BMI Calculator Guide: Formula, Categories, and How to Read Your Result
ByMuhammad Ali•Founder of KruskalCode
11:13
6 min read
Body Mass Index (BMI) is one of the most searched health maths topics online. It combines height and weight into a single number so you can compare your result with standard adult categories. The maths is straightforward once you pick either metric or imperial units and stick with that system throughout.
Explanation
BMI does not measure body fat directly. It is a quick screening ratio used in schools, sports medicine leaflets, and general wellness checks. Because it only uses height and weight, it can overestimate body fat in very muscular people and underestimate risk in some other cases. Treat your result as a rough estimate—not a diagnosis.
Formula
Metric: BMI = weight (kg) ÷ [height (m)]² Imperial: BMI = 703 × weight (lb) ÷ [height (in)]² Adult categories (WHO): under 18.5 underweight; 18.5–24.9 normal; 25–29.9 overweight; 30+ obese
Example
A student weighs 70 kg and is 175 cm tall. Convert height to metres: 175 ÷ 100 = 1.75 m. Square it: 1.75 × 1.75 = 3.0625. Divide: 70 ÷ 3.0625 ≈ 22.9. That falls in the normal range (18.5–24.9). In imperial, 154 lb at 5 ft 9 in (69 in total) gives BMI = 703 × 154 ÷ 69² ≈ 22.7—nearly the same answer when conversions are consistent.
How to use the related calculator
Open the BMI Calculator on ProMathTools, choose metric or imperial, then enter your weight and height. For metric, type kilograms and centimetres. For imperial, enter pounds plus feet and inches separately. Press calculate to see your BMI rounded to one decimal, the WHO category, and which formula was applied. If you normally think in stones or metres, convert first using our weight or height converters linked on the page.
Try the related calculator
Open toolFAQ
What is a healthy BMI?
For most adults, WHO treats 18.5 to 24.9 as the normal range. Individual health depends on many factors beyond this single number.
Why are there two formulas?
The metric formula uses kg and metres. The imperial version uses pounds and inches with the factor 703 so both systems produce the same BMI when measurements are equivalent.
Can teenagers use adult BMI categories?
Teens are often plotted on age-and-sex growth charts instead of fixed adult cut-offs. Use adult categories here as a general reference only.
Is this accurate for everyone?
No. BMI varies in usefulness across body types, ethnicity, pregnancy, and medical conditions. It is an estimate for general information—not personalised medical advice.
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About the author
Muhammad Ali. Muhammad Ali is a full-stack developer and founder of KruskalCode. He builds SaaS platforms and automation systems with React and Laravel, and helps teams ship fast, scalable tools.
Need a custom calculator, dashboard, or automation workflow? Reach out to KruskalCode.
