FortiorBody
Body fat percentage tracking: weight, body fat and lean mass trends
A single body fat reading is one sample from a noisy measurement. Fortior charts your bodyweight, body fat and lean mass over time, labels where each reading came from, and holds back weights that look like mistakes until you decide.
Why trends beat single readings
Body weight can move by a kilogram or more in a day with water, food, salt and time of day, which is often more than any real change in fat or muscle. Body fat percentage adds measurement error on top: different scales and scanners can read the same person several percentage points apart. One number mostly reflects the conditions on the day.
What carries information is the direction over weeks, measured the same way each time: the same device, a similar time of day and similar hydration. Fortior is built around that idea.
What Fortior tracks
- Weight, body fat and lean mass as latest values with their own charts for the date range you pick. Each reading keeps the name of the app or scale it came from.
- Trend. A straight-line fit through the weights in your range, shown as gaining, losing or stable per week. Under 0.25% of bodyweight per week reads slow, up to 0.6% reads steady, and above that reads fast.
- Lean mass and FFMI. If a reading has measured lean mass, Fortior uses it. Otherwise it works lean mass out from the weight and body fat in the same reading. The fat-free mass index (FFMI) is lean mass in kilograms divided by height in metres squared, and it needs your height.
- Body-fat sources. When Apple Health supplied body fat or lean mass, you can choose which sources to use.
To log a weight or body fat by hand, tap Weight in the Log section on Today. To bring readings in automatically, connect Apple Health so your scale or scanner app can write to it.
A worked example
At 80 kg (176 lb) and 18% body fat, lean mass is 80 × (1 − 0.18) = 65.6 kg (145 lb) and fat mass is 14.4 kg. At a height of 180 cm, FFMI is 65.6 ÷ 1.8² = 20.2. If a different scale had read 16% body fat the same morning, lean mass would be 67.2 kg and FFMI 20.7. Same person, same day, half a point of FFMI apart. That is why one reading isn't a verdict.
For the trend, suppose 80 kg drifts to 79.2 kg over two weeks. That is −0.4 kg per week, or 0.5% of bodyweight, and it reads as losing about 0.40 kg per week (steady).
Weights that look wrong: the outlier review
One bad weigh-in, such as shoes on or a mistyped unit, can swing your trend and every number that uses your weight. Fortior watches for isolated, severe outliers. It holds a weight back only when there are readings within three weeks on both sides, those neighbors roughly agree with each other, and the reading differs from their median by at least 10 kg or 15%, whichever is larger. A real change that stays changed is never flagged, and readings at the very start or end of your history are kept.
A held-back weight stays out of your weight trend, and out of anything that uses your weight, until you look at it. On the bodyweight chart, tap Review and choose:
- Confirm to use the reading.
- Exclude from Fortior to leave it out.
- Restore automatic to undo your choice.
Reviewed weights lists your past decisions. Only the weight is reviewed: body fat and lean mass from the same reading stay in use. Fortior never changes what Apple Health or your scale app recorded, so fix a wrong source reading there.
Measurement methods and their caveats
| Method | What it does | Main caveat |
|---|---|---|
| Smart scale or handheld | Passes a small current through the body and estimates body fat from the resistance | Hydration, food, time of day and recent exercise all change the reading |
| InBody-style scanner | Measures several body segments and reports lean and fat mass for each | Still sensitive to hydration, and best compared with your own earlier scans on the same machine |
| Photo estimate | Fortior AI gives a range from one photo shoot. It is part of Fortior Premium and needs AI turned on in Settings. | Lighting, clothing, pose and a pump all move it |
Lean mass isn't the same as muscle, and the method, hydration and the height you entered all affect the result. Fortior doesn't tell you what range is right for you.
InBody segmental results
InBody scanners report lean and fat mass for the right arm, left arm, trunk, right leg and left leg. Apple Health can't carry that segmental analysis, so you add it in Fortior: photograph the printout, choose up to three screenshots from the InBody app, or type the values. Fortior AI reads a photo or screenshots. If your InBody results reach Apple Health, the segmental values attach to that day's InBody record instead of being saved as a second reading.
Fortior then shows each region's lean mass next to the strength of that region's lifts, using your best estimated max in the four weeks up to each scan. You can see whether arms, trunk and legs are moving together.
Getting a trend you can trust
- Weigh at a consistent time, such as first thing in the morning, under similar conditions.
- Use the same scale or scanner for body fat, and compare each reading with that device's own earlier ones.
- Judge weeks, not days. Set the chart range to a few weeks or more.
- When a weight looks off, check Review before changing anything else.
- Treat body fat and lean mass as estimates, and pair them with your strength relative to bodyweight and how your training feels.
In the app: to log, open Today and tap Weight in the Log section. For trends, open Progress and choose Body & Lifestyle. Tap Review on the bodyweight chart when a reading needs a look. For regional results, open Segmental Composition in Body & Lifestyle and tap Add segmental data. Body composition is free; reading InBody results from a photo uses Fortior AI.
Body composition figures are estimates from scales, scanners or photos. They are not medical measurements, and Fortior is not medical advice. Talk to a healthcare professional about body weight or health questions.