How to Calculate Your BMI
- Select your preferred unit system — Metric (cm/kg) or Imperial (ft/in/lbs).
- Enter your height and weight.
- Your BMI, health category, and healthy weight range appear instantly.
The BMI Formula
BMI is calculated by dividing weight in kilograms by the square of height in meters:
BMI = weight (kg) ÷ height² (m²)
In imperial units: BMI = (weight (lbs) × 703) ÷ height² (inches²)
For example, a person who is 175 cm tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which falls in the normal weight range.
BMI Categories (WHO Standards)
- Severely Underweight — BMI below 16.0
- Underweight — BMI 16.0–18.4
- Normal weight — BMI 18.5–24.9
- Overweight — BMI 25.0–29.9
- Obesity Class I — BMI 30.0–34.9
- Obesity Class II — BMI 35.0–39.9
- Obesity Class III — BMI 40 and above
Limitations of BMI
BMI is a widely used screening tool, but it has important limitations you should be aware of:
- Doesn't distinguish muscle from fat. Athletes and bodybuilders often have a high BMI due to muscle mass, not excess body fat.
- Ignores fat distribution. Where body fat is stored (visceral vs. subcutaneous) matters significantly for health risk.
- Age and sex differences. Older adults typically have more fat at the same BMI than younger adults. Women generally have more body fat than men at the same BMI.
- Ethnic variation. Some health organizations use lower BMI cutoffs for Asian populations, as they face higher metabolic risk at lower BMIs.
Use BMI as a starting point for conversation with your healthcare provider, not as a definitive measure of health.
Frequently Asked Questions
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. However, healthy weight varies by age, sex, and ethnicity. Consult your healthcare provider for a complete assessment.
Is BMI accurate for everyone?
BMI is a useful screening tool but has limitations. It doesn't account for muscle mass, bone density, or fat distribution. Athletes often have a high BMI due to muscle, not fat. Older adults may have normal BMI but excess body fat.
What is the healthy weight range for my height?
The calculator shows your healthy weight range automatically. It is calculated by solving the BMI formula for weight at the lower (18.5) and upper (24.9) bounds of the normal BMI range for your height.
How is BMI different from body fat percentage?
BMI is derived only from height and weight, making it easy to calculate but imprecise. Body fat percentage directly measures how much of your body is fat vs. lean mass. It's more accurate but requires special equipment (DEXA scan, calipers, or BIA scale) to measure.