BMI Calculator
BMI Calculator – Check Your Body Mass Index & Health Status
Use our Body Mass Index (BMI) Calculator to determine your BMI and understand your weight category based on your height, weight, and age. Easily switch between Metric Units (kg, cm) and US Units (lbs, ft/in) with just a click. This tool also calculates your Ponderal Index, offering deeper insight into your body composition.
What is BMI?
BMI (Body Mass Index) is a quick screening tool that helps estimate body fat based on a person's height and weight. It's widely used to categorize individuals as:
- Underweight
- Normal weight
- Overweight
- Obese
While not a perfect measurement, BMI is a helpful starting point to assess whether your weight could pose potential health risks. Keep in mind that factors like age, sex, muscle mass, and ethnicity can affect your BMI's accuracy.
BMI Categories for Adults (20+)
According to the World Health Organization (WHO), here's how adult BMI is classified:
| Classification | BMI Range (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 - 17 |
| Mild Thinness | 17 - 18.5 |
| Normal Weight | 18.5 - 25 |
| Overweight | 25 - 30 |
| Obese Class I | 30 - 35 |
| Obese Class II | 35 - 40 |
| Obese Class III | > 40 |
BMI for Children & Teens (Age 2–20)
For children and teens, BMI is evaluated using CDC BMI-for-age percentiles:
| Category | Percentile Range |
|---|---|
| Underweight | < 5% |
| Healthy Weight | 5% – 85% |
| At Risk of Overweight | 85% – 95% |
| Overweight | > 95% |
Separate BMI growth charts are used for boys and girls.
Health Risks of Being Overweight
Carrying excess weight increases the risk of:
- Heart disease & high blood pressure
- Type 2 diabetes
- High LDL ("bad") cholesterol
- Stroke & gallbladder disease
- Sleep apnea, breathing problems
- Joint issues (e.g., osteoarthritis)
- Certain cancers (breast, colon, liver, etc.)
- Mental health issues like anxiety and depression
- Reduced quality of life and mobility
Health Risks of Being Underweight
Being underweight can lead to:
- Malnutrition and anemia
- Weak bones (osteoporosis)
- Weakened immune function
- Hormonal imbalance and reproductive issues
- Growth delays in children
- Higher surgical risks
- Increased mortality risk
If unexplained, being underweight may indicate underlying medical issues such as eating disorders.
Limitations of BMI
While useful, BMI doesn't directly measure body fat. It doesn't account for differences in muscle mass, bone density, or fat distribution. Consider these factors:
- Athletes may have a high BMI due to muscle mass
- Older adults may have more body fat at a "normal" BMI
- Children's BMI is influenced by growth and puberty stages
For the most accurate health assessment, BMI should be combined with other measurements and medical evaluations.
BMI Calculation Formula
📏 Metric (kg/m²):
BMI = weight (kg) / [height (m)]²
📐 US Units:
BMI = 703 × weight (lbs) / [height (in)]²
Example: A 5'10" person (70 in) weighing 160 lbs: BMI = 703 × 160 / 70² = 23.0
BMI Prime – A Quick Ratio Check
BMI Prime compares your BMI to the upper limit of the "normal" BMI range (25 kg/m²):
Formula:
BMI Prime = Your BMI / 25
| Classification | BMI | BMI Prime |
|---|---|---|
| Severe Thinness | < 16 | < 0.64 |
| Moderate Thinness | 16 – 17 | 0.64 – 0.68 |
| Mild Thinness | 17 – 18.5 | 0.68 – 0.74 |
| Normal | 18.5 – 25 | 0.74 – 1 |
| Overweight | 25 – 30 | 1 – 1.2 |
| Obese Class I | 30 – 35 | 1.2 – 1.4 |
| Obese Class II | 35 – 40 | 1.4 – 1.6 |
| Obese Class III | > 40 | > 1.6 |
Ponderal Index (PI) – For Height Extremes
The Ponderal Index (PI) adjusts for height more effectively than BMI, especially for very tall or short individuals. It's calculated by cubing the height instead of squaring it.
PI Formula (US Units):
PI = height (in) / ∛weight (lbs)
Example: 70 in / ∛160 lbs ≈ 12.9
PI Formula (Metric):
PI = weight (kg) / height³ (m)
Example: 72.57 kg / 1.778³ ≈ 12.9
How to Use the BMI Calculator: A Step-by-Step Guide
Step 1: Choose Your Unit System
Select either US units (pounds, feet/inches) or Metric units (kilograms, centimeters) based on your preference.
Pro Tip: If you're in the United States, US units might be more familiar. Most other countries use the metric system.
Step 2: Enter Your Personal Information
Fill in the required fields:
- Age: Enter your age (between 2-120 years)
- Gender: Select male or female
- Height: Enter your height in feet/inches (US) or centimeters (metric)
- Weight: Enter your weight in pounds (US) or kilograms (metric)
Important: Make sure to enter accurate measurements for the most precise BMI calculation.
Step 3: Calculate Your BMI
Click the "Calculate BMI" button to process your information. The calculator will automatically:
- Calculate your BMI value
- Determine your BMI category (underweight, normal, overweight, or obese)
- Calculate your BMI Prime (ratio to upper normal limit)
- Calculate your Ponderal Index (an alternative measure)
Step 4: Interpret Your Results
After calculation, you'll see your results displayed with:
- BMI Value: Your calculated Body Mass Index
- BMI Category: Your weight classification according to WHO standards
- BMI Prime: Your BMI relative to the upper limit of normal (25)
- Healthy Weight Range: The ideal weight range for your height
- Ponderal Index: An alternative measure that may be more accurate for very tall or short individuals
Understanding Results: A BMI between 18.5 and 25 is generally considered normal weight. However, BMI is just a screening tool and doesn't diagnose body fatness or health.
Step 5: Consider Additional Measurements
For a more comprehensive health assessment, consider combining your BMI results with:
- Waist circumference measurement
- Body fat percentage
- Physical fitness level
- Family history and other health factors
Beyond BMI: Remember that BMI doesn't distinguish between fat, muscle, and bone mass. Athletes may have a high BMI due to muscle weight, not excess fat.