How to Calculate BMI Correctly: Formula, Examples, Chart and Calculator
Quick answer: BMI (Body Mass Index) is calculated by dividing weight in kilograms by height in meters squared. The metric formula is BMI = weight (kg) ÷ height (m)2. In U.S. customary units, use BMI = [weight (lb) ÷ height (in)2] × 703. Adults aged 20 and older use standard adult BMI categories, while children and teens ages 2–19 require age- and sex-specific BMI-for-age percentile interpretation. Use the SmartWebHub BMI Calculator for a quick result.
Important: BMI is a screening measure, not a diagnosis. It does not directly measure body fat and cannot distinguish muscle from fat.
What Is BMI?
Body Mass Index is a number derived from height and weight. It is commonly used as a screening measure because it is simple, inexpensive, and easy to repeat. BMI can help identify broad weight categories, but it is not a complete measurement of health or body composition.
The CDC recommends considering BMI together with other health information rather than using it as a diagnosis. CDC Adult BMI guidance.
What Is the BMI Formula?
Metric BMI formula
BMI = weight (kg) ÷ [height (m)]2
If your height is in centimeters, divide it by 100 first. For example, 175 cm = 1.75 m.
A direct centimeters version is BMI = [weight (kg) ÷ height (cm)2] × 10,000.
Imperial BMI formula
BMI = [weight (lb) ÷ height (in)2] × 703
For feet and inches, convert the full height to inches first. Five feet six inches equals 66 inches.
CDC BMI calculation guidance documents these formulas.
How to Calculate BMI Step by Step
- Measure your weight.
- Measure your height.
- Convert measurements to the correct units.
- Square the height.
- Divide weight by squared height.
- For pounds and inches, multiply by 703.
- Interpret the number according to age.
Metric BMI Example
For 70 kg and 1.75 m:
70 ÷ (1.75 × 1.75) = 22.9
For an adult aged 20 or older, 22.9 is within the standard healthy-weight category.
Imperial BMI Example
For 160 lb and 5 ft 6 in:
5 ft 6 in = 66 in
(160 × 703) ÷ (66 × 66) ≈ 25.8
For an adult aged 20 or older, 25.8 is within the standard overweight category.
Adult BMI Chart for Ages 20+
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to below 25 | Healthy Weight |
| 25 to below 30 | Overweight |
| 30 or higher | Obesity |
See the CDC adult BMI categories.
How Is BMI Calculated for Children and Teens?
Children and teens ages 2–19 are still growing. Their BMI is therefore interpreted using age- and sex-specific BMI-for-age percentiles rather than adult fixed cutoffs.
- Below the 5th percentile: Underweight
- 5th to below the 85th percentile: Healthy Weight
- 85th to below the 95th percentile: Overweight
- 95th percentile or higher: Obesity
For official percentile results, use the CDC Child and Teen BMI Calculator.
How to Calculate BMI from Height and Weight
The main challenge is keeping units consistent. For metric BMI, use kilograms and meters. For U.S. customary BMI, use pounds and inches. Never mix kilograms with inches or pounds with meters.
How to Calculate BMI Manually
A basic calculator is enough. Square the height, divide the weight by the squared height, and add the 703 factor when using pounds and inches. A dedicated BMI Calculator removes some of the arithmetic and conversion work.
How to Calculate BMI Using Centimeters
When weight is in kilograms and height is in centimeters:
BMI = [weight (kg) ÷ height (cm)2] × 10,000
Example: 70 kg and 175 cm gives approximately 22.9.
How to Calculate BMI in Feet and Inches
Convert feet to inches by multiplying feet by 12 and adding the remaining inches. Then use the pounds-and-inches formula if the weight is in pounds.
Why Height Is Squared in the BMI Formula
The BMI equation uses height squared as part of its standard height-adjusted weight calculation. In practical terms, “height squared” means height multiplied by itself.
Common BMI Calculation Mistakes
- Entering centimeters as meters.
- Forgetting to square height.
- Mixing metric and U.S. customary units.
- Forgetting the 703 conversion factor.
- Applying adult cutoffs to children and teens.
Does BMI Measure Body Fat?
No. BMI does not directly measure body-fat percentage, muscle mass, or fat distribution. Highly muscular people may have a higher BMI because BMI is based on weight and height alone.
BMI for Men and Women
The adult BMI formula does not use a separate male or female multiplier. Sex is important in BMI-for-age interpretation for children and teens because the reference percentiles are sex-specific.
BMI, Pregnancy and Athletic Training
Pregnancy is a special situation where general BMI categories should not be used to assess pregnancy weight gain or fetal health. Likewise, athletes with substantial muscle mass may need additional context because BMI cannot separate muscle from fat.
How Accurate Is BMI?
The arithmetic can be calculated consistently when height and weight are measured accurately. That does not mean BMI is a perfect measurement of body composition. It is best understood as a screening number.
Why BMI Is Still Useful
BMI is quick, inexpensive, and easy to reproduce. It can help identify broad weight patterns and indicate when a fuller assessment may be useful.
How to Use a BMI Calculator
- Select the correct age group.
- Enter accurate height and weight.
- Choose matching units.
- Calculate BMI.
- Use the age-appropriate interpretation.
Frequently Asked Questions
How do you calculate BMI?
Divide weight in kilograms by height in meters squared. With pounds and inches, divide pounds by inches squared and multiply by 703.
What is the BMI calculation formula?
Metric: BMI = weight (kg) ÷ height (m)². U.S. customary: BMI = [weight (lb) ÷ height (in)²] × 703.
How do you work out BMI manually?
Measure height and weight, use consistent units, square height, then divide weight by squared height. Use ×703 for pounds and inches.
What is a healthy BMI range for adults?
For adults aged 20 and older, the standard healthy-weight category is 18.5 to below 25.
Is BMI different for children?
Yes. Ages 2–19 are interpreted using age- and sex-specific BMI-for-age percentiles.
Can athletes have a high BMI?
Yes. BMI cannot distinguish muscle from fat, so muscular people may have a higher BMI.
Is BMI a diagnosis?
No. BMI is a screening measure and should be considered with other health information.
Related SmartWebHub Tools
Calculate now: SmartWebHub BMI Calculator.