What are the assumptions of the IMC formula?

Adults 20 years or older. For children and teens, BMI is age- and sex-specific (percentile). Standard WHO BMI categories apply. Asian populations may need different cutoffs (per WHO 2004) — overweight at 23+, obese at 27.5+. Height and weight are accurate measurements (self-reported values reduce accuracy by ~1-2 BMI points on average). Standard US or metric units. The calculator converts automatically; mixing units will produce wrong results. The calculation is for screening, not diagnosis. A doctor should interpret BMI alongside other health markers.

What are the limitations of IMC?

BMI does not distinguish between muscle mass and fat mass. Athletes often have high BMI but low body fat. BMI does not account for fat distribution. Visceral (belly) fat is more dangerous than subcutaneous fat. BMI may misclassify older adults (who tend to lose muscle) and pregnant women. Ethnic differences in body composition mean standard cutoffs may not apply equally to all groups. BMI does not measure bone density, hydration, or metabolic health. Two people with the same BMI can have very different health profiles. It is a population-level screening tool, not a clinical diagnostic. Use it as a starting point, not a verdict. Pregnancy and lactation require adjusted weight gain targets, not BMI categories — use IOM 2009 guidelines instead. For very short adults (under 5 ft / 152 cm) or very tall adults (over 6 ft 4 in / 193 cm), BMI can be off by 1-2 points. Use of BMI in children requires CDC BMI-for-age growth charts, not the adult categories — interpretation changes significantly for ages 2-20. Waist circumference (men >40 in, women >35 in) is a complementary cardiometabolic risk marker that BMI alone does not capture.

Metodología

How we calculate BMI

Our methodology for the IMC calculator: the formula, step-by-step calculation, authoritative sources, and limitations. Reviewed quarterly.

Fórmula

BMI = weight (kg) / height (m)²

Paso a paso

1
Convert weight to kilograms (1 lb = 0.4536 kg).
2
Convert height to meters (1 in = 0.0254 m; 1 ft = 0.3048 m).
3
Square the height in meters (multiply it by itself).
4
Divide the weight in kilograms by the squared height.
5
The result is your BMI in kg/m². Compare to WHO categories: underweight <18.5, normal 18.5–24.9, overweight 25–29.9, obese ≥30.
6
For children and teens (ages 2–20), use the CDC BMI-for-age percentile calculator instead of the adult categories.

Fuentes autorizadas

Every claim on this page is backed by an authoritative source.

Supuestos

What we take to be true when applying this formula.

Adults 20 years or older. For children and teens, BMI is age- and sex-specific (percentile).
Standard WHO BMI categories apply. Asian populations may need different cutoffs (per WHO 2004) — overweight at 23+, obese at 27.5+.
Height and weight are accurate measurements (self-reported values reduce accuracy by ~1-2 BMI points on average).
Standard US or metric units. The calculator converts automatically; mixing units will produce wrong results.
The calculation is for screening, not diagnosis. A doctor should interpret BMI alongside other health markers.

Limitaciones

What this method does NOT capture.

BMI does not distinguish between muscle mass and fat mass. Athletes often have high BMI but low body fat.
BMI does not account for fat distribution. Visceral (belly) fat is more dangerous than subcutaneous fat.
BMI may misclassify older adults (who tend to lose muscle) and pregnant women.
Ethnic differences in body composition mean standard cutoffs may not apply equally to all groups.
BMI does not measure bone density, hydration, or metabolic health. Two people with the same BMI can have very different health profiles.
It is a population-level screening tool, not a clinical diagnostic. Use it as a starting point, not a verdict.
Pregnancy and lactation require adjusted weight gain targets, not BMI categories — use IOM 2009 guidelines instead.
For very short adults (under 5 ft / 152 cm) or very tall adults (over 6 ft 4 in / 193 cm), BMI can be off by 1-2 points.
Use of BMI in children requires CDC BMI-for-age growth charts, not the adult categories — interpretation changes significantly for ages 2-20.
Waist circumference (men >40 in, women >35 in) is a complementary cardiometabolic risk marker that BMI alone does not capture.

Nota editorial

Reviewed against WHO 2004 expert consultation and CDC 2023 guidelines. Includes pediatric, ethnic, clinical, pregnancy, and height-edge limitations. For personalized assessment, consult a physician or registered dietitian.

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Última revisión: 2026-06-15 • Reviewed by: CalcxApp editorial team