BMI Calculator
Body Mass Index, a screening number I would never treat as the whole picture.
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Learn the details
BMI is your weight divided by your height squared: kilograms ÷ metres², or 703 × pounds ÷ inches². It is a screening ratio built for comparing populations, so it cannot tell muscle from fat on any one person.
The arithmetic is the easy part. What trips people up is how much to read into the answer, so here is the honest version: where BMI earns its place, where it falls apart, and what I would read alongside it.
Where the ratio came from
BMI was never designed to grade an individual. The Belgian statistician Adolphe Quetelet worked it out in the 1830s while studying the average characteristics of whole populations, and it carried his name for more than a century before Ancel Keys compared the simple weight-for-height ratios in 1972 and found this one tracked body fatness across large groups better than the others. Squaring height is the trick that makes weights roughly comparable between someone 5 foot 2 and someone 6 foot 4, and that comparison is the entire job the ratio was hired to do. Everything we ask of it beyond that is stretching a population tool over a single person.
A worked example
Say you are 5 foot 10 inches, which is 70 inches, and you weigh 180 pounds. Square the height and you get 4,900. Multiply your weight by 703 to get 126,540, then divide by 4,900. That lands at a BMI of about 25.8, which sits just over the line into the overweight band. Nudge the same person to 170 pounds and the BMI drops to about 24.4, back inside the healthy range. Small weight changes move the number more than most people expect, which is exactly why a single reading is a starting point, not a verdict.
The standard ranges
- Under 18.5: underweight
- 18.5 to 24.9: healthy range
- 25.0 to 29.9: overweight
- 30.0 and above: obese
These cutoffs come from population health data, so they describe how weight tracks with risk across large groups, not what is right for you specifically on any given day.
Where BMI misleads
A lifter carrying a lot of lean mass can read overweight, or even obese, while being genuinely lean, and someone in the middle of the healthy range can still carry too little muscle and too much fat. To this formula, weight is just weight. It also ignores where that weight sits, and fat around your waist carries more risk than the same pounds on your hips. Age, sex, and ethnicity shift what a healthy number looks like too, which is why two people at an identical BMI can be in genuinely different places.
What to do with your number
If your BMI is inside the healthy range, it is one small green light, keep training and eating in a way you can sustain. If it reads high or low, pair it with a couple of measures that actually see your body: your body fat percentage and your waist-to-hip ratio. Those, plus how your lifts and energy are trending, tell you far more than BMI alone. If you are not sure what to aim for once you have that number, what a healthy body fat percentage looks like lays out the ranges for men and women. If a reading worries you, that is a conversation for a doctor, not a calculator.
Sources: CDC adult BMI categories
Check monthly during a cut or bulk; daily weight swings make more frequent readings noisy. If you're muscular, expect a higher BMI and lean on FFMI or body fat % instead.
BMI is one of five ways to read the same body. See all five against one set of measurements.