in the context of your blood pressure, blood sugar and history.
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Frequently asked questions
How is BMI calculated?▾
BMI is your weight in kilograms divided by your height in metres squared (kg/m²). For 75 kg at 1.75 m: 75 ÷ (1.75 × 1.75) = 24.5. The same formula in imperial uses a 703 multiplier: BMI = (weight in lb × 703) / (height in inches)². Either way the answer is the same number — only the inputs change.
What are the BMI categories the NHS uses?▾
The NHS uses the WHO bands: under 18.5 = underweight; 18.5–24.9 = healthy weight; 25–29.9 = overweight; 30 and above = obese. Obese is sometimes broken down further: 30–34.9 (Class I), 35–39.9 (Class II), 40+ (Class III). Some NHS guidance lowers the thresholds for South Asian, Chinese, and Black African heritage adults — Class I starts at 27.5 and screening for diabetes from 23 — because cardiometabolic risk rises at lower BMI in those groups.
Is BMI accurate?▾
It is a rough population-level screening tool, not a diagnosis. BMI cannot tell muscle from fat (a 100 kg rugby player and a 100 kg sedentary person at the same height get the same BMI), it doesn't account for fat distribution (visceral fat is the harmful kind, not subcutaneous), and it under- or over-estimates risk in athletes, the elderly, the pregnant, and people who are very tall or very short. Use it as one signal alongside waist measurement, blood pressure, blood-sugar tests, and how you feel.
Why is healthy BMI 18.5–24.9?▾
Population studies (originally Quetelet in 1832, refined through 20th-century actuarial and WHO data) link the lowest all-cause mortality rates with this band. Below 18.5 mortality rises (often from underlying illness or under-nutrition); above 25 it rises gradually; above 30 it rises more sharply. The exact 'healthy' band is a statistical convention, not a sharp clinical cliff.
Should children use this calculator?▾
No — BMI for under-18s is interpreted using age-and-sex centile charts, not adult bands. The NHS Healthy Weight calculator handles paediatric BMI properly. Use this calculator for adults aged 18+.
What about waist measurement?▾
Waist circumference is a useful complement to BMI because it captures fat distribution. NHS guidance: a waist over 94 cm (37 in) for men or 80 cm (32 in) for women indicates increased health risk regardless of BMI; over 102 cm (40 in) men / 88 cm (35 in) women indicates high risk. Lower cut-offs apply for South Asian heritage.