There are a ton of web based ones if you don't want to use a spreadsheet. These calculators are very popular amongst direct marketing and CPA affiliate profiteers. I hesitate to link to any here for fear of being marked a spammer, but a Google search will turn up tons.
Providing links in HN isn't seen as spam, especially if it clears up a missing point. I, for one, would appreciate any links you've got of ones you trust and use.