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XD currently uses a virtual pixel for most of its measurements (including font size), which should be the same unit of measure as a CSS pixel, or most measurements in iOS. It's roughly equal to 1 physical pixel on a 72 dpi monitor (and, incidentally, a point).
I'm afraid there's no option to change the unit currently. Which units would you find useful?
I work on iOS and Android Applications.
For Android our developer wants to have the font sizes communicate in pixel, but based on a 160 dpi view which is the base size for Android.
But I even make the test:
Saved a view from XD as PNG. Opened it in Photoshop as a 72 dpi view. And checked the size of the fonts. Its the same I use in XD.
Thank you for your help.
SO are you saying that the system XD uses is the same as point when on a 72dpi screen?
I would use PT, like Sketch does, due to diverse pixel densities across devices. My monitor, for example, is 96 dpi. So, for a 20px font to render the same size as a 20pt font I have to increase the font size to 27px. But say, another person were viewing my design with a 72 dpi screen then the 27px text with would be much larger than I intended. It would render as the equivalent size of a 27pt font.
So, this becomes a huge issue in a work space where the design may be viewed or created on multiple screens with differing pixel densities.
iOS only uses points and it is not true that pixles to points is 1:1, it grows as the number gets bigger. Here is an example of a converter to demonstrate, http://pttopx.com/
This becomes a huge issue for translating font sizes and dimensions to my iOS developers. For an Xd artboard of iPhone 6/7 is at 375x667 PIXELS, however iOS specs list it as 375x667 in POINTS. There is no conversion what so ever, but uses the same number.
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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,
In addition to Bear's answer it's useful to note that on iOS 1pt in a native app is equal to 1px in CSS, and on Android 1dp in a native app is equal to 1px in CSS. So if you have an image and display it at 100x200pt on iOS, 100x200dp on Android, and 100x200px on a web page, they will appear at the same size on the same device.
(Also note that CSS has its own pt unit, which is 1.333 (96/72) times larger than a px, however this is different from an iOS pt.)