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BMI Calculator

Calculate your Body Mass Index (BMI) to check if your weight is in the healthy range. Supports metric and imperial units with WHO category classification.

kg cm

How to Calculate BMI

BMI is a simple ratio of weight to height squared. It's used worldwide as a quick screening tool for weight categories.

BMI Formula

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = (weight (lbs) × 703) ÷ height (inches)²

Example: A person weighing 70 kg at 1.75 m tall: BMI = 70 / (1.75)² = 70 / 3.0625 = 22.9 (Normal weight)

BMI Categories (WHO)

  • Underweight: BMI < 18.5 — may indicate malnutrition or other health issues
  • Normal weight: BMI 18.5-24.9 — associated with lowest health risks
  • Overweight: BMI 25-29.9 — increased risk for cardiovascular disease
  • Obese Class I: BMI 30-34.9
  • Obese Class II: BMI 35-39.9
  • Obese Class III: BMI ≥ 40 — highest health risk category

Limitations of BMI

BMI is a population-level screening tool, not a diagnostic measure. It has several known limitations:

  • Muscle vs fat: Athletes and bodybuilders often have "overweight" BMIs despite low body fat
  • Age: Older adults tend to have more body fat at the same BMI as younger adults
  • Sex: Women typically carry more body fat than men at identical BMIs
  • Ethnicity: Health risks associated with BMI vary across ethnic groups. Some populations face higher risk at lower BMIs
  • Fat distribution: BMI doesn't indicate where fat is stored. Visceral (abdominal) fat is more dangerous than subcutaneous fat

For a more complete picture, consider using our Body Fat Calculator or TDEE Calculator alongside BMI.

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Frequently Asked Questions

What is BMI?

Body Mass Index (BMI) is a screening tool that estimates body fat based on height and weight. It is calculated as weight (kg) divided by height (m) squared.

What is a healthy BMI range?

WHO categories: Underweight < 18.5, Normal 18.5-24.9, Overweight 25-29.9, Obese ≥ 30.

Is BMI accurate for everyone?

BMI does not distinguish between muscle and fat. Athletes with high muscle mass may have a high BMI despite low body fat. It also does not account for age, sex, ethnicity, or fat distribution.

How do I calculate BMI?

Metric: BMI = weight (kg) / height (m)². Imperial: BMI = (weight (lbs) × 703) / height (inches)².

What BMI is considered obese?

A BMI of 30 or higher is classified as obese. Class I obesity is 30-34.9, Class II is 35-39.9, and Class III (severe) is 40+.

Should I use BMI or body fat percentage?

BMI is a quick screening tool. Body fat percentage (measured via calipers, DEXA, or our body fat calculator) gives a more accurate picture of body composition.