Body Fat Trend Calculator
Two readings of weight and body fat tell you something neither gives alone: whether the weight you lost was fat, and whether you kept your muscle.
Your results
Entirely dependent on the accuracy of the two body-fat figures. A 3-point error in either shifts the lean-mass result by roughly 2.5 kg — so use the same method under the same conditions for both readings, and treat the direction as more reliable than the magnitude.
The calculation this makes possible
The scale tells you total mass changed. It cannot tell you what changed. This can:
- Fat mass = weight × body fat %
- Lean mass = weight − fat mass
Do that at two points and subtract. If you lost 6 kg and 5.5 kg of it was fat, that is an excellent diet. If you lost 6 kg and 3.8 kg of it was lean mass, the rate was too aggressive or the protein and training were not there.
This is the single most useful thing you can do with body-fat measurements — far more useful than knowing the absolute number.
Why the error margin matters here
Body-fat methods carry real error: bioimpedance scales ±4–8%, tape-measure formulas ±4–6%, skinfolds ±3–5%, DEXA ±1–3%. And this calculation uses two readings, so the errors compound.
Practical consequences:
- Use the same method both times, under the same conditions — same time of day, same hydration state.
- Leave enough time between readings. Eight to twelve weeks minimum. Over two weeks the change is smaller than the error.
- Trust the direction more than the magnitude. "Mostly fat" is a reliable conclusion; "exactly 5.5 kg of fat" is not.
- Cross-check with your logbook. If your lifts held, you kept the muscle, whatever the numbers say.
Common questions
How do I know if I am losing muscle or fat?
Compare fat mass and lean mass at two points. Also check your logbook — if your lifts held, the loss was mostly fat.
Can I lose fat and gain muscle at the same time?
Yes, especially as a beginner, when returning after a break, or with fat to lose. It is slow and the scale barely moves.
How often should I take these readings?
Every eight to twelve weeks. Shorter intervals are swamped by measurement error.
My lean mass dropped. Is that muscle?
Partly, and partly water and glycogen — which fall in a deficit and return when you eat normally. Do not panic at a small drop.
Track the numbers, not just calculate them
MyFitsy stores your weight, measurements and every session, so the figures above become a trend instead of a one-off.
