Body Mass Index (BMI) Calculator
Use this quick calculator to estimate your BMI using metric or imperial units.
Understanding the body mass calculation formula
When people search for a body mass calculation formula, they usually mean the Body Mass Index (BMI) formula. BMI is a simple screening tool that compares your body weight with your height to estimate whether your weight is in a generally healthy range.
It does not directly measure body fat, but it is widely used in medicine, public health, fitness coaching, and wellness programs because it is quick, inexpensive, and easy to calculate.
The BMI formula (metric and imperial)
Metric formula
BMI = weight (kg) / [height (m)]²
- Convert height from centimeters to meters first.
- Square the height in meters.
- Divide body weight in kilograms by that squared height value.
Imperial formula
BMI = 703 × weight (lb) / [height (in)]²
- Convert total height to inches (feet × 12 + inches).
- Square the total inches.
- Multiply weight by 703, then divide by squared height.
Step-by-step examples
Example 1 (Metric)
Suppose a person weighs 70 kg and is 175 cm tall.
- Height in meters: 175 cm = 1.75 m
- Height squared: 1.75 × 1.75 = 3.0625
- BMI: 70 / 3.0625 = 22.9
Example 2 (Imperial)
Suppose a person weighs 154 lb and is 5 ft 9 in.
- Total height in inches: (5 × 12) + 9 = 69 in
- Height squared: 69 × 69 = 4761
- BMI: (703 × 154) / 4761 = 22.7
BMI categories
| BMI Value | Weight Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
Why BMI is useful
- It is fast and standardized.
- It helps identify potential health risk trends.
- It can be used for population-level analysis.
- It provides a useful starting point for a health conversation.
Important limitations of the body mass formula
BMI is not a perfect indicator for every individual. It can misclassify people with high muscle mass, differences in bone structure, fluid retention, and other body composition variables.
- Athletes may have a higher BMI due to muscle, not excess fat.
- Older adults may have normal BMI but lower muscle mass.
- Body fat distribution (especially abdominal fat) is not captured by BMI alone.
For a fuller picture, combine BMI with waist circumference, activity level, blood pressure, lab values, and guidance from a qualified clinician.
Other body-related formulas you may hear about
Basal Metabolic Rate (BMR)
Estimates daily calories burned at rest. Useful for nutrition planning, but not the same as BMI.
Body Fat Percentage
Gives more direct composition data than BMI, especially when measured with quality methods (DEXA, calipers, or validated scanners).
Waist-to-Height Ratio
A practical marker of central adiposity and cardiometabolic risk that can complement BMI.
How to use this calculator effectively
- Measure height and weight consistently (same scale, similar conditions).
- Track trends over time instead of focusing on one data point.
- Use BMI as a screening metric, not a final diagnosis.
- Pair results with healthy habits: sleep, exercise, strength training, and balanced nutrition.
Final takeaway
The standard body mass calculation formula is the BMI equation. It is a useful first-step health indicator that converts your height and weight into a single number. Use it to monitor trends, identify risk ranges, and start informed discussions with healthcare professionals.