BMI Calculator
Use this quick tool to calculate your Body Mass Index (BMI) using either metric or imperial units.
What Is BMI?
BMI stands for Body Mass Index. It is a simple screening number that compares your weight to your height. Doctors and health organizations use BMI as a quick way to group weight status into broad categories like underweight, healthy weight, overweight, and obesity.
It is not a diagnosis by itself, but it is a useful starting point for conversations about health, nutrition, and fitness.
BMI Formula: How to Calculate It
Metric Formula
If your weight is in kilograms and your height is in meters:
BMI = weight (kg) ÷ [height (m)]²
Example: 70 kg and 1.75 m
BMI = 70 ÷ (1.75 × 1.75) = 22.9
Imperial Formula
If your weight is in pounds and your height is in inches:
BMI = 703 × weight (lb) ÷ [height (in)]²
Example: 154 lb and 69 in
BMI = 703 × 154 ÷ (69 × 69) = 22.7
Step-by-Step BMI Calculation
- Measure your weight accurately (morning readings are usually more consistent).
- Measure your height without shoes.
- Choose metric or imperial formula based on your units.
- Square your height value.
- Divide weight by squared height (or use the 703 factor for imperial).
- Round to one decimal place.
BMI Categories for Adults
- Below 18.5: Underweight
- 18.5 to 24.9: Healthy weight
- 25.0 to 29.9: Overweight
- 30.0 and above: Obesity
Some sources split obesity into classes:
- Class I: 30.0–34.9
- Class II: 35.0–39.9
- Class III: 40.0 and above
How to Read Your Result
A BMI number is a screening indicator, not a full health report. It is best used with other metrics such as:
- Waist circumference
- Blood pressure
- Blood glucose and lipid levels
- Physical activity and sleep patterns
- Medical and family history
Common BMI Calculation Mistakes
- Mixing units (for example, kg with inches).
- Forgetting to square height.
- Using estimated height or old measurements.
- Rounding too early during calculation.
Important Limitations of BMI
BMI is convenient, but it has limits:
- It does not distinguish fat from muscle.
- Very muscular people may have a high BMI but low body fat.
- Older adults may have normal BMI but low muscle mass.
- It may not reflect individual differences across populations.
That is why clinicians often combine BMI with body composition, waist measurement, and lab tests.
Practical Takeaway
If you are asking “bmi how to calculate,” the process is straightforward and fast. Use a reliable formula, double-check your units, and treat the result as a screening tool. For a complete health assessment, discuss your numbers with a qualified healthcare professional.