Adult, Child & Teen BMI Calculator

What Is BMI?

Body Mass Index, usually called BMI, is a calculated measure of weight relative to height. It is widely used as a simple screening measure because the calculation is quick, inexpensive, and easy to reproduce. BMI can help identify a broad weight category, but it does not directly measure body fat and should not be treated as a diagnosis.

For adults, BMI is interpreted with standard fixed categories. For children and teens, BMI changes naturally as they grow, so the BMI number must be compared with age- and sex-specific reference data.

How to Calculate BMI

The basic BMI calculation is the same mathematical relationship for all ages:

For example, a person weighing 70 kg and measuring 1.75 m has a BMI of about 22.9. In a pounds-and-inches example, 160 lb at 66 inches gives a BMI of about 25.8.

The CDC documents both formulas and recommends accurate height and weight measurements for BMI calculation. Read the CDC BMI calculation guidance.

Adult BMI Categories for Ages 20+

These adult ranges are used for people aged 20 and older. The CDC further divides obesity into three classes at higher BMI levels. BMI is a screening measure and should be considered together with other information about health.

Source: CDC Adult BMI Categories.

BMI for Children and Teens Ages 2–19

Children and teenagers are still growing, so adult BMI cutoffs should not be used to interpret their results. For ages 2–19, BMI is interpreted with sex-specific BMI-for-age percentiles. The CDC child and teen calculator provides BMI, BMI percentile, and BMI category using growth-reference data.

Because percentile interpretation requires exact age, sex, and reference growth data, this SmartWebHub tool calculates the BMI number but directs users to the official CDC calculator for the percentile and category.

Use the official CDC Child and Teen BMI Calculator.

How to Use This BMI Calculator

1. Select 2–19 years or 20+ years.
2. Enter age. For children and teens, enter age plus any additional months.
3. For ages 2–19, select the sex used for the BMI-for-age reference.
4. Enter weight and choose kg or lb.
5. Enter height and choose cm, m, ft, or in.
6. Select Calculate BMI.
7. Adults 20+ see the standard adult category. Children and teens see the BMI number and a link to official percentile interpretation.

Common BMI Calculation Mistakes

Using centimeters as meters

If your height is 175 cm, use 1.75 m in the metric formula. Entering 175 as meters produces an invalid result.

Forgetting to square height

BMI uses height squared. Squaring 1.75 means 1.75 × 1.75 = 3.0625.

Mixing pounds with meters

Use consistent units. The metric formula expects kilograms and meters; the U.S. customary formula uses pounds and inches with 703.

Using adult cutoffs for children

Adult thresholds such as 18.5, 25, and 30 are not the correct way to interpret BMI for ages 2–19.

Does BMI Measure Body Fat?

No. BMI uses height and weight, so it cannot directly tell you your body-fat percentage, muscle mass, or body-fat distribution. A muscular person can have a higher BMI because muscle contributes to body weight.

This limitation is one reason BMI should be treated as one screening measure among several rather than as a complete description of body composition or health.

BMI and Age, Sex, Pregnancy, and Athletic Training

Age matters because the body changes over time. For children and teens, age is part of the reference used to interpret BMI. For adults, standard BMI categories are intended for people 20 and older.

Sex matters for interpreting BMI-for-age in children and teens because the growth-reference percentiles are sex-specific. For adult BMI, the formula itself uses weight and height rather than a male/female coefficient.

Pregnancy is a special circumstance in which a general BMI calculator should not be used to assess pregnancy weight gain or fetal health. Similarly, highly muscular athletes may have a BMI that does not reflect body-fat level.

Why Accurate Height and Weight Matter

Small measurement errors can change BMI because height is squared in the formula. For consistency, measure height without shoes and use a reliable scale when practical. The CDC notes that accurate height and weight measurements are important for correct BMI calculations and BMI-for-age interpretation.

Is BMI the Same as Healthy Weight?

No. BMI is a screening number that places weight relative to height into broad categories. A healthy-weight BMI category does not guarantee that every aspect of health is optimal, and a BMI outside the standard range does not by itself diagnose a disease.

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