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BMI Calculator
Enter your weight and height to calculate your BMI.
What is BMI?
Body Mass Index (BMI) is a simple screening measure that estimates body fatness from your weight and height. It doesn't measure body fat directly, but it correlates closely enough with more direct measures to be a useful, low-cost starting point for tracking weight status over time.
Adult BMI classification (WHO)
| Category | BMI range |
|---|---|
| Severe Thinness | Below 16 |
| Moderate Thinness | 16 – 17 |
| Mild Thinness | 17 – 18.5 |
| Normal | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese Class I | 30 – 35 |
| Obese Class II | 35 – 40 |
| Obese Class III | Above 40 |
Source: World Health Organization (WHO) BMI classification.
BMI for children and teens (2–20 years)
For children and teens, a single set of fixed BMI ranges doesn't work, growing bodies change shape at different rates by age and sex. Instead, the CDC plots BMI against age- and sex-specific percentiles from a reference population:
| Category | Percentile |
|---|---|
| Underweight | Below 5th percentile |
| Healthy weight | 5th – 85th percentile |
| At risk of overweight | 85th – 95th percentile |
| Overweight | Above 95th percentile |
Source: CDC BMI-for-age growth charts.
How BMI is calculated
Metric formula: BMI = weight (kg) / height (m)²
US formula: BMI = 703 × weight (lb) / height (in)²
Worked example
Metric: a person weighing 70 kg at 175 cm (1.75 m) tall: 70 / (1.75 × 1.75) = 70 / 3.0625 ≈ 22.9.
US units: a person weighing 160 lb at 5'8" (68 in) tall: 703 × 160 / (68 × 68) = 112,480 / 4,624 ≈ 24.3.
BMI Prime
BMI Prime expresses BMI as a ratio to the upper bound of the normal range (25), rather than as an absolute number: BMI Prime = BMI / 25. A BMI Prime of 1.0 sits exactly at the normal/overweight boundary, values below 1.0 fall within the normal range, and values above 1.0 exceed it. For the 22.9 example above, BMI Prime is 22.9 / 25 ≈ 0.92.
BMI and health risk
Population-level research associates BMI outside the normal range with a higher likelihood of certain health conditions. At the lower end, a BMI under 18.5 is associated with nutritional deficiency and weakened immune function. At the higher end, a BMI over 25, and more so over 30, is associated with a higher likelihood of conditions such as type 2 diabetes, cardiovascular disease, and joint strain. These are statistical associations across populations, not individual diagnoses, and BMI alone does not determine any one person's health status.
Sources: CDC, WHO.
Limitations
Adults: BMI doesn't distinguish muscle from fat, so muscular or highly athletic individuals can register as overweight despite low body fat. It also doesn't account for fat distribution, age-related muscle loss, or differences across ethnic groups, and shouldn't be used alone to assess an individual's health.
Children and teens: because body composition changes continuously through growth, a raw BMI number means little on its own for this age group. It has to be read against the age- and sex-specific percentile charts above, not the adult category table.
Frequently asked questions
Is BMI accurate for muscular or athletic people?
Less so. BMI can't tell muscle from fat, so a muscular person may show as overweight despite low body fat. See the limitations above for more on this.
How often should I check my BMI?
For most people, every few months is enough to spot a trend, BMI changes slowly and daily checks mostly reflect water weight, not real change.
What's the difference between BMI and body fat percentage?
BMI is a ratio of weight to height and says nothing about what that weight is made of. Body fat percentage measures the actual proportion of fat versus lean mass and is more precise — try our Body Fat Calculator for a measurement-based estimate.