Body Composition Tracker
Your gym hands you an InBody printout and no way to read it. Log your scans here and find out what is actually happening.
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Where these numbers are on an InBody sheet
Weight, SMM, and Body Fat Mass are the three bars in the Muscle-Fat Analysis block near the top. Percent Body Fat is in Obesity Analysis directly below it. Total Body Water is the first row of Body Composition Analysis. Visceral Fat Level and BMR appear in the Research Parameters block at the bottom of the sheet, or on the right-hand panel depending on model.
Skeletal muscle mass is not the same as lean body mass. SMM counts only the muscle attached to your skeleton, which is roughly half of lean mass; the rest is organs, bone, skin, and body water. This tracker calculates lean body mass itself as weight minus fat mass, and uses SMM separately as the muscle-specific signal.
How this works
A body composition scan is a snapshot with meaningful error attached. A series of scans taken under the same conditions is a trend line, and the trend is where the useful information lives. This tracker is built around that distinction: it treats every individual number with suspicion and every consistent direction as real.
Making your scans comparable
Bioelectrical impedance works by passing a small current through the body and measuring resistance. Fat-free mass conducts well because it is roughly 73 percent water; fat conducts poorly. The device measures impedance and infers composition from it. This means anything that changes your hydration changes your result, without any change in your actual body composition.
A shift of 1 to 2 liters of body water, which is entirely normal across a day, can move a reported body fat percentage by 2 to 3 points. If you scan dehydrated after training, you will read fatter than you are. If you scan after a high-carbohydrate day, glycogen holds extra water and you will read leaner.
To make scans comparable, standardize every one of these:
- Same time of day, ideally morning
- Fasted, or at least 3 hours after eating
- After using the bathroom
- Before training, not after
- No alcohol in the previous 24 hours
- Same device, same clothing, bare feet and hands
- For women, the same phase of the menstrual cycle, since premenstrual water retention commonly adds 1 to 4 lb
Scan every four to eight weeks. Weekly scanning produces noise that reads as signal and drives bad decisions.
Accuracy, plainly
Multi-frequency segmental devices like the InBody agree with DEXA to within roughly 3 to 5 percentage points of body fat for an individual, with a tendency to underestimate fat in leaner people and overestimate lean mass in people with higher body fat. Population-level agreement is much better than individual agreement, which is why validation studies look more favorable than your personal experience suggests.
What BIA does well is repeatability. Under standardized conditions the same device on the same person reproduces itself within about 1 to 2 percent. That precision is what makes tracking work even when the absolute number is off: if your scan reads 3 points high today, it will read 3 points high in eight weeks, and the difference between them is still real.
The practical rule: do not compare an InBody number to a DEXA number, or to a friend's InBody at a different gym. Compare your scans to your scans.
Reading the trajectory
With three or more scans, the tracker fits a least-squares regression through each metric and reports the slope as a weekly rate, along with an R-squared value indicating how well a straight line describes the data. Low R-squared with more than three scans usually means either your rate genuinely changed partway through, or your scan conditions were not standardized.
The most informative output is the partition of weight change. During weight loss, roughly 20 to 25 percent of the loss coming from lean tissue is typical and acceptable; more than that suggests the deficit is too steep, protein is too low, or resistance training is missing. During weight gain in a trained lifter, more than half the gain arriving as fat means the surplus is larger than your capacity to use it.
Realistic rates
| Situation | Plausible rate |
|---|---|
| Fat loss, sustainable | 0.5 to 1.0 percent of body weight per week |
| Muscle gain, first year of training | 1.0 to 1.5 lb per month |
| Muscle gain, intermediate | 0.5 to 1.0 lb per month |
| Muscle gain, advanced | 0.25 lb per month or less |
| Recomposition, beginner or returning | Both directions at once, realistically |
| Recomposition, lean and trained | Very slow, often indistinguishable from noise |
If your scans show 4 lb of muscle gained in six weeks, the overwhelmingly likely explanation is glycogen and water, not new contractile tissue. Muscle is built slowly and the ceiling is low. Anyone selling you a faster number is measuring hydration.
Visceral fat
Visceral adipose tissue surrounds the organs and is metabolically distinct from subcutaneous fat. It is more lipolytically active, drains directly into the portal vein, and correlates far more tightly with insulin resistance, dyslipidemia, and cardiovascular risk than total body fat does. Two people at the same body fat percentage can have very different visceral fat, and it is the difference that matters clinically.
InBody reports visceral fat as a level, usually on a 1 to 20 scale where the manufacturer treats 1 to 9 as normal and 10 or above as elevated, roughly corresponding to 100 cm squared of visceral fat area on CT. The good news is that visceral fat is preferentially mobilized early in weight loss, so it usually improves faster than the total number does.
Total body water
Body water typically runs 50 to 65 percent of body weight, higher in leaner and more muscular people because muscle is roughly three quarters water. A body water percentage that swings substantially between scans without a corresponding change in lean mass is your signal that scan conditions were not standardized, and it is the reason this tracker asks for the number. Devices reporting intracellular and extracellular water separately are worth watching too: a rising ECW to TBW ratio can indicate inflammation or fluid retention rather than tissue gain.
Related calculators
TDEE Calculator for the full energy expenditure model, including adaptive TDEE from intake data.
Macro Calculator to split the calorie target below into daily grams.
Protein Intake Calculator for meal distribution.
Body Fat Estimator as a between-scans check with a tape measure.
References
Ward LC. Bioelectrical impedance analysis for body composition assessment: reflections on accuracy, clinical utility, and standardisation. Eur J Clin Nutr. 2019;73(2):194-199.
Achamrah N, Colange G, Delay J, et al. Comparison of body composition assessment by DXA and BIA according to the body mass index: a retrospective study on 3655 measures. PLoS One. 2018;13(7):e0200465.
Neeland IJ, Ross R, Despres JP, et al. Visceral and ectopic fat, atherosclerosis, and cardiometabolic disease: a position statement. Lancet Diabetes Endocrinol. 2019;7(9):715-725.
Helms ER, Zinn C, Rowlands DS, Brown SR. A systematic review of dietary protein during caloric restriction in resistance trained lean athletes: a case for higher intakes. Int J Sport Nutr Exerc Metab. 2014;24(2):127-138.
Slater GJ, Dieter BP, Marsh DJ, et al. Is an energy surplus required to maximize skeletal muscle hypertrophy associated with resistance training? Front Nutr. 2019;6:131.
Nunes EA, Colenso-Semple L, McKellar SR, et al. Systematic review and meta-analysis of protein intake to support muscle mass and function in healthy adults. J Cachexia Sarcopenia Muscle. 2022;13(2):795-810.