Fleet Mass Calculator spreadsheet. Requires a Google account.
WARNING: this sheet doesn't magically know if a hole in your chain has been massed prior to your use of it. If your friends got rolled out despite the sheet saying they'd fit, I can only chuckle and advise more propitiations to Bob. You could, for instance, self-destruct all your yachts and pledge never to use them again; that would please Him.
Pick ships and amounts and you're good. Change the hole mass reduction percentage if you want more or less leeway in the conservative transits column.
If you already have a fleet out, use the Fleet Composition & DScan Checker subsheet.
Other EVE-related stuff.
An example of what the sheet does: