# Body fat (BF%)

A bioimpedance estimate, corrected against a DEXA scan when you have one; 2 to 3 points of day-to-day noise.

Type: body_value · Unit: % · Domain: Body Composition · Accuracy: Estimate · Importance: Core

Also called: body fat percentage, fat percentage

## In one sentence

The share of your weight that is fat, which a smart scale estimates from how easily a small current passes through you.

## Why it matters

Two people of the same weight and height can be made of very different things, and body fat is the number that says which. It carries far more information than weight: during a diet it tells you whether what you lost was fat or muscle and water, and over years it is the measure that moves with how you eat and train. In population studies body fat tracks the metabolic measures that weight alone misses, which is why Vitality Tracker gives it the largest share of the body-composition area of the Vitality Score.

## What moves it

True body fat changes slowly, by a few hundred grams a week at most. The scale's number moves much faster, because the scale does not see fat; it measures water, and infers fat from it. Anything that changes your water changes the reading: dehydration reads as more fat, a carbohydrate-rich day as less, a hard session, a sauna, an evening's alcohol and the menstrual cycle all move it by a point or two. The device itself adds a bias that is personal and constant: in 106 adults, consumer scales read body fat with a mean error of under 1.5 points on the better devices and about 3 points on the worst, but the range for an individual ran as wide as 14.5 points under to 8.6 over on a foot-to-foot scale, against 6.6 under to 4.6 over on a hand-to-foot one (Bosy-Westphal 2008). Dry feet can add two or three points on their own.

## How to measure it well

[*How to use a bioimpedance scale*](https://vitalitytracker.app/knowledge/how-to-use-a-bioimpedance-scale): same scale, same morning routine, damp soles, still feet, and the bowel question answered. Then look at the 14-day trend and ignore the morning.

## How Vitality Tracker uses it

Three things. VT corrects the scale's number against a DEXA scan when you have one, so your daily trend sits on a calibrated level rather than the scale's guess. It reads the 14-day trend, not the reading. And it shows the value as a percentile for your age and sex beside your metabolic age, so body composition shows through without being turned into an age. Body fat carries 60 percent of the body-composition area of the Vitality Score, with a 90-day shelf life, and the lean mass it implies feeds the energy engine's resting-metabolism estimate when a DEXA anchor exists. A new scale starts a new series.

## How far to trust the number

Estimate. The same scale on the same morning routine is consistent with itself (test–retest agreement of 0.92 to 0.99 for lean mass on three low-cost devices; Vasold 2019), so the direction of its trend is believable even when its level is off by several points. Never compare one scale with another, or your number with a friend's.

## Method

The scale converts impedance, height, weight, age and sex into fat through the maker's own equation, which is not published. VT's DEXA correction applies an offset learned from your scan to the scale's series (see *[DEXA](/knowledge/dexa)*). The percentile uses the Kelly 2009 NHANES reference curves (see *[Body-fat percentile](/knowledge/body-fat-percentile)*).

## Sources

1. Bosy-Westphal A, Later W, Hitze B, et al. Accuracy of bioelectrical impedance consumer devices for measurement of body composition in comparison to whole body magnetic resonance imaging and dual X-ray absorptiometry. Obes Facts 2008;1:319–24. [doi:10.1159/000176061](https://doi.org/10.1159/000176061)
2. Vasold KL, Parks AC, Phelan DML, Pontifex MB, Pivarnik JM. Reliability and validity of commercially available low-cost bioelectrical impedance analysis. Int J Sport Nutr Exerc Metab 2019;29:406–10. [doi:10.1123/ijsnem.2018-0283](https://doi.org/10.1123/ijsnem.2018-0283)
3. Kelly TL, Wilson KE, Heymsfield SB. Dual energy X-ray absorptiometry body composition reference values from NHANES. PLoS One 2009;4:e7038. [doi:10.1371/journal.pone.0007038](https://doi.org/10.1371/journal.pone.0007038).

## Related

- [Fat mass and lean body mass](https://vitalitytracker.app/knowledge/fat-mass-and-lean-body-mass)
- [Body-fat percentile](https://vitalitytracker.app/knowledge/body-fat-percentile)
- [DEXA](https://vitalitytracker.app/knowledge/dexa)
- [Bioimpedance: the smart scale](https://vitalitytracker.app/knowledge/bioimpedance)
- [How to use a bioimpedance scale](https://vitalitytracker.app/knowledge/how-to-use-a-bioimpedance-scale)
- [Why your weight and body fat wobble](https://vitalitytracker.app/knowledge/why-weight-and-body-fat-wobble)
- [Vitality Score](https://vitalitytracker.app/knowledge/vitality-score)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/body-fat