BMI CALCULATOR – CHECK YOUR BODY MASS INDEX ONLINE

Formula to calculate BMI is weight (kg) ÷ [height (m)]². Healthy BMI fall between the

range, 18.5–24.9.

For children and teens, BMI is interpreted using age- and sex-specific percentiles rather than the adult cut-offs.

For adults, most online BMI calculators are computationally accurate—they all use the same formula (weight ÷ height²) and standard categories. What varies is your input and how results are interpreted.

No—don’t rely on BMI alone.

BMI is fine for a quick screen, but it can misclassify people because it doesn’t show body-fat distribution, muscle mass, age/sex differences, or health history.

Your age doesn’t change how BMI is calculated, but it does change how we interpret it.

• Adults (20+): BMI is just weight ÷ height² and the cut-offs are the same regardless of age. Age isn’t part of the formula.


• Kids & teens (2–19): Because they’re still growing, BMI is read against age- and sex-specific percentiles (BMI-for-age). So here, age does affect how a BMI is classified.


• Older adults: With age, people typically lose muscle and may gain fat even if BMI stays the same. That body-composition shift means BMI can under- or overestimate health risk in later life, so waist size, body-fat % and strength measures become especially useful alongside BMI.

BMI can mislead in several situations because it only uses height and weight—not body fat, muscle, or fat distribution. It can be “wrong” (misclassify risk) when:

• You’re very muscular (e.g., athletes): High muscle pushes BMI up, making healthy, lean people look “overweight/obese.”


• You’re older or have low muscle mass: Age-related muscle loss can keep BMI “normal” even when body fat and health risk are high.


• Different body types or ethnic backgrounds: Health risks can appear at lower BMIs for some groups (e.g., many Asian populations) and at higher BMIs for others, so one-size-fits-all cutoffs can misclassify risk.


• Children and teens: Adult BMI cutoffs don’t apply; they need age- and sex-specific percentiles.


• Pregnancy or major fluid shifts: Pregnancy, edema, ascites, or dehydration change weight without reflecting true body fat.


• Unusual body proportions or amputations: Very short/tall stature, spinal curvature, or limb loss can distort BMI’s height–weight relationship.


• “Normal-weight obesity”: A normal BMI with high body fat—especially around the waist—still carries cardiometabolic risk.


• High bone density or large frame: Heavier bones can elevate BMI independent of fatness.