You can't throw a party without crunching numbers: How many people to invite? What time should they come? How much cash to spend? We've got answers to all that and more. Keep these digits handy, and your bash will come together in a fraction of the time -- and cost.

Party By the Numbers

200 napkins
 For a party of 15, have a supply of 200 napkins. Guests tend to take more than they need, so extras never hurt. And speaking of extras: Plan on two to three glasses a person at a cocktail party, says Lulu Powers, Los Angeles caterer and author. People drink, then put their glass down and take a new one.

$0: The amount you need to spend on favors.
 Hand out something you've collected (such as seashells) or a recipe from the night's meal. For an almost-free favor, founder Robyn Spizman suggests e-mailing guests beforehand to find out their favorite candy, then putting the goodies at their place settings. When guests arrive, they can find their seats based on the treat.

Assume that only 30% of guests will RSVP.
 Start hassling people for responses five days before the party, to give yourself time to shop for the right amount of food and beverages. "Even though I don't normally do my grocery shopping until three days before, I think, out of respect, guests should respond five days ahead," says Boston event planner Bryan Rafanelli.

Food can sit out for 4 hours.
 Food's "danger zone" is between 40 and 140 degrees -- food that's typically refrigerated or frozen should be kept below 40 degrees, and hot food should be kept above 140 degrees. The good news? You can be in the danger zone for four hours before nasties like bacteria become a concern, says private pastry chef Diana Sproveri. If your party is going to last longer, plan accordingly.

50°: The temperature at which outdoor entertaining must cease.
 "In March that's a little cold, but not in June, because the ground isn't holding the cold," says Rafanelli. "It also depends where you are; I don't do any parties outside in March in Boston, but I do in Miami. We wear sweaters and sit under heat lamps." Of course, you could also rent a heat lam -- prices vary, but Rafanelli estimates $100 plus delivery.

67%: The average invitation acceptance rate.
 "I don't care if it's Christmastime or a wedding with 350 guests," says Tara Guerard. "We always budget for two-thirds." Peter Greenberg, travel editor for CBS News, says, "In my experience -- and I'm not saying this as a man -- men have trouble committing even to lunch. I invite triple the number of men to women so that all my single women friends aren't mad at me." And if rain or snow is in the forecast. "Never underestimate the unpredictability of the weather or the predictability of your fair-weather friends," Greenberg says.

If you offer tea or coffee, 80% of guests will accept.
 It may seem like a throwback to another era, but nothing completes an evening like a cup of hot tea or coffee. Powers suggests making decaf coffee beforehand: Just pour it into a pan and heat it up in two minutes.

$5.30: The average cost of a custom-printed invitation.
 Here's a thought -- send an e-mail or make a phone call, then buy one of the following for yourself: a new shade of red lipstick, a cheap bouquet for the bedroom or a dozen freshly baked muffins for the morning after the party.