How the numbers are worked out

BMI is calculated as weight in kilograms divided by height in metres squared. If you enter feet, inches, stone, and pounds, the calculator converts those values into metric units first and then applies the same formula.

For most UK adults, the calculator uses the standard NHS-style BMI bands: underweight, healthy weight, overweight, and obese.

For people of South Asian, Chinese, Black African, and African-Caribbean family background, lower BMI cut-offs are used because health risk can rise at lower BMIs in those groups.

The healthy-weight range is calculated from the same BMI thresholds used for the selected background option. That means the displayed weight range and the category result are now based on the same rules.

The percentile feature is not part of the official NHS BMI calculator. It is an independent estimate that compares your BMI with published UK adult survey summaries.

The model uses summary statistics from the Health Survey for England and applies a normal-distribution approximation within each age and sex band.

In plain English: it is a useful directional comparison, but it is not an exact official percentile and should not be treated as one.

BMI is a screening tool, not a diagnosis.

It can be less useful for people with a lot of muscle, during pregnancy, and in some older adults.

This calculator is for adults aged 18 and over only. Children and teenagers need a different BMI method that adjusts for age and sex.

NHS adult BMI guidance: Calculate your body mass index (BMI) for adults

UK survey data: Health Survey for England