How to Calculate BMI: Formula, BMI Chart & Examples
Learn how to calculate BMI with metric and imperial formulas, examples, adult BMI categories, key limitations, and when to use a BMI calculator safely.
BMI is one of the most common health screening numbers because it needs only height and weight. Knowing how to calculate BMI helps you understand where your weight falls on standard adult BMI charts, but it is important to remember what BMI can and cannot tell you. It does not directly measure body fat, muscle, fitness, nutrition, or overall health. It is a quick screening tool that can be useful when interpreted with context.
What Is BMI?
BMI stands for Body Mass Index. It compares body weight with height and places the result into a category such as underweight, healthy weight, overweight, or obesity.
The CDC describes BMI as a calculated measure of weight relative to height and notes that it should be considered with other health factors, not used as a diagnosis by itself.
How to Calculate BMI
There are two formulas.
Metric Formula
BMI = weight (kg) ÷ height (m)²
If height is in centimeters, divide by 100 to convert it to meters.
Imperial Formula
BMI = 703 × weight (lb) ÷ height (in)²
Use total height in inches.
Use our free BMI Calculator to calculate your BMI instantly.
Metric Example
A person weighs 72 kg and is 1.75 m tall.
Step 1 - Square height: 1.75 × 1.75 = 3.0625
Step 2 - Divide weight by height squared: 72 ÷ 3.0625 = 23.5
BMI = 23.5
Imperial Example
A person weighs 170 lb and is 5 feet 9 inches tall.
Step 1 - Convert height: 5 feet 9 inches = 69 inches
Step 2 - Square height: 69 × 69 = 4,761
Step 3 - Multiply weight by 703: 170 × 703 = 119,510
Step 4 - Divide: 119,510 ÷ 4,761 = 25.1
BMI = 25.1
Adult BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity Class 1 |
| 35.0 to 39.9 | Obesity Class 2 |
| 40.0 or higher | Obesity Class 3 |
These standard adult categories apply to most adults age 20 and older. Children and teens use BMI percentiles based on age and sex.
BMI Quick Reference by Height and Weight
| Height | Weight | Approximate BMI | Category |
|---|---|---|---|
| 5’4” | 120 lb | 20.6 | Healthy weight |
| 5’4” | 160 lb | 27.5 | Overweight |
| 5’8” | 150 lb | 22.8 | Healthy weight |
| 5’8” | 190 lb | 28.9 | Overweight |
| 6’0” | 180 lb | 24.4 | Healthy weight |
These are examples only. Your exact BMI depends on your precise height and weight.
BMI Limitations
BMI is useful because it is fast, but it misses important details:
- It does not distinguish muscle from fat.
- It does not show where body fat is carried.
- It may misclassify athletes.
- It does not account for pregnancy.
- It does not measure blood pressure, cholesterol, blood sugar, or fitness.
For a fuller picture, combine BMI with waist measurement, medical history, physical activity, lab markers, and professional guidance. The Calorie Calculator and TDEE Calculator can help if you are planning nutrition changes.
Common Mistakes
Using height in centimeters in the meter formula is a common error. Convert centimeters to meters first.
Forgetting to square height will produce a completely wrong result.
Treating BMI as a diagnosis overstates what the number can do. BMI is a screening tool.
Using adult BMI charts for children is incorrect because children and teens need percentile charts.
When to Recalculate BMI
You do not need to calculate BMI every day. For most adults, recalculating after a meaningful weight change, during an annual checkup, or when starting a health plan is enough. If you are tracking progress, use the same scale, similar clothing, and similar timing so the trend is easier to compare.
Frequently Asked Questions
How to calculate BMI in kg and cm?
Convert height from centimeters to meters, then use BMI = weight in kilograms divided by height in meters squared. For example, 70 kg and 170 cm becomes 70 ÷ 1.70² = 24.2.
How to calculate BMI in pounds and inches?
Use BMI = 703 × weight in pounds ÷ height in inches squared. For example, 160 lb and 66 inches gives 703 × 160 ÷ 66² = 25.8.
What is a normal BMI?
For most adults age 20 and older, a BMI from 18.5 to 24.9 is categorized as healthy weight. This range is a screening category, not a complete health assessment.
Is BMI accurate?
BMI is useful for quick screening but not perfect for individuals. It can misclassify muscular people, older adults, pregnant people, and those with different body composition patterns. Use it alongside other health measures.
Can BMI be used for children?
The formula can be calculated for children, but interpretation is different. Children and teens use BMI-for-age percentiles that account for age and sex. Adult category cutoffs should not be used for people under 20.
The Bottom Line
To calculate BMI, divide weight in kilograms by height in meters squared, or use 703 times pounds divided by inches squared. The result can place you in a standard adult category, but it should be interpreted with other health information.
Use our free BMI Calculator to calculate your number and see the adult category instantly.
Medical note: This article is for informational purposes only and is not medical advice. Consult a qualified healthcare professional about personal health concerns.
How to Calculate: Step-by-Step Guide
Measure your weight
Use a scale to find your weight in kilograms or pounds. For accuracy, measure in the morning.
Measure your height
Stand against a wall and measure your height in meters or inches.
Apply the BMI formula
If using metric: weight (kg) ÷ height (m)². If using imperial: 703 × weight (lb) ÷ height (in)².
Interpret your result
Compare your number to standard adult BMI categories (e.g., 18.5-24.9 is healthy).