What Is BMI and How Is It Calculated? Formula, Chart & Limitations
A clear, complete explanation of Body Mass Index — the formula, a worked example, the standard BMI chart, and why BMI isn't the full picture on its own.
Published 7 Sep 2026 · 13 min read
BMI, or Body Mass Index, is one of the most widely used health screening numbers in the world — doctors, insurers and fitness apps all reference it. It's also one of the most misunderstood, since a single number can't capture everything about a person's health. Here's exactly what BMI is, the formula behind it, how to read the chart, and the limitations worth knowing before relying on it too heavily.
📖 What is BMI?
Body Mass Index (BMI) is a number calculated from a person's height and weight, used as a simple screening tool to estimate whether someone's weight falls into a healthy range for their height. It was developed in the 1830s as a population-level statistical measure, not an individual diagnostic tool — which is part of why it has well-documented limitations when applied to any one specific person, covered further down.
🧮 The BMI formula
Metric (kg, m)
BMI = weight (kg) ÷ height (m)²
Divide your weight in kilograms by your height in meters, squared.
Imperial (lb, in)
BMI = (weight (lb) ÷ height (in)²) × 703
Divide weight in pounds by height in inches squared, then multiply by 703.
🔢 A worked example
Say someone weighs 70 kg and is 1.75 m tall.
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9
In imperial units, that's roughly 154 lb at 5'9" (69 inches): BMI = (154 ÷ 69²) × 703 = (154 ÷ 4761) × 703 ≈ 22.7 (small rounding differences between unit systems are normal).
A BMI of 22.9 falls within the "normal weight" range, covered in the chart below.
📊 The standard BMI chart
Below 18.5
Underweight
18.5 – 24.9
Normal weight
25.0 – 29.9
Overweight
30.0 and above
Obese
These ranges are the standard adult categories used by most major health organizations. Categories for children and teenagers are different and are based on age- and sex-specific percentiles rather than fixed numbers, since body composition changes significantly during growth.
⚠️ What BMI doesn't tell you
BMI is a widely used screening tool, but it has well-documented limitations worth understanding:
- It doesn't distinguish muscle from fat. A muscular athlete can have a high BMI despite low body fat, since BMI only factors in total weight, not composition.
- It doesn't account for fat distribution. Where fat is carried on the body matters for health risk, and BMI doesn't capture this at all.
- It doesn't adjust for age, sex or bone density. Two people with identical BMI can have very different body compositions.
- It's a population-level tool. It was designed to track trends across large groups, not to serve as a precise individual diagnosis.
For these reasons, BMI is best used as a quick, general starting point rather than a complete picture of someone's health — doctors typically combine it with other measurements and context before drawing conclusions.
❓ Frequently Asked Questions
Disclaimer: This article is for general educational purposes only and is not medical advice. BMI is a screening tool, not a diagnosis. Consult a doctor or qualified healthcare provider for personal health guidance.
Want to calculate your own BMI? Try the BMI Calculator → or ask the Quinet Community.