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.
BMI in Real Life
Understanding how BMI applies to everyday situations helps you interpret your result:
What BMI numbers look like in real people
| Height | Weight | BMI | Category |
|---|---|---|---|
| 5'0" (152 cm) | 100 lbs (45 kg) | 19.5 | Normal |
| 5'4" (163 cm) | 130 lbs (59 kg) | 22.3 | Normal |
| 5'6" (168 cm) | 155 lbs (70 kg) | 25.0 | Overweight |
| 5'8" (173 cm) | 170 lbs (77 kg) | 25.8 | Overweight |
| 5'10" (178 cm) | 190 lbs (86 kg) | 27.3 | Overweight |
| 6'0" (183 cm) | 220 lbs (100 kg) | 29.8 | Overweight |
| 6'2" (188 cm) | 250 lbs (113 kg) | 32.0 | Obese Class I |
Notice how small weight changes at the boundary (like 154 vs 155 lbs at 5'6") can shift categories. This is why BMI should be one of several health indicators, not the only one.
Health risks by BMI range
Research shows these conditions become more likely as BMI increases:
- BMI 25-30 (Overweight): Slightly elevated risk of type 2 diabetes, high blood pressure, and joint problems
- BMI 30-35 (Obese Class I): Moderate risk of heart disease, diabetes, sleep apnea, and certain cancers
- BMI 35-40 (Obese Class II): High risk of multiple conditions, including cardiovascular disease and metabolic syndrome
- BMI 40+ (Obese Class III): Very high risk, often qualifying for medical intervention
However, these are population-level statistics. Individual risk depends on genetics, lifestyle, diet, exercise, and other factors.
When BMI gives misleading results
- Athletes and bodybuilders: A male powerlifter at 5'9" and 220 lbs has a BMI of 32.5 (obese), but may have 15% body fat — well within the healthy range
- Elderly adults: A 75-year-old woman at 5'4" and 125 lbs has a BMI of 21.5 (normal), but may have lost significant muscle mass and have high body fat percentage
- People with disabilities: Wheelchair users or amputees may have altered weight-height relationships that BMI doesn't capture accurately
- Pregnant and breastfeeding women: BMI calculations don't apply during pregnancy or while carrying extra weight from breastfeeding
How to improve your BMI assessment
Combine BMI with these simple measurements for a better picture:
- Waist circumference: Measure at the navel. Men: under 40" (102 cm). Women: under 35" (88 cm). Above these thresholds indicates elevated risk regardless of BMI
- Waist-to-height ratio: Divide waist by height. Should be under 0.5. Example: 32" waist ÷ 68" height = 0.47 (healthy)
- Body fat percentage: Use a smart scale with bioelectrical impedance or get a DEXA scan. Healthy ranges: Men 10-20%, Women 18-28%
- Blood work: Check cholesterol, blood sugar, and blood pressure. These direct measurements matter more than BMI alone
BMI Quick Reference
Common BMI values and what they mean for your health:
| BMI | Category | Health Risk | Typical Appearance |
|---|---|---|---|
| Below 16 | Severe thinness | High | Very underweight, possible malnutrition |
| 16-17 | Moderate thinness | Elevated | Noticeably underweight |
| 17-18.5 | Mild thinness | Low-moderate | Thin but often healthy |
| 18.5-22 | Normal (lower) | Lowest | Lean, often athletic |
| 22-25 | Normal (upper) | Low | Average build |
| 25-27 | Overweight (lower) | Moderate | Slightly heavy |
| 27-30 | Overweight (upper) | Moderate-high | Clearly overweight |
| 30-35 | Obese Class I | High | Obese |
| 35-40 | Obese Class II | Very high | Severely obese |
| 40+ | Obese Class III | Extremely high | Morbidly obese |
The lowest mortality risk in most studies is in the 22-25 BMI range. However, "healthy" varies by age, sex, and individual circumstances.
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+).