BMI Calculator

What is BMI Calculator?

Body Mass Index (BMI) is a number derived from your weight and height. It is widely used as a quick screening tool to identify possible weight categories that may be linked to health risk in adults. BMI does not measure body fat directly and does not distinguish muscle from fat, but it is simple, inexpensive, and standardized across countries. Health organizations such as the World Health Organization publish BMI ranges for adults: underweight, normal weight, overweight, and obesity classes. These cutoffs help researchers and clinicians compare populations. For individuals, BMI is only one signal—waist circumference, blood pressure, blood tests, activity level, and family history also matter. Our calculator applies the standard mathematical definitions so you can reproduce textbook and clinical examples exactly, using either metric or US customary inputs.

How It Works

You choose metric (kilograms and centimeters) or imperial (pounds and feet plus inches). For metric input, we convert height from centimeters to meters by dividing by 100, then compute BMI as weight divided by height squared. For imperial input, we combine feet and inches into total inches, then apply the standard conversion factor 703 multiplied by weight in pounds divided by height in inches squared. The result is shown to one decimal place and mapped to WHO adult categories for quick interpretation. A short disclaimer reminds you that BMI is not a diagnosis.

Formula

Metric (kg, m):  BMI = weight_kg / (height_m)²     where height_m = height_cm / 100

Imperial (lb, in):  BMI = 703 × weight_lb / (height_in)²     where height_in = feet×12 + inches

Example (metric): 70 kg, 175 cm → height_m = 1.75 → BMI = 70 / (1.75)² ≈ 22.9

Example (imperial): 154 lb, 5 ft 9 in → height_in = 69 → BMI = 703×154 / (69)² ≈ 22.7

Formula Explained

The metric form is the definition of BMI in SI units: kilograms per square meter. The imperial form exists because many people in the United States measure weight in pounds and height in feet and inches. The factor 703 is exactly (0.45359237 kg per lb) / (0.0254 m per in)² after simplifying, so that plugging pounds and inches into the formula yields the same BMI as converting to metric first. Squaring height in the denominator reflects the fact that body mass scales with volume (three dimensions) while the index normalizes for height in two dimensions in this simplified model—hence dividing by height squared.

Example

Person A: 80 kg, 180 cm height_m = 1.80 BMI = 80 / (1.80)² = 80 / 3.24 ≈ 24.7 → normal weight range Person B: 200 lb, 5 ft 8 in height_in = 5×12 + 8 = 68 BMI = 703 × 200 / (68)² = 140600 / 4624 ≈ 30.4 → obesity class I Always verify inputs; small measurement errors in height change BMI noticeably because height is squared.

Tips & Best Practices

✓Measure height without shoes and weight with minimal clothing for more consistent results.
✓If you are very muscular, BMI may be high even with low body fat—use clinical context.
✓For children and teens, BMI percentiles by age and sex are required—this tool uses adult cutoffs only.
✓Use the same unit system you are comfortable with; the math is equivalent when converted correctly.
✓Discuss sustained BMI changes or health goals with a qualified professional.

Common Use Cases

•Quick self-check of weight-for-height before a general checkup discussion
•Homework or coursework comparing BMI calculations in metric and imperial units
•Wellness programs that use BMI as one of several screening metrics
•Understanding how published WHO or CDC category boundaries map to your numbers
•Reproducing textbook examples with transparent formulas

Frequently Asked Questions

In metric units, BMI = weight in kilograms divided by height in meters squared: BMI = kg / m², where height in meters is centimeters divided by 100. In US customary units, BMI = 703 × weight in pounds / (height in inches)². Both give the same BMI when converted consistently.

We use standard WHO adult categories: underweight below 18.5, normal weight 18.5 to under 25, overweight 25 to under 30, and obesity from 30 upward (with classes I, II, and III at higher thresholds). These apply to adults; children and teens use age- and sex-specific charts.

BMI is a population-level screening tool. It may overestimate body fat in muscular people and underestimate it in some older adults. It does not measure body composition directly. For personal health decisions, consult a healthcare professional.

Use whichever you know best. The calculator converts internally using exact formulas. Metric uses kilograms and centimeters; imperial uses pounds and feet plus inches (total height in inches is feet×12 plus inches).

No. Results are for educational purposes only. Always seek professional medical advice for diagnosis, treatment, or diet planning.