BMI calculator with the healthy weight range for your height
Enter your height and weight in whichever units you think in — centimetres and kilograms, feet and inches, stones and pounds — and get your body mass index, the World Health Organization category it falls in, and the weight range that would put you in the healthy band for your height. The arithmetic is shown so you can check it.
How to use the BMI Calculator
- Choose metric or imperial so a bare number is read correctly.
- Type your height — “175”, “1.75 m” and “5ft 9in” all work.
- Type your weight — “70 kg”, “11st 4lb” or “160 lb”.
- Press Calculate BMI to see the index, the category and your healthy weight range.
The formula
Body mass index is weight in kilograms divided by height in metres squared:
BMI = kg ÷ m²
Worked example: someone who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9, which sits in the healthy band.
If you prefer imperial units there is an equivalent form that avoids converting first: BMI = 703 × pounds ÷ inches². The 703 is simply the conversion factor 0.45359237 kg per pound divided by 0.0254 m per inch squared. A person of 5ft 9in (69 in) weighing 160 lb gets 703 × 160 ÷ 4,761 = 23.6.
WHO categories for adults
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight (pre-obesity) |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and over | Obesity class III |
Healthy weight range for a height
Rearranging the formula gives the weight band that corresponds to a BMI of 18.5 to 24.9 at your height: weight = BMI × m². At 1.75 m that is 18.5 × 3.0625 = 56.7 kg at the bottom and 24.9 × 3.0625 = 76.3 kg at the top — a range of nearly 20 kg, which is why a single "ideal weight" number is misleading.
Where BMI stops being useful
BMI is a screening ratio, not a measurement of body fat, and it was designed to describe populations rather than individuals. It cannot tell muscle from fat, so a trained athlete or a heavy manual worker can score in the overweight or obese range with very little body fat. It is not valid for children and teenagers, who are assessed against age-and-sex BMI centile charts instead, nor during pregnancy. In older adults, height loss and reduced muscle mass both distort the result.
Family background matters too: UK guidance from NICE advises lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean family background, using BMI 23.0 to signal increased risk and 27.5 for high risk, because cardiometabolic risk rises at a lower BMI in these groups. NICE also suggests using waist-to-height ratio alongside BMI for anyone with a BMI under 35 — keeping your waist to less than half your height is the simple version of that check. Treat this calculator as a starting point for a conversation with a clinician, not a diagnosis.
Frequently asked questions
What is a healthy BMI?
For most adults the WHO healthy band is 18.5 to 24.9. Thresholds are lower for some ethnic groups — UK guidance uses 23.0 as the increased-risk point for people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean family background.
Does BMI work for athletes?
Not well. Muscle is denser than fat, so a muscular person can have a BMI in the overweight range with low body fat. Body-fat measurement or waist circumference is a better guide.
Can I use this for a child?
No. Children and teenagers are assessed with BMI-for-age centile charts that take sex and age into account, not the adult categories.
How do I convert stones and pounds to kilograms?
One stone is 6.35029318 kg and one pound is 0.45359237 kg, so 11st 4lb is (11 × 14 + 4) × 0.45359237 = 71.7 kg. The calculator accepts “11st 4lb” directly.
Privacy
This tool runs entirely inside your browser using WebAssembly and the Canvas/File APIs. Your files are never uploaded to ToolFlint or any third party; you can verify this in your browser's network tab or by switching to airplane mode after the page loads. Read how we process files.
Last updated 2026-09-23.