Body Mass Index — BMI — is one of the most widely used health screening tools in the world. Doctors, nurses, and health organizations use it to quickly assess whether a person's weight is appropriate for their height. In this guide you will learn exactly how to compute it yourself, understand what your number means, and know its limitations.
What Is BMI?
BMI is a numerical value derived from a person's weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet as a simple way to describe the "average man" statistically. It was never originally designed as a clinical health tool — but over the past century, the medical community adopted it as a quick, low-cost screening method for weight-related health risks.
BMI does not directly measure body fat. It is a proxy — a number that correlates reasonably well with body fat percentage in population studies, even though it can be misleading for specific individuals. Understanding both its usefulness and its limits is essential for interpreting your result correctly.
The Formula
Height in meters — then squared (multiplied by itself)
Imperial version: BMI = 703 × Weight (lb) ÷ Height (in)²
Metric: weight in kg, height in meters (not centimeters). Convert cm to m by dividing by 100. Example: 170 cm = 1.70 m.
Imperial: weight in pounds, height in total inches. Example: 5 ft 8 in = 68 inches. Multiply by 703.
Step-by-Step Computation
Measure your weight and height accurately
Use a reliable scale for weight and measure height without shoes. For the most accurate result, measure in the morning before eating and wearing light clothing. Small measurement errors are magnified when squaring height — a 1 cm error in height can shift your BMI by 0.3–0.5 points.
Convert units if needed
If using metric: convert height from centimeters to meters by dividing by 100. If using imperial: convert height to total inches (feet × 12 + remaining inches) and ensure weight is in pounds.
Square your height
Multiply your height in meters (or inches) by itself. Example: 1.70 m × 1.70 m = 2.89 m². This is the denominator in the BMI formula.
Divide weight by height squared
Divide your weight (in kg) by the squared height (in m²). The result is your BMI — a unitless number typically between 15 and 40 for most adults.
Look up your BMI category
Compare your result to the WHO BMI classification table. Note that Asian populations may use slightly lower thresholds — more on this below.
Worked Examples
Example 1: Metric (Normal Weight)
Example 2: Metric (Overweight)
Example 3: Imperial (Underweight)
Example 4: Metric (Obese)
🧮 Quick BMI Calculator
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BMI Categories (WHO Classification)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Risk of malnutrition, osteoporosis, anemia |
| 18.5 – 24.9 | Normal weight | Lowest health risk for most adults |
| 25.0 – 29.9 | Overweight | Increased risk of cardiovascular disease |
| 30.0 – 34.9 | Obese Class I | High risk — diabetes, hypertension |
| 35.0 – 39.9 | Obese Class II | Very high risk — multiple comorbidities |
| 40.0 and above | Obese Class III (Morbid) | Extremely high risk — life-threatening |
Asian BMI Thresholds
The standard WHO BMI thresholds were developed primarily using data from Western populations. Research has consistently shown that people of Asian descent — including Filipinos, Chinese, Japanese, and Koreans — carry higher proportions of body fat at lower BMI values, and face increased health risks at lower BMI cutoffs.
| Category | WHO (Global) Threshold | WHO Asian Threshold |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Normal weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25.0 – 29.9 | 23.0 – 27.4 |
| Obese | 30.0 and above | 27.5 and above |
The Philippine Department of Health and many Filipino clinicians use the Asian BMI thresholds. If your BMI is between 23 and 27.4, you may be classified as overweight under these guidelines even though the standard WHO table would classify you as normal weight. Ask your doctor which threshold applies to your clinical assessment.
BMI for Children and Teens
BMI is calculated the same way for children and teens (ages 2–19) but it is interpreted differently. Instead of fixed cutoff numbers, children's BMI is compared to growth charts using percentiles — because healthy weight ranges vary significantly with age and sex during development.
| Percentile Range | Category |
|---|---|
| Below 5th percentile | Underweight |
| 5th to 84th percentile | Healthy weight |
| 85th to 94th percentile | Overweight |
| 95th percentile and above | Obese |
A child's BMI percentile tells you how their BMI compares to other children of the same age and sex. Our BMI Calculator supports pediatric BMI calculation using CDC percentile charts for children aged 2–19.
Limitations of BMI
BMI is a useful population-level screening tool but has well-documented limitations for individuals:
- Athletes and muscle mass: Muscle is denser than fat. A very muscular person may have a high BMI but very low body fat — classified as "overweight" despite being extremely fit.
- Older adults: As people age, muscle mass decreases and fat mass increases even if weight stays the same. An older person may have a "normal" BMI but unhealthy body fat distribution.
- Body fat distribution: BMI doesn't tell you where fat is stored. Abdominal (visceral) fat carries significantly higher health risk than fat stored in the hips and thighs.
- Sex differences: Women naturally carry more body fat than men at the same BMI. The same BMI value represents different health risks for men and women.
- Ethnic differences: Health risk thresholds differ across ethnic groups as described above.
- Pregnancy: BMI is not meaningful during pregnancy as weight gain is expected and healthy.
Waist circumference — measures abdominal fat directly. Risk increases above 88 cm (35 in) for women and 102 cm (40 in) for men.
Waist-to-hip ratio — compares waist to hip circumference, indicating fat distribution pattern.
Body fat percentage — measured by DEXA scan, hydrostatic weighing, or bioelectrical impedance — directly measures fat vs lean mass.
Frequently Asked Questions
What is a healthy BMI?
For adults using WHO global standards, a BMI between 18.5 and 24.9 is considered normal weight. For Asian populations including Filipinos, many clinicians use 18.5–22.9 as the normal range.
Can I have a normal BMI but still be unhealthy?
Yes. This is sometimes called "normal weight obesity" or "skinny fat" — having a normal BMI but an unhealthy proportion of body fat, often combined with low muscle mass. Waist circumference and body fat percentage give a more complete picture.
Does BMI change with age?
The BMI formula itself doesn't change, but the health implications shift. For older adults (65+), some research suggests that a slightly higher BMI (around 23–27) may be associated with better outcomes than the standard normal range.
How often should I check my BMI?
For most healthy adults, checking BMI once or twice a year as part of a general health review is sufficient. Day-to-day weight fluctuations from water and food don't meaningfully change your BMI.
1. Formula: BMI = Weight (kg) ÷ Height (m)² — or 703 × Weight (lbs) ÷ Height (in)²
2. Normal range (WHO global): 18.5 – 24.9
3. Asian/Filipino normal range: 18.5 – 22.9
4. BMI does NOT directly measure body fat — it's a proxy
5. Athletes may appear "overweight" by BMI despite being very healthy
6. For children, BMI is interpreted using age-and-sex-specific percentiles
7. Always use BMI alongside other measurements — not in isolation
⚖️ Try the Full BMI Calculator
Our BMI Calculator supports adults and children, metric and imperial, with CDC percentile charts for ages 2–19.
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