Putting screen densities into the correct bucket

The reason I asked is because I have created the following value directory resources. (...) In the dimens.xml I have different margins and set the dp depending on the bucket size. (...) I am interested to know if this is the correct way to do this.

I'm not sure why you want different margins specified in dp depending on the density. Specifying the margin as dp once, for the baseline density, already handles all other densities for you, meaning that the physical size of the margin will be the same when displayed on any device.

If you used px instead of dp (but don't), then you would have to do the scaling for different screens yourself.

Scaling down into the buckets i.e if the ppi is 300 that would go into the hdpi bucket as it less then 320?

Yes, but not because it is less than 320. If there was a rule of thumb I would say it is rounding to the nearest generalized density. See this illustration of how Android roughly maps actual densities to generalized densities (figure is not exact):

Relevant part of the documentation is this:

Each generalized size and density spans a range of actual screen sizes and densities. For example, two devices that both report a screen size of normal might have actual screen sizes and aspect ratios that are slightly different when measured by hand. Similarly, two devices that report a screen density of hdpi might have real pixel densities that are slightly different. Android makes these differences abstract to applications, so you can provide UI designed for the generalized sizes and densities and let the system handle any final adjustments as necessary.

So again, you shouldn't really care how Android does this if you are just writing an app. What you should care about is:

• specify all layout dimension values in dp or with wrap_content/match_parent, as appropriate (text can be in sp to additionally match the user preference, but nothing other than text),
• think about different layouts depending on physical size and orientation of the screen,
• provide bitmap resources for different densities, just to avoid blurry or pixelated artifacts (because Android will scale them to have the right physical size if you use dp or wrap_content).

Android will lookup the best matching resource, and then transparently handle any scaling of the dp units, as necessary, based on the actual density of the screen in use. The conversion of dp units to screen pixels is simple: px = dp * (dpi / 160).

Note the actual density as opposed to generalized density. The latter is only a convenience for the developers, since it would be impossible to provide drawables for every screen out there. This way developers need to provide only 3 or 4 sets of graphics, while Android picks the closest fit and adjusts it further for the needs of that particular device. (Nowadays it's possible to use one vector drawable instead of many pre-scaled raster graphics, meaning better quality and less size.)

Is this the correct way to work out buckets for screen sizes.

No, it is not. According to Google device metrics all devices you listed fall into buckets higher than you expected:

Galaxy S3    NA        NA
Nexus 4     318     xhdpi
Nexus 5X    424    xxhdpi
Nexus 5     445    xxhdpi
Nexus 6     493   xxxhdpi
Nexus 6P    515   xxxhdpi

I took some other devices from that list, and plotted how different devices are falling into density buckets depending on their actual physical density.

Chromebox 30            101      mdpi
Chromebook 11           135      mdpi
Samsung Galaxy Tab 10   149      mdpi
Nexus 7 '12             216     tvdpi
Android One             218      hdpi
Chromebook Pixel        239     xhdpi
Nexus 9                 288     xhdpi
Nexus 10                299     xhdpi
Moto X                  312     xhdpi
Nexus 4                 318     xhdpi
Nexus 7 '13             323     xhdpi 
Moto G                  326     xhdpi
Dell Venue 8            359     xhdpi
LG G2                   424    xxhdpi
Nexus 5X                424    xxhdpi
HTC One M8              441    xxhdpi
Nexus 5                 445    xxhdpi
Nexus 6                 493   xxxhdpi
Nexus 6P                515   xxxhdpi
LG G3                   534    xxhdpi

You can see, that with some notable exceptions, the rule that the closest generalized density is selected, holds.

The exceptions being Nexus 6 and 6P, that are listed as xxxhdpi, even though LG G3 has a higher physical density and still is far from 640px/in. Android One is hdpi but it is only slightly denser than Nexus 7 '12 which is tvdpi. Chromebox 30 and Chromebook Pixel (admittedly, not Android) are assigned to buckets mdpi and xhdpi even though they are physically lower than ldpi and hdpi, respectively.

I am interested to know if this is the correct way to do this.

You are mostly correct.

The problem lies in this part:

The reason I asked is because I have created the following value directory resources:

values-hdpi/dimens
values-xhdpi/dimens
values-xxhdpi/dimens
values-xxxhdpi/dimens

In the dimens.xml I have different margins and set the dp depending on the bucket size i.e.

<dimen name="network_quantity_margin_top">100dp</dimen>

The purpose of dp gets defeated by defining folders like values-hdpi/dimens. Density Pixels, by design, are device-agnostic - 100dp on a device with dpi = 240 will look just as wide/long on a device with dpi = 480. So, if you want your app to look consistent, do not provide different dimensions for different screen densities.

The correct way to think about this is to realize that the only resource that is affected by varying screen densities is drawable. A drawable on a screen with dpi = 240 will look twice as big compared to a screen with density = 480. I am sure that you're providing folders like drawable-hdpi, drawable-xhdpi etc. to deal with this. For everything else, and especially dimensions, use dp. For text sizes, use scaled-pixels - sp.

More importantly, you should worry about the range of different screen sizes that are available for android. How would you use all the extra screen real-estate on a 10 inch device compared to a 5 inch phone? Qualifiers such as -normal, -large, xlarge should be of more interest to you.

To summarize:

• consider all devices of a certain screen size the same - their screen densities are irrelevant when using density pixels.
• for every drawable resource you use, place their scaled versions in the buckets you wish to support. Remember that, if you don't provide resources for a certain bucket (say drawable-hdpi), android will scale down your drawables from drawable-xhdpi folder (provided drawable-xhdpi is defined). The reverse is also true: if you have placed all your drawables in drawable-xhdpi, android would scale-up your drawables on a xxhdpi device. The result will be blurry graphics - because of scaling-up.

I know that its a bit of a steep slope here :). So, if you need to clarify some more, leave me a comment.

From the Android documentation:

Supporting Multiple Screens

In some cases, you will need to express dimensions in dp and then convert them to pixels. Imagine an application in which a scroll or fling gesture is recognized after the user's finger has moved by at least 16 pixels. On a baseline screen, a user's must move by 16 pixels / 160 dpi, which equals 1/10th of an inch (or 2.5 mm) before the gesture is recognized. On a device with a high-density display (240dpi), the user's must move by 16 pixels / 240 dpi, which equals 1/15th of an inch (or 1.7 mm). The distance is much shorter and the application thus appears more sensitive to the user.

To fix this issue, the gesture threshold must be expressed in code in dp and then converted to actual pixels. For example:

// The gesture threshold expressed in dp
private static final float GESTURE_THRESHOLD_DP = 16.0f;

// Get the screen's density scale
final float scale = getResources().getDisplayMetrics().density;
// Convert the dps to pixels, based on density scale
mGestureThreshold = (int) (GESTURE_THRESHOLD_DP * scale + 0.5f);

// Use mGestureThreshold as a distance in pixels...

The DisplayMetrics.density field specifies the scale factor you must use to convert dp units to pixels, according to the current screen density. On a medium-density screen, DisplayMetrics.density equals 1.0; on a high-density screen it equals 1.5; on an extra-high-density screen, it equals 2.0; and on a low-density screen, it equals 0.75. This figure is the factor by which you should multiply the dp units on order to get the actual pixel count for the current screen. (Then add 0.5f to round the figure up to the nearest whole number, when converting to an integer.) For more information, refer to the DisplayMetrics class.