Free BMI Calculator for Adults and Children
Enter your height and weight above to get your BMI result instantly. This calculator uses WHO standards for adults and CDC growth chart percentiles for children aged 2–17.
BMI Calculator
Body Mass Index — Adults & Children · WHO-standard classifications
What is BMI?
BMI — Body Mass Index — is a number calculated from your height and weight. It’s one of the most widely used tools to screen whether someone’s weight is in a healthy range for their height. It doesn’t measure body fat directly. But for most people, it gives a reliable starting point for understanding where their weight sits and whether it could be affecting their health.
The WHO BMI categories for adults are:
For children and teens aged 2–17, BMI is calculated the same way — but the result is read differently. Because body fat naturally changes as kids grow, and differs between boys and girls, a child’s BMI is compared to other children of the same age and sex using CDC growth charts.
The result is given as a percentile:
| Percentile | Category |
|---|---|
| Below 5th | Underweight |
| 5th to 84th | Healthy weight |
| 85th to 94th | Overweight |
| 95th and above | Obese |
BMI for children and teens
For children and teens aged 2–17, BMI is calculated the same way — but the result is read differently. Because body fat naturally changes as kids grow, and differs between boys and girls, a child’s BMI is compared to other children of the same age and sex using CDC growth charts.
The result is given as a percentile:
How to calculate BMI
The BMI formula uses weight and height. In metric units:
BMI = weight (kg) ÷ height (m)²
In imperial units (pounds and inches):
BMI = (weight in lbs × 703) ÷ height in inches²
Example: A person who weighs 70 kg and stands 1.75 m tall has a BMI of: 70 ÷ (1.75 × 1.75) = 22.9 — Normal weight range.
The calculator above does all of this for you in both metric and imperial units.
What BMI doesn't tell you
BMI is a useful screening tool, but it has real limits. After 7+ years of coaching, I’ve seen this cause confusion more times than I can count — and it’s worth being clear about. It doesn’t account for muscle mass. A 90 kg athlete with low body fat and a 90 kg sedentary person can have the exact same BMI. The number doesn’t know the difference.
If you’re training seriously, your BMI may read as overweight even when your body composition is excellent. It doesn’t distinguish where fat is stored. Visceral fat — the fat around your organs — carries far higher health risk than fat stored under the skin. BMI doesn’t separate the two.
It’s less accurate at the extremes of height. Very tall or very short people can get BMI readings that don’t match their actual body fat levels. In my coaching practice, I use BMI as a starting reference — not a verdict. It tells me where to look, not what the final answer is. For a fuller picture, tracking waist circumference, body fat percentage, and training performance alongside BMI gives you much more useful data.
Frequently asked questions
For adults aged 18 and older, a BMI between 18.5 and 24.9 is considered a healthy weight. Below 18.5 is underweight. Between 25 and 29.9 is overweight. A BMI of 30 or above falls into the obese category, which is further divided into three classes by the WHO.
The BMI calculation is the same for men and women. However, women naturally carry more body fat than men at the same BMI. This is a known limitation of the standard formula. At the same BMI reading, two people with different body compositions can have very different health risk profiles.
In children aged 2–17, BMI is measured against CDC growth chart percentiles rather than fixed numbers. A healthy weight falls between the 5th and 84th percentile for a child’s age and sex. Our calculator includes a dedicated child and teen mode that accounts for this.
Yes — particularly if you have significant muscle mass. Muscle is denser than fat, so highly trained individuals often read as overweight or even obese on standard BMI charts. If you’re actively training, use BMI alongside other markers like waist circumference and body fat percentage for a more accurate picture.
Once every 4–8 weeks is enough for most people tracking progress. Checking too frequently creates noise — daily weight fluctuations from water retention, food timing, and training can make the numbers misleading if you read too much into short-term changes.
Use our other free calculators to build a complete picture of your fitness:
- Track your calorie burn per workout based on your weight and intensity
- Find your daily protein target for muscle growth or fat loss
- Calculate your daily water intake based on your body weight and activity level