I found this buried deep in an appendix of the Mapnik XML Schema Reference, and I thought it so useful I am reposting it here: Zoom level Scale denominator 0 559,082,264 1 279,541,132 2 139,770,566 3 69,885,283 4 34,942,642 5 17,471,321 6 8,735,660 7 4,367,830 8 2,183,915 9 1,091,958 10 545,979 11 272,989 12 136,495 13 68,247 14 34,124 15 17,062 16 8,531 17 4,265 18 2,133 19 1,066 20

**Update 2012-02-02 17:57 -0800:I just wrote a post about another way to do this. I really like QGIS. It’s a powerful GIS that runs on Linux (among other operating systems) and doesn’t require incredible amounts of CPU. One criticism I had of it was that it did not provide the same ‘click-and-go’ basemap experience one can get with ArcGIS for making simple maps (i.e. Add Basemap, select Bing, DeLorme, &c. and you’re done).