Qt has API for querying physical DPI:

QPaintDevice::physicalDpiX()
QPaintDevice::physicalDpiY()
QScreen::physicalDotsPerInch()
Quick: Screen.pixelDensity

Which are documented to return the number of physical pixels per inch/millimeter. This was simple, until the introduction of the logical pixels (device independent pixels). Now there are two behavior options:

1) The API returns a constant value: display physical dots per inch

This value does not change if/when the device pixel ratio changes. Physical dots per inch is, after all, a physical (constant) property of the display. It is reasonable that an API that previously returned a hardware characteristic continues to report the same value, independent of the software configuration.

2) The API returns a value suitable for creating physically sized UI: the number of logical pixels per mm (or inch)

The value is divided by the device pixel ratio internally. This enables writing code like this:

item.with = 2 * Screen.pixelDensity

The item will now be two mm across on all displays (provided the display provides accurate physical measurements). The above code works on dpr == 1 displays; it is reasonable to expect that it continues to do so on other displays.

Qt Currently implements option 2. A patch for changing it to option 1 is here: https://codereview.qt-project.org/#/c/161333/