Calculate Your Body Mass Index
Understanding Body Mass Index (BMI)
What is BMI?
Body Mass Index (BMI) is a numerical value calculated from your weight and height. It provides a simple way to categorize individuals as underweight, normal weight, overweight, or obese. BMI is a screening tool that can indicate whether a person might have health issues related to their weight.
BMI Formula
The BMI calculation is the same for both adults and children and is based on the following formulas:
Metric Units: BMI = weight (kg) / (height (m))²
Imperial Units: BMI = 703 × weight (lbs) / (height (in))²
BMI Table for Adults
| BMI | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Possible nutritional deficiency and osteoporosis |
| 18.5 – 24.9 | Normal weight | Low risk (healthy range) |
| 25.0 – 29.9 | Overweight | Moderate risk of developing heart disease, high blood pressure, stroke, diabetes |
| 30.0 and Above | Obese | High risk of developing heart disease, high blood pressure, stroke, diabetes |
BMI Chart for Adults
BMI for Children and Teens (Age 2-20)
For children and teens, BMI is age- and sex-specific and is often referred to as BMI-for-age. The CDC provides growth charts to determine the BMI percentile for children.
| BMI Percentile | Category |
|---|---|
| Less than 5th percentile | Underweight |
| 5th to less than 85th percentile | Healthy weight |
| 85th to less than 95th percentile | Overweight |
| 95th percentile or greater | Obese |
Risks Associated With Being Overweight
- High blood pressure
- Type 2 diabetes
- Coronary heart disease
- Stroke
- Gallbladder disease
- Osteoarthritis
- Sleep apnea and breathing problems
- Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
- Low quality of life
- Mental illness such as clinical depression, anxiety
- Body pain and difficulty with physical functioning
Risks Associated With Being Underweight
- Malnutrition, vitamin deficiencies, anemia
- Osteoporosis from too little vitamin D and calcium
- Decreased immune function
- Growth and development issues in children and teens
- Potential complications from surgery
- Fertility issues in women
Limitations of BMI
While BMI is a useful screening tool, it has limitations:
- Doesn’t directly measure body fat
- May overestimate body fat in athletes and others with muscular builds
- May underestimate body fat in older persons and others who have lost muscle
- Doesn’t account for differences in body composition between ethnic groups
- Doesn’t distinguish between fat, muscle, and bone mass
BMI Prime
BMI Prime is a simple modification of BMI. It is the ratio of a person’s BMI to the upper BMI limit (25). A BMI Prime less than 0.74 is underweight, between 0.74 and 0.99 is optimal, and 1.00 or greater is overweight.
Ponderal Index
The Ponderal Index (PI) is similar to BMI but adjusts for height differently. It’s calculated as weight in kilograms divided by height in meters cubed. Some researchers suggest it may be more accurate than BMI for very tall or short individuals.
BMI Frequently Asked Questions
BMI Frequently Asked Questions
1. Is BMI an accurate measure of health?
BMI is a useful screening tool but doesn’t directly measure body fat or account for muscle mass, bone density, or body composition.
2. Should children use the same BMI categories as adults?
No, children and teens need age- and sex-specific BMI percentiles because their body composition varies as they grow.
3. Why doesn’t BMI work for athletes?
BMI may classify muscular athletes as overweight because muscle weighs more than fat, even at low body fat percentages.
4. How often should I check my BMI?
For most adults, checking every 3-6 months is sufficient unless you’re actively trying to change your weight.
5. Can BMI be different for men and women?
The same BMI ranges apply to both, but women typically have more body fat than men at the same BMI.
6. Does BMI change with age?
BMI interpretation may need adjustment for older adults who naturally lose muscle mass as they age.
7. What’s a healthy BMI for seniors?
Some research suggests a slightly higher BMI (up to 27) may be healthier for older adults than the standard range.
8. Are there ethnic differences in BMI interpretation?
Yes, some ethnic groups may have higher health risks at lower BMIs, leading to adjusted guidelines in some countries.
9. What’s better than BMI for measuring health?
Waist circumference, waist-to-hip ratio, or body fat percentage measurements often provide more accurate health assessments.
10. Can I have a normal BMI but still be unhealthy?
Yes, factors like poor diet, inactivity, high body fat percentage with low muscle mass, or fat distribution can affect health regardless of BMI.