How many guests can I expect if I invited 40? Is there a rule of thumb to go by?

Just wondered... I remember hearing 1/3 of those invited usually come, or something like that... does anyone know? Or if you entertain often, tell me your experience with that. This is for a graduation party, and I didn't put RSVP in the invitations because most people fail to respond these days. Thanks.
How many guests can I expect if I invited 40? Is there a rule of thumb to go by?
It depends on lots of things. But if you are serving food and those invited know it, you may have way over 40. That is my experience!
Reply:usually, half of the people come, but it always varies. there's really no rule of thumb to go by, but if you invited 40, then you should buy enough for about 35 people, just in case!
Reply:abt 30-35


but it varies in some of the parties we hve at hme there are 40/40, 30/40 or 20/40


so hedge ur bets prediction 30-35
Reply:usually half ..but if most of the guests you invited are kids who are also graduating and the party is on the same day as the graduation, it will be considerably less..Some will have several parties to go to so they may only be there for a short visit.