Which unit does Adobe XD use for pixel sizing?

Hello there,

The pttopx converter you linked is using the conversion of number of pixels = number of points * (pixels per inch / points per inch), where the pixels per inch (or DPI) can change, and 72 is the typical number of points per inch. This whole problem of converting design units to a physical size is tricky, and so, at least for now, XD kind of sidesteps it.

It's easiest to think of XD as unit-less, kind of like a vector graphic, and focus instead on the relationships between elements. Let's say you design an iPhone 6/7 artboard at 375x667 units, and it uses type with a 10 unit font size. That relationship is the same, no matter what physical size your design is scaled to. You could rasterize your design to an image where each unit maps to 1, 2, 3, or even more physical pixels. And this mapping happens quite frequently, when you render to the screen, or export a bitmap image, or preview the design on your phone. On MacOS/iOS this abstract "unit" is called a Point (see Points vs Pixels​​, and PaintCode's Ultimate Guide to iPhone Resolutions), which typically matches up with the 1/72 inch definition above (for example, when printing), but not always (displaying on a monitor). So in the above design, since you are designing with the mapping of 1 XD unit to 1 iOS point, the artboard dimensions you can give to a developer are 375x667 points, and the font size would be 10 points. The key here is that you will need to always think about what that final mapping is, from an abstract XD unit to the final development one, be it pixels, points, or something else.

Down the road, we're hoping we can more thoroughly investigate the problem of design to display unit conversion, but this is how the application currently works.

Hope that helps,

-Bear