Health & Fitness · Guide
BMI Calculator Guide: How to Read Your Body Mass Index
Body Mass Index (BMI) is one of the most widely used health numbers in the world. It compares your weight to your height with a single formula, giving a rough idea of whether you are in a healthy range. It is quick and free — but it is also a blunt instrument. This guide explains how BMI is calculated, what each category means, and where the number falls short so you can interpret it intelligently.
In this guide
How BMI is calculated
BMI is weight in kilograms divided by the square of height in meters. Using the imperial system, multiply your weight in pounds by 703, then divide by your height in inches squared. The result is a single number you can compare against standard categories.
Understanding the categories
A BMI under 18.5 is underweight, 18.5 to 24.9 is a healthy range, 25 to 29.9 is overweight, and 30 or above is obesity. The World Health Organization uses these cutoffs for adults, and the same ranges apply to both men and women.
Where BMI falls short
BMI cannot tell fat from muscle. A muscular athlete can register as overweight despite being lean, while an older adult with low muscle mass can have a normal BMI with too much body fat. It also ignores where fat sits on your body — waist circumference and body-fat percentage add important context.
Key takeaways
- BMI is weight ÷ height², quick and free to compute.
- Healthy range: 18.5–24.9 for most adults.
- BMI can misclassify muscular or low-muscle bodies.
- Combine BMI with waist size and body fat for context.
Frequently asked questions
What is a good BMI?
A BMI between 18.5 and 24.9 is generally considered healthy for adults, with 25–29.9 classed as overweight and 30+ as obesity.
Is BMI accurate for athletes?
Not always. Because BMI ignores body composition, it can overstate body fat in muscular people and understate it in people with low muscle mass.
What should I do if my BMI is high?
Treat BMI as a starting point. Use the Body Fat Calculator and your waist measurement for context, then talk to a professional about a realistic plan.
Put these numbers to work
BMI, BMR, daily calories, ideal weight and body fat — instantly.
BMI Calculator
Calculate your Body Mass Index and healthy weight range.
Body Fat Calculator
Estimate body fat percentage with the U.S. Navy method.
Ideal Weight Calculator
Find your ideal weight range using four medical formulas.
Daily Calorie Calculator
Calculate the calories you need to maintain, lose or gain weight.