Body fat percentage gets treated as a precise, authoritative number, but the methods used to estimate it outside a lab vary enough that the number deserves more skepticism than it usually gets.
How it's typically estimated
Common methods include skinfold calipers (measuring the thickness of a fold of skin at specific sites), bioelectrical impedance (sending a small electrical current through the body, common in smart scales), and formula-based estimates using measurements like waist and neck circumference alongside height. Each has a meaningfully different margin of error, and none of them matches the accuracy of a lab method like DEXA scanning or hydrostatic weighing.
Why estimates disagree with each other
Hydration level, time of day, recent food and drink, and even room temperature can shift a bioelectrical impedance reading by a noticeable margin — which is why the same person can get different numbers from the same smart scale on different days without any real change in body composition. Skinfold and formula-based methods carry their own separate sources of error, which is why different methods on the same person, on the same day, can disagree meaningfully.
What to actually track
Given the noise in any single reading, the trend over weeks or months — using the same method consistently — is far more meaningful than any single number in isolation. A body fat calculator is useful for this kind of consistent, repeatable tracking, even though the absolute number carries real uncertainty.
The bottom line
Treat any body fat percentage estimate as a rough, method-dependent approximation for tracking direction over time — not a precise clinical measurement.
Try it yourself
Put this into practice with our body fat calculator.