web mapping - Disk space and numbers of tiles for world layer

I'm thinking about testing tiles generation of a world layer for webmapping.

In this context, I want to know how I can calculate the number of tiles (256x256 for tilecache if I remember) I will need for n levels in WGS84 coordinates.

Maybe I will use a composite solution with pregenerated tiles and on the fly generation but I have to know at the end the available disk space if every tiles are generated and the number of files it will represent.

There are two goals, time it will take and disk space needed.

Any informations are welcome

Edit:

I've found this script to calculate the number of tiles to generate depending on zoom levels and your data extent. See this gist https://gist.github.com/1675606

Answer

Here is excel sheet in Google Docs from GeoSolutions team (http://geo-solutions.blogspot.com/2010/12/estimating-time-and-space-required-to.html)

https://spreadsheets4.google.com/ccc?key=tyCIm7rz8753DUGC9FyOXaw#gid=0