How to Calculate BMI and What the Number Means
Published 2026-11-02
Body mass index (BMI) is still the first number many clinics and searchers want: weight relative to height, in one figure. It is cheap to compute and easy to compare. It is also easy to over-read.
Use the free tool
Run these formulas with your own numbers. Math stays in your browser.
Open BMI Calculator →This page shows the adult formula, the standard categories, and why a high BMI is not the same as “too much fat.” Our BMI calculator does the arithmetic; the body-fat and ideal-weight tools add composition and other formulas.
Use the free BMI calculator
Enter height and weight in US or metric units. The tool returns BMI, the adult category and a healthy-weight range for that height. Nothing is uploaded.
BMI formula
Metric is direct. US customary uses pounds and inches with a constant so the result matches the metric definition.
BMI = kg ÷ (m)² or kg ÷ (cm ÷ 100)²
BMI = 703 × lb ÷ (in)²
Adult categories (WHO / CDC style)
These cutoffs are for non-pregnant adults. They are screening labels, not diagnoses.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0–34.9 | Obesity class I |
| 35.0–39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
Worked examples
Metric: 70 kg and 170 cm → 70 ÷ 1.70² = 70 ÷ 2.89 ≈ 24.2 (healthy weight).
US: 5 ft 10 in (70 in) and 170 lb → 703 × 170 ÷ 70² = 119,510 ÷ 4,900 ≈ 24.4.
When BMI is a poor fit
Athletes and people with more muscle can sit in “overweight” with a healthy waist and bloodwork. Older adults can sit in “healthy” while losing muscle. BMI does not know where fat sits — abdominal fat is the more useful clinical flag. A 5 ft 10 in adult at 170 lb is about 24.4; the same height at 200 lb is about 28.7 (overweight on the adult chart) even if the extra weight is mostly training mass. That is why the BMI calculator also shows a healthy-weight range for the height you typed — it is a band, not a target weight for every body.
Children and teens need age- and sex-specific percentiles, which this adult tool does not compute. Pregnancy changes weight for a reason; do not chase a pre-pregnancy BMI without a clinician. “BMI calculator” is a high-volume search because clinics and forms still ask for the number — treat it as a starting flag, then open the body-fat calculator if composition is the real question and the calorie calculator if the next question is intake.
- Use waist or the body-fat calculator when composition matters.
- Use the ideal-weight calculator to see Devine, Robinson, Miller and Hamwi side by side.
- Use the calorie calculator if the next question is “how much should I eat?”
Use the free tool
Run these formulas with your own numbers. Math stays in your browser.
Open BMI Calculator →Frequently Asked Questions
Is BMI the same in every country?
The formula is the same. Some Asian guidelines use lower overweight cutoffs. This site uses the common 18.5 / 25 / 30 adult chart.
Do I need to fast?
No. BMI is only height and weight.
Why 703 in the US formula?
It converts pounds and inches into the same units as kg/m².
Can BMI diagnose obesity-related disease?
No. It is a screening ratio. Blood pressure, lipids, glucose and waist add the clinical picture.
Is this medical advice?
No. Discuss your number with a clinician if you are unsure.