How is the BMI result rounded?

We round the BMI to one decimal place using standard rounding (for example, 24.95 becomes 25.0). The category is then read from the rounded number against the adult thresholds below.

Do metric and US units give the same BMI?

Yes. BMI is a single value, so converting your measurements between systems produces the same number apart from tiny rounding differences. The calculator uses the standard metric and US formulas shown on this page.

Where is my data processed?

All calculations run in your browser using JavaScript. Your height, weight, and age are never sent to a server. The optional BMI history is saved only in your own browser using local storage, and you can clear it at any time.

BMI Calculator Methodology

This page explains exactly how our BMI calculator works: the formulas, how we round, the adult category thresholds we use, how we handle children and teens, and how your data is processed. BMI is a screening measure, not a diagnosis, so we keep the method transparent and the interpretation conservative.

The BMI formulas

BMI compares your weight to your height. We use the standard formulas:

Metric: BMI = weight (kg) / (height (m) x height (m))
US customary: BMI = (weight (lb) / (height (in) x height (in))) x 703

For US height entered as feet and inches, we convert to total inches (feet x 12 + inches) before applying the formula. Heights given in centimetres are converted to metres (cm / 100).

Rounding

The result is rounded to one decimal place. We read your category from that rounded value, which is also how most public health tools present BMI.

Adult category thresholds (ages 20+)

For adults, we use the categories published by the U.S. Centers for Disease Control and Prevention (CDC):

Adult BMI category thresholds used by this site
Category	BMI range
Underweight	Below 18.5
Healthy Weight	18.5 - 24.9
Overweight	25.0 - 29.9
Obesity	30.0 and above
Class 1 Obesity	30.0 - 34.9
Class 2 Obesity	35.0 - 39.9
Class 3 Obesity	40.0 and above

These ranges are screening categories based on population data. A number inside or outside the healthy range is a signal to consider alongside other information, not a diagnosis.

Children and teens (ages 2-19)

Adult categories do not apply to anyone aged 2 to 19. Children and teens are still growing, so BMI for this group must be interpreted using age- and sex-specific percentiles from growth charts. Our tools can calculate the BMI number, but for an official category we direct families to the child BMI calculator guidance and the official CDC Child and Teen BMI Calculator. See also the BMI calculator by age.

Privacy model

Every calculation runs entirely in your browser. We do not receive, store, or sell the measurements you enter. If you use the BMI tracker, the history is kept only in your browser's local storage on your device, and you can delete individual entries or clear everything at any time.

Limitations

BMI does not measure body fat directly and does not account for muscle mass, fat distribution, age, or sex. It can over- or under-state body fat for some people. For a fuller picture, read our guide to BMI limitations and our comparison of BMI vs body fat percentage. For personal health questions, speak with a qualified healthcare provider.

How we research and review this content is described in our editorial policy.