BMI Calculator

You’re signing up for a charity 10K, and the registration page asks for your BMI. Or maybe you’re at an annual physical and want a quick, objective way to track whether your weight is trending up or down as your routine changes. Body Mass Index (BMI) is a simple screening number that relates weight to height—useful for quick context, but not a full health diagnosis.

What Is BMI Calculator?

A helpful context fact: BMI is dimensionless, meaning it has no units—so a BMI of 24.0 means the same thing whether you measured in pounds/inches or kilograms/centimeters, as long as the formula is applied correctly.

The Formula (Metric and US)

BMI = weight_kg / height_m²  
- Convert height from centimeters to meters: height_m = height_cm / 100  
- Square height: height_m² = height_m × height_m  
- Divide weight by squared height

BMI = (weight_lbs / height_in²) × 703  
- Convert height to total inches: height_in = (feet × 12) + inches  
- Square height in inches: height_in² = height_in × height_in  
- Multiply by 703 to account for unit conversion

Once BMI is calculated, it’s typically rounded to one decimal place and mapped to a BMI category: - Underweight: BMI < 18.5 - Normal: 18.5 ≤ BMI < 25 - Overweight: 25 ≤ BMI < 30 - Obese: BMI ≥ 30

These are the adult categories used by CDC (Gold source above). Note: BMI interpretation differs for children/teens (percentiles) and may differ by clinical context.

Step-by-Step Examples (Real Numbers)

Example 1 (Metric): 77 kg, 178 cm 1) Convert height to meters: - height_m = 178 / 100 = 1.78 m 2) Square the height: - height_m² = 1.78 × 1.78 = 3.1684 3) Divide weight by squared height: - BMI = 77 / 3.1684 = 24.3 (rounded to one decimal) 4) Category: 24.3 falls in 18.5–24.9 → Normal

Example 2 (US): 170 lb, 5'0" (60 inches) 1) Convert height to total inches: - height_in = (5 × 12) + 0 = 60 in 2) Square the height: - height_in² = 60 × 60 = 3600 3) Apply the US BMI formula: - BMI = (170 / 3600) × 703 - BMI = 0.04722… × 703 = 33.2 (rounded) 4) Category: 33.2 → Obese

Example 3 (US): 150 lb, 5'7" (67 inches) 1) Total inches: - height_in = (5 × 12) + 7 = 67 in 2) Square height: - height_in² = 67 × 67 = 4489 3) Calculate BMI: - BMI = (150 / 4489) × 703 - BMI = 0.03342… × 703 = 23.5 (rounded) 4) Category: 23.5 → Normal

Example 4 (Metric): 92 kg, 165 cm 1) height_m = 165 / 100 = 1.65 m 2) height_m² = 1.65 × 1.65 = 2.7225 3) BMI = 92 / 2.7225 = 33.8 (rounded) 4) Category: 33.8 → Obese

Pro Tip: If you’re close to a category cutoff (like 24.9 vs 25.0), tiny measurement differences matter. Weighing yourself at the same time of day and measuring height accurately (no shoes, standing tall) makes BMI tracking more consistent.

Common Mistakes to Avoid

Common Mistake #2: Forgetting to convert feet/inches into total inches. The formula needs height in inches squared. For 5'7", height is 67 inches—not 5.7 or 57.

Common Mistake #3: Squaring the wrong value. BMI depends on height². People sometimes divide by height (not squared) or square the feet portion only. The correct step is: compute total height (meters or inches), then square it.

Common Mistake #4: Over-interpreting BMI as body fat percentage. BMI is a screening index, not a direct measure of body composition. A muscular athlete can have a high BMI without excess fat, while someone with low muscle mass may have a “normal” BMI but higher health risk. CDC explicitly notes BMI is a screening measure and should be considered alongside other assessments (Gold source above).

When to Use BMI (and When Not To)

Good times to use BMI: 1) Tracking weight trends during a lifestyle change (new training plan, diet shift, postpartum recovery) where the goal is consistent monitoring. 2) Getting a quick screening category before discussing weight-related risk factors with a clinician (blood pressure, lipids, glucose). 3) Comparing population-level benchmarks (e.g., workplace wellness reporting or public health targets). 4) Setting general health goals when you don’t have access to body fat testing.

