BMI Calculator

The BMI formula

Body mass index divides weight by the square of height, using metric units:

In imperial units, the equivalent formula multiplies by a constant to correct for the different units: BMI = 703 × weight (lb) ÷ [height (in)]². Both formulas produce the same BMI value for the same person — this calculator uses the metric version internally.

Standard BMI categories

These are the World Health Organization's general adult cutoffs. Some national health bodies use slightly adjusted thresholds for specific populations.

Step-by-step: getting an accurate reading

1. Measure height without shoes, standing straight against a flat wall.
2. Weigh yourself in light clothing, ideally at a consistent time of day (first thing in the morning is common, since weight fluctuates through the day).
3. Enter both figures in consistent units — this calculator expects centimeters and kilograms.
4. Read the result as a general screening figure, not a precise health verdict — see the FAQ below on why BMI has real limitations.

Metric vs. imperial: how this calculator handles both

BMI's underlying formula is defined in metric units — kilograms and meters — but the tool accepts imperial input (pounds and inches or feet) and converts internally before calculating, so the result is identical either way. If you've seen a slightly different imperial-only formula elsewhere (weight in pounds × 703, divided by height in inches squared), that's a rearranged version of the same underlying metric formula, adjusted with a conversion constant — not a different method.

BMI across different groups

The standard BMI categories shown above were developed primarily using adult population data and are generally not applied the same way to children, teenagers, pregnant women, or elderly adults, all of whom have different healthy body-composition ranges. Children and teens are typically assessed using age- and sex-specific percentile charts rather than the fixed adult thresholds. If you're calculating BMI for anyone outside a typical adult range, treat the standard categories with extra caution and consult a pediatrician or relevant specialist instead. The same caution applies to older adults, where some research suggests a slightly higher healthy range may be more appropriate, reflecting natural age-related changes in muscle and bone density that the standard scale doesn't separately account for.

Pregnancy is another case where standard BMI categories don't apply in the usual way, since healthy weight gain during pregnancy is expected and tracked against different benchmarks entirely — a pre-pregnancy BMI is what's typically used as the starting reference point, with an obstetrician guiding appropriate weight gain from there rather than the general categories on this page.

Why BMI is a starting point, not a full picture

BMI was designed in the 1830s as a population-level statistic, not an individual diagnostic tool, and it has well-known blind spots. It can't distinguish muscle from fat, so a muscular athlete can register as "overweight" despite having low body fat, while someone with low muscle mass and higher body fat can land in the "normal" range despite carrying more health risk. It also doesn't account for where fat is distributed on the body, age-related muscle loss, or differences between individuals that a single height-and-weight ratio can't capture. Clinicians typically use BMI alongside other measurements — waist circumference, body composition, blood pressure, bloodwork — rather than as a stand-alone verdict. Treat a BMI result here as one data point worth discussing with a healthcare provider if you have questions about it, not as a conclusion on its own.

Frequently asked questions