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Published 13 Sep 2026
No household method measures your body fat percentage. Every option estimates it, each in its own way, and they disagree with each other by more than the change most people are trying to detect. That one fact should shape how you use the number.
Four methods are within reach of most people, and each one is wrong in its own way.
| Method | What it measures | Where it goes wrong |
|---|---|---|
| Skinfold calipers | fat directly under the skin at set sites, fed into a formula | assumes a relationship with total fat that varies between people; consistent in practised hands, much less so between two testers |
| Bio-impedance (scales with handles, or the ones you stand on) | how a small current travels through you | it reads hydration as much as fat, so morning and evening, or before and after a salty meal, give different numbers |
| DEXA | two X-ray energies; the closest to a direct measurement outside a lab | a clinic appointment with a cost, and it still varies between machines and between scans on the same machine |
| Tape formulas | circumferences into a published equation | crude, but a shrinking waist is real information whatever the formula turns it into |
Not your percentage. The gap between methods. Put the same person through calipers, a bio-impedance scale and a tape formula on the same morning and the three answers can differ by more than that person would be pleased to achieve in two months of careful work.
That is the practical heart of this. Measure with one method in January and another in March and the difference you see is mostly the methods disagreeing, not your body changing. You can produce progress you did not make, or hide progress you did.
So: pick one method, use it under the same conditions each time, and read the trend rather than the value. A scale that reads a little high but reads the same way every Tuesday morning is more useful than an accurate one-off.
Fitener uses your body fat percentage, when you give it one, to work from your lean mass instead of your total weight. That changes which equation sets your resting energy figure, and from there it moves your calorie target and your protein target with it. The BMR calculator shows both equations side by side and says which one applies to the numbers you type in.
Which means an error here does not stay put. A body fat figure that is well off shifts the whole plan built on top of it, in the direction of the error. That is the real reason to care where the number comes from.
It is also why the app does not guess one. Leave the field empty and it falls back to a rule based on body weight alone, deliberately more conservative than pretending to know a composition it has not been told.
Three habits turn the figure into something useful rather than something misleading.
Body composition is a health matter as well as a training one. If you are being assessed for something clinical, if you are pregnant, or if a measurement has left you worried, that is a conversation with a doctor rather than a calculation. This page is about method error in healthy adults who train, and it does not go further than that.
DEXA comes closest of the methods available outside a laboratory, and it still varies between machines and between scans. For tracking change, consistency with one method beats accuracy from a method you use once.
For a single true value, not really; they read your hydration as much as your fat. For a trend, used at the same time of day under the same conditions, they are more useful than their reputation suggests.
Because they measure different things and convert them with different assumptions. The gap between methods is usually wider than the change you are looking for, which is why mixing them makes your data unreadable.
Rarely. Every month or two is plenty, because no household method can resolve a week of real change, and measuring more often mainly produces noise.
It uses it to work from your lean mass instead of your weight, which changes which equation sets your resting energy figure and moves your calorie and protein targets with it. The BMR calculator shows both equations and says which one applies.
The calculators behind this: BMR calculator · protein page
More reading: body recomposition · all posts
Fitener runs these rules on the data you log, so the targets move when you do, and it puts what you ate next to them. One app for the training and the eating.