BMI Calculator — Body Mass Index (Metric & Imperial)

What is BMI?

Body Mass Index (BMI) is your weight divided by your height squared. It's a quick, free way to check whether your weight sits in a healthy range for your height — no special equipment, just two numbers.

Enter your height and weight above and the result updates instantly, with the category it falls into and the healthy weight range for your height.

BMI is a screening tool, not a diagnosis. It's meant for adults 20+, and it doesn't apply to children, pregnancy, or athletes (see Limitations).

The formula

$BMI = \frac{\text{weight (kg)}}{\text{height (m)}^2}$

With pounds and inches, multiply by 703:

$BMI = 703 \times \frac{\text{weight (lb)}}{\text{height (in)}^2}$

That's the whole calculation — divide weight by height-squared in consistent units. The 703 factor just rescales imperial units to the metric result.

Going deeper. BMI Prime is your BMI ÷ 25 — above 1.0 means above the healthy ceiling. The Ponderal Index (weight ÷ height³) behaves better at very short or tall heights, where BMI drifts.

BMI categories (WHO, adults)

Obesity is sometimes split into Class I (30–34.9), II (35–39.9), and III (≥40). For people of South/East Asian descent, risk rises earlier — many guidelines treat BMI ≥ 23 as overweight.

Limitations

BMI is a ratio of weight to height. It never sees what the weight is made of, so it can mislead:

• Muscle reads as excess. Athletes and lifters can score "overweight" with low body fat.
• It ignores fat location. Belly (visceral) fat is riskier than hip/thigh fat; BMI treats them the same.
• Age and sex shift things. Older adults lose muscle; women carry more fat than men at the same BMI.
• Not for everyone. Children, teens, pregnancy, and some ethnic groups need different cutoffs.

Better companions: waist circumference, waist-to-hip ratio, and body-fat measures like calipers, bioimpedance, or a DEXA scan.

FAQ

What's a "good" BMI? For most adults, 18.5–24.9.

Is BMI accurate? As a population screen, yes. For one person — especially a muscular one — pair it with a waist or body-fat measure.

Can I use it for kids? No. Children use age- and sex-based percentiles; ask a pediatrician.

Does BMI differ for men and women? Same formula and cutoffs; body composition differs, which is a limitation, not a separate formula.

Code

Compute BMI (metric) in your language of choice:

1=ROUND(weight_kg / (height_cm / 100)^2, 1)
2

1def bmi(kg, cm): return round(kg / (cm / 100) ** 2, 1)
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1const bmi = (kg, cm) => +(kg / (cm / 100) ** 2).toFixed(1);
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1double bmi(double kg, double cm) { return Math.round(kg / Math.pow(cm / 100, 2) * 10) / 10.0; }
2

1double Bmi(double kg, double cm) => Math.Round(kg / Math.Pow(cm / 100, 2), 1);
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1double bmi(double kg, double cm) { return std::round(kg / std::pow(cm / 100, 2) * 10) / 10; }
2

1func bmi(kg, cm float64) float64 { return math.Round(kg/math.Pow(cm/100, 2)*10) / 10 }
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1fn bmi(kg: f64, cm: f64) -> f64 { (kg / (cm / 100.0).powi(2) * 10.0).round() / 10.0 }
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1function bmi($kg, $cm) { return round($kg / ($cm / 100) ** 2, 1); }
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1def bmi(kg, cm) = (kg / (cm / 100.0)**2).round(1)
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1bmi <- function(kg, cm) round(kg / (cm / 100)^2, 1)
2

History

Belgian polymath Adolphe Quetelet devised the weight-over-height-squared ratio in the 1830s (the "Quetelet Index"). Physiologist Ancel Keys renamed it Body Mass Index in 1972 after showing it tracked body fat better than rival formulas. It's been the default population weight screen ever since.

References

• World Health Organization — Body mass index (BMI)
• U.S. CDC — About Adult BMI
• NIH / NHLBI — Calculate Your BMI