Body-fat measurements
"My scale says 24 percent body fat. Is that right?" Probably not to within several points — and the same is true of most ways of getting that number.
The methods, by accuracy
- Four-compartment model. The research reference: it measures body density, total water and bone mineral separately and combines them. Not available outside a laboratory; every other method is judged against it.
- DXA. Dual-energy X-ray absorptiometry; the clinical reference, accurate to a couple of percentage points for a person, sensitive to hydration and to the manufacturer's software. A scan costs money and a small dose of radiation, and is worth it if you want a real number once.
- Bod Pod. Air-displacement plethysmography; close to DXA in accuracy for most people, without radiation.
- Skinfolds. Callipers in trained hands are reasonable for tracking one person over time; between people and between measurers the error grows fast.
- Bioimpedance (BIA). The bathroom scales and wristbands. They send a small current and infer fat from resistance, which depends heavily on hydration, on when you last ate, exercised or drank, and on the equation the manufacturer chose. A systematic review comparing BIA devices against the four-compartment model finds that agreement varies widely, that consumer devices are the simplest class and err the most, and that for an individual the error is large — several percentage points either way — while the trend under constant conditions is more usable than the number.
What the app does without a measurement
When no DXA or Bod Pod result is entered, the app estimates body fat from BMI, age and sex with a 1991 population equation whose standard error is about four percentage points. That is a deliberately modest estimate used only to derive fat-free mass for an energy floor; the BMI calculator shows the same figure so you can see what the app is working from. A measured value replaces it.
The honest use
If you want one true number, get a DXA or Bod Pod scan. If you want to know whether a programme is working, weigh under the same conditions, measure your waist monthly, and — if you like the scale's body-fat reading — read it the same way every time and watch its direction, never its value. A reading that moves three points in a week has measured your hydration.