BMI Calculator — How to Calculate Your Body Mass Index
Learn how to calculate BMI, what your result means, and the limitations of BMI as a health metric. Use our free BMI calculator for instant results.
Body Mass Index (BMI) is one of the most widely used screening tools for assessing weight relative to height. Understanding your BMI takes 30 seconds — and understanding what the number actually means takes a few more minutes.
What is BMI?
BMI is a numerical value calculated from your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s as a population-level statistical tool, not as a diagnostic measure for individuals.
Today it is used by doctors, researchers, and health organizations as a quick screening indicator — not a complete picture of health.
How to calculate BMI
Metric formula:
BMI = weight (kg) ÷ height (m)²
Imperial formula:
BMI = (weight (lbs) × 703) ÷ height (inches)²
Example (metric): A person weighing 70 kg and 1.75 m tall:
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
Use our BMI Calculator to get your result instantly without doing the math manually.
BMI categories (WHO standard)
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Obese (Class III) |
These ranges apply to adults 18 and older. Different thresholds apply to children and teenagers (age and sex-specific percentiles are used instead).
BMI by age and sex
The standard BMI categories above apply to adults of all ages and both sexes. However, the health implications of a given BMI value can differ:
- Older adults (65+) — research suggests a BMI of 25–27 may be associated with lower mortality risk in older populations compared to younger adults
- Women tend to have more body fat than men at the same BMI
- Athletes and muscular individuals — BMI overestimates body fat because muscle is denser than fat
What a "healthy" BMI actually means
A BMI in the normal range (18.5–24.9) does not guarantee good health, and a BMI outside this range does not guarantee poor health. BMI is a screening tool, not a diagnosis.
Factors BMI does not account for:
- Muscle mass — a bodybuilder may have a BMI of 28 and very low body fat
- Fat distribution — where fat is stored matters more than total amount (visceral fat around organs is higher risk than subcutaneous fat)
- Bone density — heavier bone structure increases BMI without health risk
- Age-related changes — muscle mass naturally decreases with age
- Ethnicity — some Asian populations have higher health risks at lower BMI values; some organizations recommend lower thresholds for these groups
BMI limitations — what health experts say
The American Medical Association officially recognized in 2023 that BMI has significant limitations as a health measurement and should not be used alone to assess health status.
BMI was designed as a tool for studying populations, not diagnosing individuals. For a more complete picture of health, consider also measuring:
- Waist circumference (risk increases above 88 cm / 35 in for women, 102 cm / 40 in for men)
- Waist-to-height ratio (aim for below 0.5)
- Body fat percentage (via DEXA scan, hydrostatic weighing, or bioelectrical impedance)
- Blood pressure, cholesterol, and blood glucose
BMI for children and teens
For people under 18, BMI is calculated the same way but interpreted differently. Instead of fixed categories, the result is compared against age and sex-specific growth charts as a BMI-for-age percentile:
| Percentile | Category |
|---|---|
| Below 5th | Underweight |
| 5th to 84th | Healthy weight |
| 85th to 94th | Overweight |
| 95th and above | Obese |
Related health calculators
Once you know your BMI, these calculators give you a more complete picture:
- BMR Calculator — calculate your Basal Metabolic Rate (calories your body burns at rest)
- Calorie Calculator — estimate your daily calorie needs based on activity level
- Age Calculator — calculate your exact age for health tracking
Frequently asked questions
What is a good BMI for my age? The standard healthy BMI range (18.5–24.9) applies to all adults regardless of age. However, for adults over 65, some research supports a slightly higher optimal range (23–27).
Can I have a high BMI and still be healthy? Yes. Athletes, bodybuilders, and people with high muscle mass often have BMIs in the overweight range while having low body fat and excellent health markers. BMI should always be considered alongside other health metrics.
What is morbid obesity? Morbid obesity (Class III obesity) is defined as a BMI of 40 or above. It is associated with significantly increased risk of type 2 diabetes, heart disease, sleep apnea, and other conditions.
Is BMI accurate for all ethnicities? No. Research shows that people of Asian descent have increased health risks at lower BMI values. The WHO recommends additional cutoff points for Asian populations: overweight at 23.0 and obese at 27.5.
Summary
BMI is a quick, useful starting point for understanding weight relative to height — but it is one data point among many. Use our BMI Calculator to find your number, understand which category you fall into, and use that as a starting point for a broader conversation about health rather than a final verdict.