Guide
How to Calculate BMI — Formula, Ranges & What It Means
Body Mass Index (BMI) is a widely used screening tool that estimates body fat based on height and weight. While it has limitations, it remains the simplest and most common way to categorize weight status. This guide explains the formula, what the numbers mean, and when BMI may not tell the full story.
Last updated: April 17, 2026
What Is BMI?
BMI stands for Body Mass Index. It was developed in the early 19th century by Belgian mathematician Adolphe Quetelet as a simple way to classify people's weight relative to their height. Today it's used globally by physicians, public health agencies, and insurance companies as a quick screening tool.
BMI is not a direct measurement of body fat. Instead, it's a mathematical ratio that correlates reasonably well with more precise body composition measurements at a population level. For individuals, it should be interpreted alongside other health indicators.
The BMI Formula
The BMI formula is straightforward: BMI = weight (kg) ÷ height (m)². For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
If you're using imperial units: BMI = (weight in pounds × 703) ÷ (height in inches)². For example, a person who weighs 154 lbs and is 68.9 inches tall has a BMI of (154 × 703) ÷ (68.9 × 68.9) = 22.8.
You can skip the math entirely by using our free BMI Calculator at /calculators/bmi-calculator, which handles both metric and imperial units automatically.
BMI Categories
- Underweight: BMI below 18.5 — May indicate malnutrition, eating disorders, or other health conditions that cause unintended weight loss.
- Normal weight: BMI 18.5 to 24.9 — Generally associated with the lowest health risks related to weight.
- Overweight: BMI 25.0 to 29.9 — May increase risk for cardiovascular disease, type 2 diabetes, and high blood pressure.
- Obese (Class I): BMI 30.0 to 34.9 — Significantly elevated health risks.
- Obese (Class II): BMI 35.0 to 39.9 — High health risks; medical intervention often recommended.
- Obese (Class III): BMI 40.0 and above — Severe health risks; sometimes called morbid obesity.
Limitations of BMI
BMI does not distinguish between muscle mass and fat mass. Athletes with high muscle mass may have a BMI in the 'overweight' range while having very low body fat. Conversely, elderly individuals may have a 'normal' BMI but carry excess fat due to age-related muscle loss.
BMI also doesn't account for fat distribution. Visceral fat (around the abdomen) is more strongly associated with health risks than subcutaneous fat (under the skin). Two people with identical BMI values can have very different health risk profiles based on where their body stores fat.
For children and teenagers, BMI is calculated the same way but interpreted differently using age-specific and sex-specific percentile charts, since body composition changes significantly during growth and development.
When to Use BMI
- As a quick screening tool during routine health checkups.
- For population-level studies and public health statistics.
- As a starting point for discussions about weight management with a healthcare provider.
- When more precise tools (DEXA scan, hydrostatic weighing) are not available.
- Not as a sole diagnostic tool — always consider it alongside other measurements like waist circumference, blood pressure, cholesterol levels, and activity level.
Frequently Asked Questions
- Q: Is BMI accurate for athletes? — BMI often overestimates body fat in muscular individuals. Athletes should use body fat percentage measurements instead.
- Q: Does BMI differ by ethnicity? — Research shows that health risks at the same BMI may differ between ethnic groups. Some organizations use adjusted thresholds for Asian populations (overweight starting at 23, obese at 27.5).
- Q: How often should I check my BMI? — For most adults, once or twice a year is sufficient unless you're actively working on weight management.
- Q: Can children use the same BMI chart? — No. Children and teens need age/sex-specific BMI-for-age percentile charts, available from the CDC or WHO.
Take Action
Tools and pages referenced in this guide
Keep Reading
More calculators guides and comparisons
How to Calculate Percentage Increase and Decrease Correctly
Clear formulas and real-world examples for percentage increase, decrease, and change in business, finance, and everyday math.
How to Calculate Compound Interest
Learn the compound interest formula, the Rule of 72, and how compounding frequency affects growth. Free calculator included.
Profit Margin vs Markup — What's the Difference?
Understand the critical difference between profit margin and markup with formulas, conversion table, and industry examples.
Follow Updates
Get new tools and guides as they ship
Follow our updates page for new launches, privacy-first workflows, and editorial guides. RSS is live now, and email digests appear when a deployment has a configured provider.