Android how to calculate assets sizes

Question

Sorry if the question title is confusing, but if I knew how to say ot correctly, I would've googled my question :D So I am having difficulties understanding how to create different assets for different screen sizes, for example I have a working file in Photoshop of a button which is 550x550 pixels, and I want to have something like an universal formula to calculate how to make different image sizes for the different screens. Also, it would help if there is some quick function in Photoshop for creating them

Answer 1

I often use this site to calculate quickly the various sizes for density:

http://labs.rampinteractive.co.uk/android_dp_px_calculator/

Also Cut&Slice allows you to create the different assets with Photoshop CS6

Answer 2

From the Android docs:

To create alternative bitmap drawables for different densities, you should follow the 3:4:6:8 scaling ratio between the four generalized densities. For example, if you have a bitmap drawable that's 48x48 pixels for medium-density screen (the size for a launcher icon), all the different sizes should be:

36x36 for low-density
48x48 for medium-density
72x72 for high-density
96x96 for extra high-density

I found that here: http://developer.android.com/guide/practices/screens_support.html