How to Calculate BMI
Learn how to calculate Body Mass Index (BMI) step by step. Understand BMI categories and what your result means.
Body Mass Index (BMI) is a simple calculation based on your weight and height. It is widely used as a screening tool to categorize individuals into weight status categories. Calculating your BMI takes just a few seconds and provides useful information about your weight relative to your height.
BMI Formula
The BMI formula differs slightly depending on your measurement system. For metric units: BMI = weight (kg) / height (m) squared. For imperial units: BMI = weight (lb) x 703 / height (in) squared. For example, a person who is 175 cm (1.75 m) tall and weighs 70 kg has a BMI of 70 / (1.75 x 1.75) = 22.9.
BMI Categories
The World Health Organization defines these categories: Underweight (below 18.5), Normal weight (18.5 to 24.9), Overweight (25 to 29.9), and Obese (30 and above). Each category is associated with different health risk levels. Being in the normal weight category generally indicates lower risk for weight-related health conditions.
Limitations to Keep in Mind
BMI is a useful screening tool but has limitations. It does not account for muscle mass, bone density, age, sex, or ethnic background. Athletes may have a high BMI due to muscle mass, not excess fat. Elderly individuals may have a higher BMI due to muscle loss. Use BMI as one indicator among many, not as a definitive health assessment.
Step-by-Step Instructions
1Measure your weight
Step on a scale to measure your weight in kilograms (or pounds). For the most accurate result, weigh yourself in the morning before eating.
2Measure your height
Stand straight against a wall and measure your height in meters (or inches). Remove shoes and any headwear for accuracy.
3Calculate BMI using the formula
For metric: divide your weight in kg by your height in meters squared. For imperial: multiply your weight in pounds by 703, then divide by your height in inches squared.
4Interpret your result
Compare your BMI to the standard categories. A BMI of 18.5-24.9 is normal weight. Below 18.5 is underweight. 25-29.9 is overweight. 30 or above is obese.
Tips and Tricks
- •BMI is a screening tool, not a diagnostic — consult a healthcare provider for personalized advice.
- •Use Tool Elixir's BMI calculator for instant results without manual calculation.
- •Track your BMI over time rather than focusing on a single measurement.
- •Consider waist circumference alongside BMI for a more complete health picture.