Advanced BMI Calculator for Adults, Children & Pregnancy

Body Mass Index (BMI) Calculator

A comprehensive tool for assessing weight status in adults, children, and during pregnancy.

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What is Body Mass Index?

Body Mass Index (BMI) is a simple calculation using a person's height and weight. The formula is BMI = kg/m², where kg is a person's weight in kilograms and m² is their height in metres squared.

It is a widely used, inexpensive tool to estimate body fat and screen for weight categories that may lead to health problems. However, it does not diagnose body fatness or the health of an individual directly.

Formula and Example

Metric: Metric Formula: BMI = Weight (kg) / [Height (m)]²

Imperial: Imperial Formula: BMI = [Weight (lbs) / (Height (in))²] x 703

BMI Categories for Adults

For adults 20 years old and older, BMI is interpreted using standard weight status categories (as per WHO). These categories are:

BMI RangeWeight Status
<16Severe Thinness
16 - 16.9Moderate Thinness
17 - 18.4Mild Thinness
18.5 - 24.9Normal Weight
25 - 29.9Overweight
30 - 34.9Obese Class I
35 - 39.9Obese Class II
≥40Obese Class III (Morbid Obesity)

BMI Use in Pregnancy

During pregnancy, a woman's BMI is NOT used to assess current weight status. Instead, her pre-pregnancy BMI is used to determine the recommended amount of weight she should gain during pregnancy to ensure a healthy outcome for both mother and baby.

Pre-pregnancy BMIRecommended Weight Gain
Underweight (<18.5)28–40 lbs (13–18 kg)
Normal (18.5–24.9)25–35 lbs (11–16 kg)
Overweight (25–29.9)15–25 lbs (7–11 kg)
Obese (≥30)11–20 lbs (5–9 kg)

BMI for Children & Teens

For children and teens (aged 2-19), BMI is interpreted differently. It is calculated using the same formula, but the result is then plotted on a BMI-for-age percentile chart.

Medical Warnings & Limitations

While BMI is a useful screening tool, it is not a perfect measure of health. It has several limitations:

  • It does not distinguish between fat and muscle mass. A very muscular person may have a high BMI but low body fat.
  • It does not account for the distribution of body fat (e.g., abdominal fat vs. peripheral fat), which is a key risk factor.
  • It may not be accurate for certain ethnic groups, body builders, or the elderly.

Frequently Asked Questions

Yes, for adults, the BMI categories are the same for both men and women.

A high BMI is associated with an increased risk for numerous health conditions, including type 2 diabetes, heart disease, high blood pressure, and certain types of cancer.

Yes. A person can have a normal BMI but still have excess visceral fat or other metabolic risk factors. BMI is one piece of a larger health assessment.

For most adults, checking once a year during a routine check-up is sufficient unless you are on a specific weight management program.

Both are important. Waist circumference provides information about abdominal fat, which is a strong independent predictor of health risk. They are best used together.

Consult a healthcare professional. They can perform a comprehensive assessment and recommend a personalized plan that may include dietary changes, exercise, and other interventions.

Official Sources & Further Reading