What is BMI and how is it calculated?

BMI stands for Body Mass Index. It is calculated by dividing your weight in kilograms by the square of your height in metres. A BMI of 18.5–24.9 is considered a healthy range for most adults.

Is BMI an accurate measure of body fat?

BMI is a screening estimate, not a direct measure of body fat. It does not account for muscle mass, bone density, age or fat distribution. Athletes may have a high BMI despite low body fat, and older adults may be at risk despite a normal BMI.

What BMI is considered healthy?

For most adults, a BMI between 18.5 and 24.9 is considered a healthy range. A BMI below 18.5 suggests underweight, 25–29.9 is overweight, and 30 or above falls in the obesity range.

Does BMI apply to children?

Standard adult BMI ranges do not apply to children. For children and teenagers, BMI is assessed differently using age- and sex-specific percentile charts. Consult a paediatrician for guidance.

Can I use metric and imperial units with your BMI calculator?

Yes. Asterisks Fit BMI Calculator supports both metric (kilograms and centimetres) and imperial (pounds, feet and inches) units. The calculation uses standard metric values internally for accuracy.

Free BMI Calculator

Calculate your Body Mass Index (BMI) using your height and weight. Understand what your BMI means and how it compares to healthy reference ranges. Supports both metric and imperial units.

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Understanding Your BMI Result

BMI Below 18.5

Underweight

May indicate insufficient calorie or nutrient intake. Medical guidance is advised.

BMI 18.5 – 24.9

Healthy Weight

Associated with lower risk of many weight-related health conditions for most adults.

BMI 25 – 29.9

Overweight

Modest diet and exercise improvements can help move towards a healthier range.

BMI 30 and above

Obesity Range

Associated with increased health risks. Working with a healthcare professional is recommended.

BMI Formula Explained

Metric formula:

BMI = weight (kg) ÷ height² (m²)

Imperial formula:

BMI = [weight (lbs) ÷ height² (inches²)] × 703

Example calculation

A person weighing 70 kg at 170 cm: BMI = 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2 (Healthy Weight)

Limitations of BMI

BMI is a useful population-level screening tool, but it has real limitations at the individual level:

BMI does not distinguish between muscle and fat mass. Athletes often have a high BMI despite having low body fat.
It does not account for fat distribution. Abdominal fat carries higher health risk than fat stored elsewhere.
Age, sex, and ethnicity can affect what is considered a healthy BMI in practice.
Older adults may have a healthy BMI but still have unfavourable body composition.

For a more complete picture, combine BMI with body fat percentage and ideal weight range assessments.

How the Calculation Works

Asterisks Fit BMI calculations and weight classifications are based on standardized anthropometric guidelines from global health authorities:

World Health Organization (WHO) BMI Guidelines:Classifies adult weight status into underweight (<18.5), healthy weight (18.5–24.9), overweight (25.0–29.9), and obese (≥30.0) based on epidemiological data linking BMI to long-term chronic disease risk.
Quetelet Index (1832): Formulated by mathematician and statistician Adolphe Quetelet, the body mass index equation (BMI = kg/m²) provides a simple, highly reproducible mathematical estimate of population-level body fatness.
National Institutes of Health (NIH) Standards: Adopts the same BMI classification system to screen for potential weight-related health risks, while acknowledging clinical limitations in athletes and older adults.

Formula based on the published equation. Methodology reviewed by the Asterisks editorial team. Last content review: June 2025.

Last Updated: June 2026

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Frequently Asked Questions

Medical Disclaimer: BMI is a screening estimate and does not diagnose health conditions. This tool is for informational purposes only and is not a substitute for professional medical advice. Full disclaimer →