BMI Calculator
Enter your height and weight to calculate your Body Mass Index. Supports both metric and imperial units.
BMI is a screening tool, not a diagnostic measure. It does not directly assess body fat or health. Consult a healthcare provider for personalized health advice.
How the BMI Calculator Works
BMI (Body Mass Index) is a simple calculation using your height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and remains the most widely used screening tool for weight categories.
The formula
- Metric: BMI = weight (kg) ÷ height² (m²)
- Imperial: BMI = weight (lbs) ÷ height² (in²) × 703
Example (metric): A person who is 175 cm tall and weighs 70 kg: Height in meters = 1.75. BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.9 (Normal weight).
Example (imperial): A person who is 5'8" (68 inches) and weighs 150 lbs: BMI = 150 ÷ (68)² × 703 = 150 ÷ 4624 × 703 = 22.8 (Normal weight).
BMI categories
The World Health Organization (WHO) defines these standard BMI categories for adults:
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 16.0 | Severe thinness | High — malnutrition risk |
| 16.0 – 16.9 | Moderate thinness | Elevated |
| 17.0 – 18.4 | Mild thinness | Low — but monitor |
| 18.5 – 24.9 | Normal weight | Lowest risk range |
| 25.0 – 29.9 | Overweight (pre-obese) | Moderately increased |
| 30.0 – 34.9 | Obese Class I | High |
| 35.0 – 39.9 | Obese Class II | Very high |
| 40.0 and above | Obese Class III | Extremely high |
The calculator shows your BMI number, your category, and a visual indicator of where you fall on the scale.
Understanding Your BMI Result
What BMI does well
BMI correlates reasonably well with body fat for most people in large population studies. It's a quick, free, non-invasive screening tool that gives a general sense of whether your weight is in a healthy range for your height. Major health organizations (WHO, CDC, NIH) all use BMI as a first-line assessment.
What BMI gets wrong
- Muscle vs. fat. BMI can't tell the difference. A muscular athlete who is 5'10" and 200 lbs has a BMI of 28.7 (overweight) despite having very low body fat. If you lift weights regularly, your BMI may overestimate your health risk.
- Fat distribution. Belly fat (visceral fat) is more dangerous than fat stored elsewhere. Two people with the same BMI can have very different health risks depending on where they carry weight. Waist circumference captures this better.
- Age and sex. Women naturally have more body fat than men at the same BMI. Older adults lose muscle mass, so their BMI may underestimate body fat. Some researchers recommend age-adjusted BMI thresholds.
- Ethnicity. Asian populations tend to have higher health risks at lower BMI values. Some countries use adjusted thresholds: overweight starts at BMI 23 (not 25) for people of Asian descent.
Better alternatives and complements
For a more complete health picture, combine BMI with:
- Waist circumference — above 40" (men) or 35" (women) indicates elevated risk regardless of BMI
- Waist-to-hip ratio — above 0.90 (men) or 0.85 (women) indicates abdominal obesity
- Body fat percentage — measured by DEXA scan, bioelectrical impedance, or calipers
- Blood markers — cholesterol, blood sugar, blood pressure give direct health data
Worked Examples
Example 1: Average adult (imperial)
Jane is 5'4" (64 inches) and weighs 145 lbs. She wants to know her BMI:
- BMI = 145 ÷ (64)² × 703
- = 145 ÷ 4096 × 703
- = 24.9 — top of the Normal range
Jane is right at the boundary between Normal and Overweight. A gain of just 1-2 lbs would push her into the Overweight category.
Example 2: Athletic build (metric)
Marcus is 183 cm tall and weighs 95 kg. He works out 5 times a week and has visible muscle definition:
- BMI = 95 ÷ (1.83)²
- = 95 ÷ 3.3489
- = 28.4 — Overweight by BMI
Despite being classified as "overweight," Marcus likely has a healthy body fat percentage given his muscle mass. His waist measurement (32") is well within the healthy range, confirming that BMI overestimates his risk.
Example 3: Finding a target weight
Sarah is 5'6" (66 inches) and currently weighs 180 lbs (BMI 29.0 — Overweight). She wants to know what weight puts her at BMI 24.9 (top of Normal):
- Target weight = 24.9 × (66)² ÷ 703
- = 24.9 × 4356 ÷ 703
- = 154.4 lbs
Sarah would need to lose about 26 lbs to reach the top of the Normal BMI range. A safe rate of loss is 1-2 lbs per week, so this would take roughly 13-26 weeks.
Frequently Asked Questions
BMI = weight (kg) ÷ height² (m²). For imperial: weight (lbs) ÷ height² (in²) × 703. It estimates body fatness from height and weight. A BMI of 18.5-24.9 is normal weight.
It's a useful screening tool but has limitations. It can't distinguish muscle from fat, doesn't account for fat distribution, and doesn't consider age, sex, or ethnicity. Use it as a starting point, not a diagnosis.
Underweight: below 18.5. Normal: 18.5-24.9. Overweight: 25-29.9. Obese Class I: 30-34.9. Obese Class II: 35-39.9. Obese Class III: 40+. Based on WHO guidelines.
18.5 to 24.9 is the standard healthy range. Some research suggests the lowest mortality risk is at 22-25. Older adults (65+) may benefit from a slightly higher BMI (25-27).
Metric: divide kg by meters squared. 70 kg ÷ (1.75m)² = 22.9. Imperial: divide lbs by inches squared, multiply by 703. 150 lbs ÷ (65 in)² × 703 = 24.9.
Doesn't measure body composition — muscular people may be classified overweight. Doesn't account for fat distribution (belly fat is riskier). Age, sex, and ethnicity also affect the BMI-health relationship.
Waist circumference and waist-to-hip ratio are better predictors of health risk. Body fat percentage (DEXA, bioelectrical impedance) gives a more direct picture. Combining BMI with waist measurement is the most practical approach.
The calculation is the same, but results are interpreted using age-and-sex percentile charts. Overweight is the 85th-95th percentile; obese is 95th percentile or above. This calculator is designed for adults (18+).