BMI Calculator – Body Mass Index & Weight Status Tool
Use our BMI Calculator to quickly calculate your Body Mass Index (BMI) and determine your weight category (underweight, normal, overweight, or obese) based on your age, height, and weight. This tool supports both metric units and US customary units and also computes the Ponderal Index for enhanced accuracy, especially for individuals at extreme height ranges.
What is BMI?
Body Mass Index (BMI) is a widely-used screening tool to assess body fat based on height and weight. It helps identify whether an individual is within a healthy weight range. Although BMI doesn't measure body fat directly, it’s a useful estimate for most people and can indicate whether further health assessments are necessary.
BMI categories include:
- Underweight
- Normal weight
- Overweight
- Obese (Class I, II, III)
Note: BMI ranges vary slightly by region, age, and health organization guidelines.
BMI Chart for Adults (Aged 20+)
Based on the World Health Organization (WHO) standards:
| Category | BMI Range (kg/m²) |
| Severe Thinness | < 16 |
| Moderate Thinness | 16 – 17 |
| Mild Thinness | 17 – 18.5 |
| Normal | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese Class I | 30 – 35 |
| Obese Class II | 35 – 40 |
| Obese Class III | > 40 |
BMI for Children and Teens (Age 2–20)
As recommended by the CDC (Centers for Disease Control and Prevention):
| Category | BMI Percentile Range |
| Underweight | < 5% |
| Healthy Weight | 5% – 85% |
| At Risk of Overweight | 85% – 95% |
| Overweight | > 95% |
Separate BMI growth charts are available for:
- Boys
- Girls
Health Risks of Being Overweight
According to the CDC, being overweight increases the risk of:
- High blood pressure
- Type 2 diabetes
- Stroke and heart disease
- High LDL (“bad”) cholesterol and triglycerides
- Gallbladder disease
- Sleep apnea and breathing issues
- Osteoarthritis
- Certain cancers (e.g., breast, colon, kidney)
- Reduced quality of life and mental health
- Increased mortality risk
Health Risks of Being Underweight
Potential risks of low BMI include:
- Malnutrition and anemia
- Osteoporosis and bone fractures
- Weakened immune system
- Fertility and menstrual issues in women
- Growth problems in children and teens
- Surgical complications
- Increased risk of death
Limitations of BMI
While BMI is a useful screening tool, it has limitations:
- Does not account for muscle mass vs fat mass
- Can misclassify athletes and muscular individuals
- May not accurately reflect body composition in older adults
- Less reliable for children during growth spurts
For a comprehensive health analysis, combine BMI with other metrics like waist-to-hip ratio, body fat percentage, and Ponderal Index.
BMI Formula
US Customary Units:
BMI=703×Weight (lbs)(Height (in))2BMI = \frac{703 \times \text{Weight (lbs)}}{(\text{Height (in)})^2}BMI=(Height (in))2703×Weight (lbs)
Metric Units:
BMI=Weight (kg)(Height (m))2BMI = \frac{\text{Weight (kg)}}{(\text{Height (m)})^2}BMI=(Height (m))2Weight (kg)
Example (160 lbs, 5'10"):
BMI = 23.0
BMI Prime – Quick Ratio Evaluation
BMI Prime is your BMI divided by the upper normal BMI limit (25 kg/m²).
BMI Prime=Your BMI25\text{BMI Prime} = \frac{\text{Your BMI}}{25}BMI Prime=25Your BMI
| Classification | BMI | BMI Prime |
| Severe Thinness | < 16 | < 0.64 |
| Normal | 18.5–25 | 0.74–1.00 |
| Overweight | 25–30 | 1.00–1.20 |
| Obese Class III | > 40 | > 1.60 |
This ratio allows for fast comparison across populations and regions with different BMI norms.
Ponderal Index (PI) – For Very Tall or Short Individuals
The Ponderal Index improves accuracy where BMI may fail, especially for individuals at extreme height ranges.
Formula (US Units):
PI=Height (in)Weight (lbs)3PI = \frac{\text{Height (in)}}{\sqrt[3]{\text{Weight (lbs)}}}PI=3Weight (lbs)Height (in)
Formula (Metric):
PI=Weight (kg)Height (m)3PI = \frac{\text{Weight (kg)}}{\text{Height (m)}^3}PI=Height (m)3Weight (kg)
Example (5’10”, 160 lbs): PI = 12.9
Start Using the BMI Calculator Now
Ready to find out your BMI and weight category?
Use the BMI calculator above with either Metric Units or US Units to get instant results, including your BMI value, weight classification, and Ponderal Index.