To determine the amount of money required to pay for a weekly magazine's fiction, enter the relevent data above the line, and click the Calculate button. Results appear below the line. (Don't enter numbers below the line.) For comparison purposes, the approximate numbers for Strange Horizons for 2003 and 2010 are provided in the right-hand columns.
I threw this calculator together in a hurry; it doesn't have nice features like error-checking or letting you specify a budget and having it calculate other numbers. The intent is to let you play with various scenarios and see how they affect budget. It should be flexible enough to use for a variety of magazine models, especially if you ignore terms like "weeks" in the field labels. You can even simulate a flat rate for reprints by pretending that all reprints are the same length.
The SFWA pro pay rate for original fiction has been entered already, but you can change it if you like.