Little bit of research for my short stories book. How hard is it really to sell a story to a top professional SF market? 

Methodology: I consulted The Submission Grinder (thegrinder.diabolicalplots.com), and considered only those markets that are SFWA-recognised (since these are longer established and are likely to give the most conservative figure for acceptances). Most of these have submission numbers in the high hundreds – if not over 1000 – recorded in the database, so the figures should be reasonably accurate.

Things that could make them less accurate:

1. People who submit successfully may be more likely to use the paid alternative to The Grinder (duotrope.org). 

2. People may not record all of their rejections (but they are highly likely to record an acceptance to such prestigious markets). 

I ran the figures only for science fiction, since most of the fantasy markets are also science fiction markets, but not necessarily vice versa. (The only SFWA-recognised fantasy short story market that isn’t also an SF market is Beneath Ceaseless Skies, with an acceptance rate of 4.71%.) 

Results:

Acceptance rates (in percent) for the 12 SFWA-recognised markets for SF short stories: 

Daily Science Fiction 9.89

Unlikely Story 8.1

AE: The Canadian Science Fiction Review 7.69

Escape Pod 2.95

Orson Scott Card’s Intergalactic Medicine Show 2.77

Analog 2.75

Lightspeed 2.43

Grantville Gazette: Universe Annex 2.08

Strange Horizons 1.86

Fantastic Stories of the Imagination 1.74

Asimov’s 0.81

F&SF 0.53

Average 3.63

However, consider this, which I’ve heard from several editors: Ninety percent of what is submitted to these magazines is completely unpublishable drivel. If you can actually write a story, you should multiply the number by 10 to get a more realistic figure. 

That still leaves your chances with F&SF at about 1 in 20.