BMI Calculator

What your BMI result means

Body Mass Index is a simple screening number calculated from your height and weight. It does not measure body fat directly, and it does not account for muscle mass, bone density, age or sex, but it is the same starting-point measure used by the World Health Organization, the CDC and the NIH to flag whether a person's weight is likely to be a health concern worth a closer look. The calculator above sorts your result into one of four broad WHO categories, and Obesity is further split into Class I, II and III, matching the classification used on the full BMI chart.

A result outside the 18.5 to 25 "Normal weight" range is not a diagnosis. It is a signal that is worth discussing with a doctor or another qualified healthcare provider, especially alongside other factors such as waist circumference, activity level, and family health history. Athletes and very muscular people in particular can show a high BMI while carrying very little body fat, because muscle is denser than fat and the formula cannot tell the difference.

The categories themselves come directly from the World Health Organization's adult classification, the same source used by the CDC and the NIH's own calculators: Underweight below 18.5, Normal weight from 18.5 to 24.9, Overweight from 25 to 29.9, and Obese at 30 and above, further split into Class I (30-34.9), Class II (35-39.9) and Class III (40 and above). See the full BMI chart for every band laid out in one table, including the more granular thinness classifications below 18.5.

Three worked examples

A few real calculations using the exact formula this calculator runs, so you can sanity-check your own result:

1. 5'4" (64 in), 130 lb: BMI = 703 x 130 / 64^2 = 22.3, Normal weight. Healthy weight range for this height: 108-146 lb.
2. 5'10" (70 in), 195 lb: BMI = 703 x 195 / 70^2 = 28.0, Overweight. Healthy weight range for this height: 129-174 lb.
3. 170 cm (1.70 m), 90 kg: BMI = 90 / 1.70^2 = 31.1, Obese Class I. Healthy weight range for this height: 53.5-72.3 kg.

BMI compared to other ways of measuring body composition

BMI is popular because it only needs two numbers you almost certainly already know, but it is far from the only measure researchers and clinicians use. Waist-to-height ratio (waist circumference divided by height) is often cited as a better predictor of cardiovascular risk than BMI alone, because it captures where fat is carried, not just how much a person weighs relative to their height; a common rule of thumb is keeping your waist under half your height. Body fat percentage, measured with skinfold calipers, bioelectrical impedance scales, or a DEXA scan, measures fat mass directly rather than inferring it from weight, which is exactly the gap that lets BMI misclassify very muscular people as "overweight." Waist-to-hip ratio is another common alternative, again focused on fat distribution rather than total mass.

None of these alternatives are built into this calculator, on purpose: each requires either a tape measure at a specific body location, a specialized scale, or clinical equipment, and mixing them into a single BMI tool would blur what each measurement actually tells you. BMI's real advantage is that it takes ten seconds and needs nothing more than a scale and a tape measure (or a health record), which is why it remains the standard first-pass screening number, with these other measures used as follow-ups when a closer look is warranted.

Two more worked examples, including a category change

Two more real calculations, chosen to show how a small change in either number can move the result:

1. 5'6" (66 in), 150 lb: BMI = 703 x 150 / 66^2 = 24.2, Normal weight, close to the Overweight boundary at 25. Just 6 more pounds at the same height (156 lb) pushes BMI to 25.2, Overweight, which shows how a modest weight change near a boundary can flip the labeled category even though very little has actually changed physically.
2. 163 cm (1.63 m), 55 kg: BMI = 55 / 1.63^2 = 20.7, Normal weight, comfortably in the middle of the WHO Normal band (18.5 to 24.9), unlike the boundary case above.

Imperial or metric, your choice

Click the Imperial or Metric button above the calculator to switch units at any time; your entered values stay in place and the result recalculates instantly. Imperial mode uses feet and inches for height and pounds for weight, the everyday format for the US. Metric mode uses centimeters and kilograms, the format used almost everywhere else. The math behind both is the same BMI formula, just expressed with a different unit conversion factor: see the BMI formula page for the exact equations in both systems, worked out step by step.

Why this calculator also shows a healthy weight range and BMI Prime

A single BMI number only tells you where you land right now. Two extra numbers make that result more useful in practice. The healthy weight range takes your exact entered height and works out the weight span (in pounds or kilograms) that would put you inside the WHO "Normal weight" band, so you have a concrete target range instead of just an abstract index number. BMI Prime is simply your BMI divided by 25 (the upper edge of the normal range), which gives a quick dimensionless ratio: under 1.0 means you are below the normal-weight ceiling, and it makes it easy to compare how far above or below that line two different BMI values sit, without doing the division yourself.

Both numbers are most useful when you track them over time rather than reading them once. If you are working toward a weight goal, the healthy weight range gives you a concrete target band instead of a single number to hit exactly, which is more realistic since a healthy weight is genuinely a range, not one precise figure. BMI Prime is particularly handy for a fast side-by-side comparison: if a checkup six months ago gave a BMI Prime of 1.05 and today's gives 0.98, that single ratio tells you the trend moved in the right direction without needing to recompute two separate BMI values and compare them manually.

This calculator is for adults

The WHO BMI categories used here (Underweight, Normal weight, Overweight, Obese Class I-III) apply to adults, and are the same for men and women; there is no separate men's or women's chart in the WHO adult standard, see the BMI chart for the full explanation of why that is. BMI for children and teenagers (ages 2 to 19) is worked out completely differently: instead of the same four fixed BMI bands used for adults, a child's or teen's BMI is compared against a percentile growth chart for their exact age and sex, since a healthy BMI for an 8-year-old is very different from a healthy BMI for an 18-year-old. That calculation needs a different reference dataset entirely, so this calculator intentionally does not attempt it; the CDC's own BMI Calculator for Children and Teens is the right tool for that age range.