What is BMI and how is it calculated?

BMI (Body Mass Index) is calculated by dividing your weight in kilograms by your height in meters squared: BMI = weight(kg) / height(m)^2. It is a screening tool used to categorize weight status.

What are the BMI categories?

The standard BMI categories are: Underweight (below 18.5), Normal weight (18.5 to 24.9), Overweight (25 to 29.9), and Obese (30 and above).

Is BMI an accurate measure of health?

BMI is a useful screening tool but does not directly measure body fat or account for muscle mass, bone density, age, or sex. Athletes may have a high BMI due to muscle mass. Consult a healthcare professional for a complete assessment.

Does the BMI calculator work for children?

This calculator uses adult BMI categories. For children and teens (ages 2-19), BMI is interpreted using age- and sex-specific percentiles. Consult a pediatrician for accurate assessment.

BMI Calculator

Calculate your Body Mass Index (BMI) to check if your weight is in a healthy range. Supports both metric (kg/cm) and imperial (lbs/inches) units. Instant results, 100% free.

FreeNo Signup100% PrivateMetric & ImperialLast updated: March 2026

Height (cm)

Weight (kg)

Enter your height and weight above to calculate your BMI

What Is Body Mass Index (BMI)?

Body Mass Index (BMI) is a numerical value derived from a person’s weight and height. It was devised by Belgian mathematician Adolphe Quetelet in the 19th century and remains one of the most widely used screening tools for categorizing body weight. The formula divides weight in kilograms by the square of height in meters (BMI = kg / m^2), producing a single number that health professionals use as a quick indicator of whether a person’s weight falls within a healthy range.

BMI is used by the World Health Organization (WHO), the Centers for Disease Control and Prevention (CDC), and healthcare systems worldwide to identify potential weight-related health risks at the population level. A BMI under 18.5 is classified as underweight, 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight, and 30 or above as obese. These thresholds correlate statistically with increased risks of conditions such as type 2 diabetes, cardiovascular disease, and certain cancers.

Limitations of BMI

While BMI is a convenient and cost-free screening tool, it has important limitations. It does not distinguish between lean muscle mass and body fat, meaning athletes and very muscular individuals may register as overweight or obese despite having low body fat. It also does not account for fat distribution — visceral fat (around the organs) is more dangerous than subcutaneous fat (under the skin), yet BMI cannot differentiate between the two. Additionally, BMI thresholds may not apply equally across all ethnic groups, as body composition and health risks vary among populations.

When to Use a BMI Calculator

A BMI calculator is most useful as a quick, first-pass check on your weight relative to your height. It is ideal for adults aged 20 and older who want a general sense of whether they fall within a healthy weight range. For a more complete picture, consider supplementing BMI with waist circumference measurements, body fat percentage tests, or consultation with a healthcare professional who can evaluate your individual risk factors.

How to Calculate Your BMI

Choose Your Unit System

Select either Metric (kilograms and centimeters) or Imperial (pounds and feet/inches) using the toggle at the top of the calculator. The calculator will switch input fields automatically.

Enter Your Height and Weight

Type your height and weight into the corresponding fields. For imperial, enter feet and inches separately. Results update instantly as you type — no button needed.

Read Your BMI and Category

Your BMI value, category (Underweight, Normal, Overweight, or Obese), and a colored indicator bar appear immediately. Use the category ranges below the bar to understand where your BMI falls.

TweakFiles vs Other BMI Calculators

See how TweakFiles BMI Calculator compares to other popular options.

Feature	TweakFiles	Calculator.net	NIH BMI	Mayo Clinic
Price	Free	Free (ads)	Free	Free
Metric + Imperial	Yes	Yes	Imperial only	Imperial only
Instant Results	Yes	Button click	Button click	Button click
Visual Indicator Bar	Yes	No	No	No
No Ads	Yes	No	Yes	Yes
No Signup	Yes	Yes	Yes	Yes
Privacy (client-side)	Yes	No	No	No

Comparison based on publicly available features as of 2025. All trademarks belong to their respective owners.

Frequently Asked Questions

BMI (Body Mass Index) is calculated by dividing your weight in kilograms by the square of your height in meters: BMI = weight(kg) / height(m)^2. It is a widely used screening tool to categorize weight status into underweight, normal, overweight, and obese ranges.

The World Health Organization defines four main BMI categories for adults: Underweight (BMI below 18.5), Normal weight (18.5 to 24.9), Overweight (25 to 29.9), and Obese (30 and above). These categories help identify potential health risks associated with body weight.

BMI is a useful population-level screening tool, but it has limitations for individuals. It does not distinguish between muscle and fat mass, so athletes or muscular people may have a high BMI without excess fat. It also does not account for age, sex, bone density, or fat distribution. Always consult a healthcare professional for a comprehensive health assessment.

This calculator uses standard adult BMI categories. For children and teenagers aged 2 to 19, BMI must be interpreted using age- and sex-specific percentile charts because body composition changes during growth. Consult your pediatrician for accurate child BMI assessment.

Yes, this BMI calculator is 100% free with no signup required. All calculations happen entirely in your browser — no data is sent to any server, and nothing is stored. Your health information stays completely private on your device.