Free BMI calculator
Calculate your Body Mass Index using metric or imperial units. See your BMI category based on WHO guidelines instantly.
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What is BMI?
Body Mass Index, or BMI, is a simple calculation based on height and weight. The standard formula is:
BMI = weight (kg) ÷ height² (m)
It is used as a screening tool to identify potential weight-related risk in adults. BMI does not measure body fat directly, but it offers a quick way to place a height and weight combination into a standard category.
BMI is useful as a starting point, not a full health assessment. It can overestimate body fat in people with high muscle mass and underestimate it in older adults who have lost muscle. It also does not account for differences in body composition, fat distribution, age, sex, or ethnicity.
BMI categories (WHO)
| Category | BMI range |
|---|---|
| Severe thinness | < 16 |
| Moderate thinness | 16 – 16.9 |
| Mild thinness | 17 – 18.4 |
| Normal weight | 18.5 – 24.9 |
| Pre-obese | 25 – 29.9 |
| Obese class I | 30 – 34.9 |
| Obese class II | 35 – 39.9 |
| Obese class III | ≥ 40 |
Frequently asked questions
What is a healthy BMI for adults?
The World Health Organization (WHO) defines a healthy BMI as 18.5–24.9. A BMI below 18.5 is considered underweight, 25–29.9 is pre-obese (overweight), and 30 or above is classified as obese. These thresholds apply to most adults but may not be appropriate for all ethnic groups.
Is BMI accurate for women?
BMI uses the same formula for men and women, but women naturally carry more body fat at the same BMI. This means BMI may slightly underestimate health risk in women. It is still a useful screening tool, but should be combined with other measures like waist circumference for a fuller picture.
Why does BMI not account for muscle mass?
BMI only measures weight relative to height — it cannot distinguish between fat and muscle. A muscular athlete may have a BMI in the overweight range despite having very low body fat. For a more accurate assessment, consider a body fat percentage measurement or DEXA scan.
What is BMI prime?
BMI prime is your BMI divided by 25 (the upper limit of the normal range). A BMI prime of 1.0 means you are exactly at the top of the healthy range. Values below 1.0 are normal or underweight; above 1.0 indicates overweight or obese. It makes it easy to compare across different BMI thresholds.
How do I calculate BMI manually?
In metric units: divide your weight in kilograms by your height in metres squared. For example, 70 kg ÷ (1.75 × 1.75) = 22.9. In imperial units: (weight in pounds × 703) ÷ (height in inches)². Our calculator does this automatically for both unit systems.
Can BMI be used for children?
Standard adult BMI thresholds do not apply to children. For ages 2–19, BMI-for-age percentiles (from growth charts) are used instead. A child is considered healthy if their BMI falls between the 5th and 85th percentile for their age and sex. This calculator is designed for adults only.
Related articles
Is BMI Accurate for Athletes? Why the Number Often Misses the PointAthletes often get BMI results that seem misleading because muscle changes the picture. This guide explains whether BMI is accurate for athletes and why body-fat context usually matters more.
BMI for Women vs Men: What Changes, What Doesn’t, and What to WatchBMI uses the same formula for women and men, but body composition differences still matter. This guide explains how BMI applies across sexes and where body-fat context becomes more useful.
Is BMI Accurate for Most People? What It Tells You and What It MissesBMI is one of the most widely used health screening metrics, but it has clear limits. This guide explains how accurate BMI really is, when it helps, and when other measures matter more.