BMI Calculator
Enter your height and weight in metric or imperial units to get your BMI and where it sits in the standard adult ranges — underweight, healthy, overweight or obese. The calculator also shows the healthy weight band for your exact height, so you can see how far you are from the 18.5–24.9 zone in real terms.
How it works
BMI = weight in kilograms ÷ (height in metres)². Someone who is 5ft 9in (1.75 m) and 12st 8lb (80 kg) has a BMI of 80 ÷ 3.0625 = 26.1.
Standard adult categories (WHO): under 18.5 underweight · 18.5–24.9 healthy · 25–29.9 overweight · 30–34.9 obese (class I) · 35–39.9 obese (class II) · 40+ obese (class III).
Imperial entries are converted first: 1 stone = 6.35029 kg, 1 inch = 2.54 cm. So 12st 8lb is 12.57 stone × 6.35029 ≈ 79.8 kg, and 5ft 9in is 69 inches × 2.54 ≈ 175.3 cm — the calculator does these conversions automatically, which avoids the most common manual error of mixing unit systems mid-calculation.
Alongside the BMI figure you get the healthy weight band for your height, worked out by multiplying your height in metres squared by 18.5 and by 24.9. At 1.75 m the band runs from 18.5 × 3.0625 ≈ 56.7 kg to 24.9 × 3.0625 ≈ 76.3 kg — so the person in the example above (80 kg, BMI 26.1) is about 3.7 kg above the top of the healthy range, a far more usable number than the category label alone.
The categories are the WHO bands for adults, so the result is not designed for children or teenagers. The cut-offs are also not universal even among adults: NHS guidance sets a lower overweight threshold of 23 for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, because health risks rise at lower BMIs in these groups.
Because the bands have sharp cut-offs, a small change can move you between categories — at 1.75 m the difference between a BMI of 24.9 and 25.0 is roughly 0.3 kg. Treat the category as a signal, not a verdict: BMI is a screening measure that ignores muscle mass, age and waist size, so a muscular person can read as overweight with low body fat, and borderline results deserve a proper health check rather than a self-diagnosis.
FAQ
What is a healthy BMI in the UK?
For most adults a BMI of 18.5 to 24.9 is considered the healthy range. NHS guidance also notes that people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds face higher health risks at lower BMIs, with an overweight threshold of 23 rather than 25 — so the standard bands are not one-size-fits-all.
What weight should I be for my height?
Multiply your height in metres squared by 18.5 and 24.9 to get the healthy band. At 5ft 6in (1.68 m) that is 18.5 × 2.82 ≈ 52 kg (8st 3lb) to 24.9 × 2.82 ≈ 70 kg (11st), and the calculator shows this range automatically for the height you enter. Aim for somewhere inside the band rather than a single “ideal” number.
Is BMI accurate if I train with weights?
It can overstate fatness in muscular people: muscle is denser than fat, so a rugby player or regular lifter can have a BMI over 25 with low body fat. BMI is designed as a population screening tool, not an individual diagnosis — waist measurement, body-fat percentage and a GP’s assessment give a much better picture for athletic builds.
How do I calculate BMI myself?
Divide your weight in kilograms by your height in metres squared. At 1.68 m and 65 kg: 1.68 × 1.68 = 2.82, and 65 ÷ 2.82 ≈ 23.0 — inside the healthy 18.5–24.9 range. If you think in imperial, convert first (1 stone = 6.35029 kg, 1 inch = 2.54 cm) or simply let the calculator accept stones and feet directly.
What BMI counts as overweight or obese?
Under the WHO adult classification, 25–29.9 is overweight and 30 or above is obese, split into class I (30–34.9), class II (35–39.9) and class III (40+). Below 18.5 is classed as underweight — also outside the healthy range and just as worth discussing with a GP as a high reading.
Why has a small weight change moved me into a different category?
The bands have hard boundaries, so crossing 25.0 or 30.0 by a fraction changes the label without meaningfully changing anything else. At 1.75 m, one whole BMI point is about 3 kg, so a tenth of a point is roughly 300 g — well within ordinary day-to-day weight fluctuation. Watch the trend of your BMI over weeks rather than the category on any single day.