How the measurement works
Devices with a barometric altimeter detect the air pressure changes that occur while going up or down. From a height change large enough to be recognized as relevant, iOS derives an estimated number of floors.
Individual steps of a staircase are not counted separately, what gets measured is the overall height change across a continuous stretch.
Why values can differ
Because the estimate relies on an assumed height per floor, it can diverge from the actual ceiling height of a specific building. User reports and support-forum discussions suggest the internally assumed floor height does not match every country and building exactly, and Apple has not published a detailed, official figure for it.
Weather-related pressure changes, a device that is not being carried, or a very short flight of stairs can also mean a floor is not recorded or a day shows zero even though stairs were climbed.
A zero value is not automatically a bug
If no floors show up on a given day, that does not necessarily mean something is broken. It can simply mean no sufficiently large, continuously recognized height change occurred, or the device was not being carried at the time.
Tips
- Expect an estimate, not an exact count of individual stairs.
- Carry the iPhone on your body so height changes are detected more reliably.
- Do not treat a missing or low value as an automatic app error.