Calculate logical pixels from millimeters - dart

Question

I have a design with widths, heights, paddings... in millimeters. I'm now trying to figure out how to convert those numbers to the logical pixel system Flutter uses.
I found the docs for the device pixel ratio but I'm not sure how to interpret that number and I quote:
The Flutter framework operates in logical pixels, so it is rarely necessary to directly deal with this property.
So I am not sure if this is the way to go.
My question comes down to this: Is there an easy way to calculate from millimeter to logical pixels that works for both Android and iOS?

Answer 1

Flutter's unit is DP or dip (aka density independent pixels).
Like it's name is implying, it's independent from the screen pixel ratio.
What's the difference with millimeters ? None really important.
Converting mm>dp or dp>mm is similar to mm>inch/inch>mm.
The relationship between them is fairly simple :
1 inch = 25.4 mm = 160.0 DP
Which means 1 mm = 6.299 DP

Answer 2

I would say the current accepted answer is not really accurate.
You can print the number of logical pixels for real phones, like this:
maxWidth = MediaQuery.of(context).size.width;
maxHeight = MediaQuery.of(context).size.height;
print("maxWidth = $maxWidth / maxHeight = $maxHeight");
I would appreciate if someone with real iPhones and iPads could run this code and tell me the resulting maxWidth and maxHeights. So far, with Android phones, I've compiled the results comparing with the real phone sizes. What I got is this:
Galaxy S6 ➜ 5.668537826 dp/mm for height, and 5.668537826 dp/mm for width.
Galaxy S7 ➜ 5.668537826 dp/mm for height, and 5.668537826 dp/mm for width.
Galaxy S9 ➜ 5.223614747 dp/mm for height, and 5.585946405 dp/mm for width.
Pixel XL ➜ 5.612956709 dp/mm for height, and 6.007177748 dp/mm for width.
See my spreadsheet here: https://docs.google.com/spreadsheets/d/1zmGyeKSf4w4B-bX4HSY4oSuh9RkIIkFwYSd3P9C7eX8/edit?usp=sharing
Update:
Android docs (https://developer.android.com/training/multiscreen/screendensities#TaskUseDP) say "One dp is a virtual pixel unit that's roughly equal to one pixel on a medium-density screen (160dpi; the "baseline" density)". That's obviously not true in practice for Samsung and Google phones. Or it is, if you pay attention to the word "roughly".
Flutter docs (https://api.flutter.dev/flutter/dart-ui/FlutterView/devicePixelRatio.html) say it's 3.8 logical pixels per mm, which is obviously very false.

Answer 3

Logical pixel is the ratio of dots per unit distance(mm), so you have to change your question to How many dots per mm represents 1 logical pixel?
As it is mentioned here
Flutter follows a simple density-based format like iOS. Assets might
be 1.0x, 2.0x, 3.0x, or any other multiplier. Flutter doesn’t have dps
but there are logical pixels, which are basically the same as
device-independent pixels. The so-called devicePixelRatio expresses
the ratio of physical pixels in a single logical pixel.
And as mentioned 1.0x logical pixel ratio represents mdpi in Android density qualifiers. And according to this, mdpi ≃ 160DPI and as dpi represents the number of individual dots that can be placed in a line within the span of 1 inch (2.54 cm) so:
160dpi = 160 dots per inch = 6.299 dots per mm
And as mdpi ≃ 160DPI and 1 logical pixel represents mdpi so:
1.0x logical pixel ratio ≃ 6.299 dots per mm

Answer 4

To display a widget at a real size.
double millimeterToSize(double millimeter) => millimeter * 160 * 0.03937;