What is BMI?
Body Mass Index, usually shortened to BMI, is a simple screening number that compares your weight with your height. Doctors, insurers and researchers use it as a quick first check because it needs only two measurements and works the same way for almost every adult. A BMI in the middle range is linked, across very large studies, with the lowest average risk of weight-related health problems, while values far above or below that range are linked with higher average risk. That is a population pattern, not a verdict on any one person.
BMI is deliberately simple, and that simplicity is both its strength and its weakness. It does not measure body fat directly, it cannot tell muscle from fat, and it says nothing about where fat is carried. A trained athlete may have an overweight BMI with very little body fat, and an older adult may have a normal BMI while carrying little muscle. Treat the number on this page as a starting point for a conversation, not an answer by itself. This calculator is for information only and is not medical advice.
The BMI formula
Divide your weight in kilograms by your height in metres, then divide by your height again. In imperial units the same value is 703 × weight (lb) ÷ height (in)2. This page converts imperial entries to metric first, so both systems give the same result for the same person. The ponderal index shown in the results uses height cubed instead of squared and is sometimes preferred for very tall or very short people.
Understanding the inputs and the extra numbers
Enter your real height without shoes and a recent, honest weight for the most useful result. The healthy weight range reverses the formula for your height: it is the weight range that would put you between BMI 18.5 and 24.9. BMI Prime expresses your BMI as a ratio of the top of the normal range, so 1.00 means exactly BMI 25 and 0.91 means about 9% below it. Small day-to-day weight changes of a kilogram or two are mostly water and food, so judge BMI by its trend over weeks, not by a single morning.
Worked example
A person who weighs 70 kg and is 175 cm tall has a height of 1.75 m. BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9, inside the normal range of 18.5 to 24.9. The healthy weight range for 175 cm is 18.5 × 3.0625 to 24.9 × 3.0625, about 56.7 to 76.3 kg. BMI Prime is 22.86 ÷ 25 = 0.91.
How to use this calculator
- Choose metric or imperial units.
- Enter your weight and your height without shoes.
- Read your BMI and category in the headline panel.
- Check the gauge and the healthy weight range for your height.
- Look at the WHO table below to see where your result sits among all categories.
Limitations of BMI and healthier context
BMI was designed for populations, not for judging individuals, and several groups should interpret it with extra care. It tends to overestimate body fat in muscular people and underestimate it in older adults who have lost muscle. It is not used in the same way for children, whose results are compared with growth charts by age and sex, and it should not be applied during pregnancy. Waist circumference, fitness, blood pressure, blood sugar and family history all add context that a single ratio cannot provide.
If your BMI is outside the normal range and you are unsure what, if anything, to do about it, the sensible next step is a conversation with a qualified health professional who knows your full picture. Small, steady habits, enough sleep, regular movement and a varied diet matter far more than chasing a precise number. Use this page for information only, not as medical advice or as a personal target set by a formula.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.