Times to be cautious or use additional measures: - Highly muscular individuals (BMI may overestimate fatness). - Older adults with muscle loss (BMI may underestimate risk). - Pregnancy (weight gain is expected and BMI categories aren’t applied the same way). - Children and teens (BMI-for-age percentiles are used instead of adult cutoffs). The CDC provides separate guidance for youth BMI interpretation. (CDC, Gold: [source removed])

A practical alternative or add-on is waist circumference, which can better reflect abdominal fat in some cases; clinicians often combine multiple measures for risk assessment.

To wrap up: BMI is easy to calculate manually if you remember the formulas and unit conversions, but it’s faster to rely on an automatic calculation when switching between metric and US units or when you want consistent rounding and category labeling. Manual math is great for understanding what drives the number; automated calculation is best for quick, repeatable check-ins.

Authoritative Sources

This calculator uses formulas and reference data drawn from the following sources:

- CDC — Physical Activity - NIH — National Institute of Diabetes - NIH — National Heart, Lung, and Blood Institute

BMI = weight (kg) / height (m)²

Body Mass Index (BMI) is a ratio designed to scale body mass to stature so that people of different heights can be compared on a similar basis. The key idea is that body mass tends to increase roughly with body volume, while height is a linear measure. If two people have similar body proportions, volume (and therefore mass) grows approximately with the cube of height, but in practice adult body mass across populations correlates strongly with height squared rather than height cubed. Using height² in the denominator creates a simple, useful screening index that tracks body size relative to height without requiring direct body-fat measurement. That’s why the formula uses height squared: it normalizes weight by a two-dimensional scaling of stature, producing a number that is broadly comparable across adults.

In the metric version, weight is measured in kilograms (kg) and height is measured in meters (m). If you enter height in centimeters (cm), it must be converted to meters first: height (m) = height (cm) / 100. The calculator’s metric logic does exactly that, then computes BMI and rounds to one decimal place. The variables are: w = weight_kg (kg), h = height_cm/100 (m), and bmi = w/(h×h). The output also includes a category based on standard adult cutoffs: Underweight if BMI < 18.5, Normal if 18.5 ≤ BMI < 25, Overweight if 25 ≤ BMI < 30, and Obese if BMI ≥ 30.

BMI = weight (lb) / height (in)² × 703

In the imperial version, weight is in pounds (lb) and height is in inches (in). Because pounds and inches are not coherent SI units, a conversion factor is needed to make the imperial calculation match the metric definition. Starting from the metric BMI, substitute 1 lb = 0.45359237 kg and 1 in = 0.0254 m. Then BMI = (lb×0.45359237) / (in×0.0254)² = lb / in² × (0.45359237 / 0.00064516) ≈ lb / in² × 703.0696. The calculator uses the conventional rounded factor 703. Height can be entered as feet and inches, then converted to total inches: totalInches = feet×12 + inches.

Example 1 (metric): Suppose height_cm = 178 and weight_kg = 77. Convert height: h = 178/100 = 1.78 m. Square height: h² = 1.78×1.78 = 3.1684. Compute BMI: bmi = 77 / 3.1684 = 24.301… Round to one decimal: 24.3. Category: 24.3 is less than 25 and at least 18.5, so it is Normal.

Example 2 (imperial): Suppose feet = 5, inches = 10, weight_lbs = 170. Convert height to inches: totalInches = 5×12 + 10 = 60 + 10 = 70 in. Square height: 70² = 4900. Compute BMI: bmi = 170 / 4900 × 703 = 0.0346939… × 703 = 24.389… Round to one decimal: 24.4. Category: Normal.

You can also convert between systems directly if needed: weight_kg = weight_lb × 0.45359237 and height_cm = height_in × 2.54 (since 1 in = 2.54 cm). Using the same Example 2, weight_kg = 170×0.45359237 = 77.1107 kg, height_cm = 70×2.54 = 177.8 cm, which yields essentially the same BMI after rounding.

Limitations and edge cases matter. BMI is a screening tool, not a diagnostic of body fat or health; muscular individuals may be classified as overweight, and older adults with low muscle mass may appear “normal” despite higher fat percentage. It is also less reliable for children and teens because body composition changes with age; pediatric assessment typically uses BMI-for-age percentiles rather than adult cutoffs. Very short or very tall body proportions, pregnancy, and certain medical conditions can also reduce interpretability. Finally, rounding to one decimal can shift someone near a cutoff (for example, 24.95 rounds to 25.0), so if you’re close to a boundary, it’s worth looking at the unrounded value and considering other clinical measures (waist circumference, blood pressure, labs) alongside BMI.

• NIH — National Heart, Lung, and Blood Institute
• CDC — Healthy Weight
• Mayo Clinic
• World Health Organization
• CDC — Physical Activity