HOW TO CALCULATE BMI — COMPLETE BODY MASS INDEX GUIDE 2026

BMI — Body Mass Index — is the most widely used screening tool for weight status worldwide. Your doctor uses it, health insurance companies use it, and sports scientists use it. But what does your BMI number actually mean, how accurate is it, and what should you do with the result? This complete guide covers everything.

What is BMI?

BMI is calculated by dividing your weight in kilograms by your height in metres squared. The formula is: BMI = weight (kg) ÷ height (m)². For example, someone weighing 70kg at 1.75m tall has a BMI of 22.9 — in the healthy range. You can calculate yours instantly with our free BMI Calculator.

BMI Categories — What Your Number Means

• Below 18.5 — Underweight: May indicate nutritional deficiency, malabsorption, or other health conditions. Medical assessment recommended.
• 18.5 to 24.9 — Healthy weight: Lowest statistical risk of weight-related diseases across most population studies.
• 25.0 to 29.9 — Overweight: Increased risk of cardiovascular disease, hypertension, and type 2 diabetes. Lifestyle changes may help.
• 30.0 and above — Obese: Significantly elevated health risk across multiple conditions. Medical advice strongly recommended.

BMI Formula — How to Calculate Manually

To calculate BMI manually: first convert your height to metres (e.g. 170cm = 1.70m), then square it (1.70 × 1.70 = 2.89), then divide your weight by that number (70 ÷ 2.89 = 24.2). Or use our free calculator which does it instantly.

The Real Limitations of BMI — What It Cannot Tell You

BMI is a population-level screening tool, not a personal health diagnosis. Every medical professional knows its significant limitations:

• Cannot distinguish muscle from fat: A professional athlete or bodybuilder often has a high BMI despite extremely low body fat. By BMI standards, many Olympic athletes would be classified as overweight or obese.
• Ignores fat distribution: Belly fat (visceral fat) is far more dangerous than fat stored in the hips and thighs. BMI cannot measure where fat is stored.
• Age and sex differences: Women naturally carry more essential body fat than men. Older adults lose muscle mass which can mask overweight status in BMI readings.
• Ethnicity variations: Research shows that people of Asian descent face higher health risks at lower BMI values, while some African populations show lower risk at higher BMI values.

What to Do After Calculating Your BMI

BMI is a starting point for a conversation with your doctor — not a diagnosis. If your BMI is outside the healthy range, consult a healthcare professional for a complete assessment that considers blood pressure, cholesterol, blood sugar, fitness level, family history, and other relevant factors. A single number cannot capture the full picture of your health.