How to use the BMI calculator
- Choose Metric (kg, cm) or US (lb, ft/in) units.
- Enter your height and weight. Results update as you type.
- Read your BMI, your category and where you sit on the scale.
- Check the healthy weight range for your height.
Formula
What BMI measures
Body mass index compares your weight with your height to give one number that screens for weight categories linked to health risk. It’s quick, free and used by doctors worldwide as a starting point, not as a diagnosis.
How BMI is calculated
In metric units:
BMI = weight (kg) ÷ [height (m)]²
In US units, the constant 703 converts pounds and inches:
BMI = 703 × weight (lb) ÷ [height (in)]²
Worked example
A person who is 175 cm tall and weighs 70 kg:
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9, which falls in the healthy weight range.
For the same height, the healthy range runs from 18.5 × 3.0625 = 56.7 kg to 24.9 × 3.0625 = 76.3 kg.
Adult BMI categories (WHO)
| 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 and above | Obesity class III |
What BMI doesn’t tell you
- Body composition. Muscle is denser than fat; strength athletes often show a high BMI.
- Fat distribution. Fat around the waist carries more risk than fat elsewhere. A waist-to-height ratio below 0.5 is a useful companion check.
- Age and sex differences. Older adults tend to carry more fat at the same BMI.
- Pregnancy. BMI isn’t meaningful during pregnancy.
Next steps
If your BMI is outside the healthy range, a sensible next step is to understand your daily energy needs. The calorie calculator estimates how many calories maintain, lose or gain weight at your activity level. For personal advice, talk to a doctor or registered dietitian.
Frequently asked questions
What is a healthy BMI?
For adults, the World Health Organization classifies a BMI from 18.5 to 24.9 as a healthy weight, under 18.5 as underweight, 25 to 29.9 as overweight and 30 or above as obesity.
Is BMI accurate for athletes?
Not always. BMI cannot distinguish muscle from fat, so muscular people may be classed as overweight while being lean. Waist circumference or body-fat measurements give useful extra context.
Can children use this BMI calculator?
No. Children and teenagers are assessed with BMI-for-age percentiles that account for growth and sex. This calculator uses adult cut-offs only.
Do BMI cut-offs differ by ethnicity?
Some health bodies recommend lower thresholds for people of South Asian, Chinese and other Asian backgrounds, for example treating 23 or above as increased risk, because health risks appear at lower BMI levels.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value of 1.00 or less means you are at or below that limit.