BMI Calculator
Use metric or imperial units to calculate your body mass index instantly.
What is BMI?
Body Mass Index (BMI) is a quick number used to estimate whether your weight is in a typical range for your height. It is widely used in clinics, public health reports, fitness apps, and personal health tracking.
The question “how do you calculate body mass index bmi” really comes down to one formula: divide weight by height squared. The exact equation changes slightly based on the unit system you use (metric or imperial), but the idea is the same.
BMI formula (metric and imperial)
Metric formula
If your height is entered in centimeters, convert to meters first:
Imperial formula
The factor 703 converts pounds and inches to the metric-equivalent scale used by the BMI model.
Step-by-step examples
Example 1: Metric
- Weight: 70 kg
- Height: 175 cm = 1.75 m
- BMI = 70 / (1.75 × 1.75) = 22.86
A BMI of 22.9 is in the “normal weight” range for adults.
Example 2: Imperial
- Weight: 154 lb
- Height: 5 ft 9 in = 69 in
- BMI = 703 × 154 / (69 × 69) = 22.74
A BMI of 22.7 is also in the “normal weight” category.
Adult BMI categories
For most adults, BMI is interpreted using common cutoffs:
- Below 18.5: Underweight
- 18.5 to 24.9: Normal weight
- 25.0 to 29.9: Overweight
- 30.0 and above: Obesity
These ranges are screening thresholds and do not tell the full story of your health by themselves.
Why BMI is useful (and why it has limits)
Why it helps
- Fast and easy to calculate
- Low-cost way to screen population-level risk
- Useful for tracking trends over time
Its limitations
- Does not directly measure body fat percentage
- Does not show fat distribution (like visceral abdominal fat)
- Can overestimate risk in very muscular people
- Can underestimate risk in people with low muscle mass
Because of these limits, clinicians often combine BMI with waist circumference, blood pressure, blood glucose, lipid tests, and lifestyle assessment.
Special cases: children, teens, athletes, and older adults
Children and teens
BMI for children and adolescents is age- and sex-specific and is interpreted using percentile charts rather than adult cutoff numbers.
Athletes
Athletes with high muscle mass can have a higher BMI without high body fat. In these cases, additional measurements are important.
Older adults
Age-related changes in muscle and bone mass can affect BMI interpretation. A healthcare provider can help place the number in context.
How to use your BMI result wisely
- Use BMI as a starting point, not a final verdict.
- Track it over time rather than obsessing over one value.
- Combine it with waist size, activity level, diet quality, sleep, and lab markers.
- Talk with a professional if your result is outside the typical range.
Quick FAQ
Is BMI accurate?
It is reasonably accurate for broad screening in many adults, but not perfect for individuals.
How often should I calculate BMI?
Monthly or quarterly is usually enough for trend tracking unless your clinician advises otherwise.
What is a healthy BMI?
For most adults, 18.5 to 24.9 is considered a normal range, but health status depends on more than BMI alone.
Bottom line
If you are asking “how do you calculate body mass index bmi,” the answer is straightforward: divide weight by height squared, using the appropriate formula for your units. Use the calculator above for a fast result, then interpret that number as one part of your overall health picture.