Know Your BMI - Free & Instant
Body Mass Index (BMI) is a quick screening tool that estimates whether your weight is healthy for your height. Used by the CDC and WHO, BMI helps identify potential health risks related to underweight, overweight, and obesity. This calculator gives your exact BMI plus the category it falls in.
What Is BMI?
BMI stands for Body Mass Index. It is a single number calculated from your height and weight that estimates body fatness for most adults. BMI is the standard tool used by doctors, insurance companies, and public health agencies to screen for weight-related health risks.
It is not a direct measure of body fat - muscular athletes may have high BMI without excess fat. But for the general population, BMI is a reliable, low-cost screening tool.
The BMI Formula
Metric: BMI = weight (kg) ÷ height (m)2
Imperial: BMI = [weight (lb) ÷ height (in)2] × 703
Both formulas give the same result - just different units.
BMI Categories (CDC Guidelines)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk (nutritional, immune) |
| 18.5 - 24.9 | Normal weight | Lowest risk |
| 25.0 - 29.9 | Overweight | Moderate risk |
| 30.0 - 34.9 | Obesity Class I | High risk |
| 35.0 - 39.9 | Obesity Class II | Very high risk |
| 40.0 and above | Obesity Class III | Extremely high risk |
Worked Example
A person who is 5′9″ (69 inches) and weighs 165 lb:
BMI = [165 ÷ (69)2] × 703 = [165 ÷ 4761] × 703 = 24.4
Category: Normal weight.
How to Use This Calculator
- Select your preferred unit (metric or imperial).
- Enter your height.
- Enter your weight.
- View your BMI and category instantly.
- Optionally enter age and gender for a more refined interpretation (BMI ranges differ for children).
BMI for Children and Teens
BMI for people under 20 uses age- and sex-specific percentiles rather than fixed categories. A child at the 85th percentile is considered overweight; 95th percentile or higher is obese. Always use a pediatric BMI chart for anyone under 20.
Limitations of BMI
- Athletes: Muscle weighs more than fat. NBA players often register as "overweight" on BMI.
- Elderly: Loss of muscle mass can mask fat gain.
- Ethnic differences: Some populations have higher health risks at lower BMI values.
- Body fat distribution: BMI does not distinguish between belly fat and hip fat, which have different health implications.
Use BMI as a screening tool, not a diagnosis. Your doctor may also measure waist circumference, body fat percentage, and other metrics.
What To Do With Your BMI Result
- Underweight: Consult your doctor about nutrition and possible underlying conditions.
- Normal: Maintain your current habits. Focus on regular exercise and balanced nutrition.
- Overweight: Losing 5-10% of body weight improves blood pressure, cholesterol, and blood sugar.
- Obesity: Speak with a healthcare provider about medical, dietary, and lifestyle interventions.
Frequently Asked Questions
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy by CDC standards.
Is BMI accurate for athletes?
No. BMI overestimates body fat for muscular individuals. Athletes should use body fat percentage instead.
What is a good BMI for a woman?
The same range applies to men and women: 18.5-24.9 is healthy. Women naturally carry more body fat than men at the same BMI.
Does BMI change with age?
For adults, the categories stay the same. But for people over 65, a slightly higher BMI (23-28) may actually be protective.
Can I use BMI for children?
Children and teens require age- and sex-specific BMI percentile charts, not the adult categories.
What is the difference between BMI and body fat?
BMI is a ratio of weight to height. Body fat percentage directly measures fat tissue. Both tell you different things about health.