Planning a party: The right math to do

When planning any cocktail party, from an impromptu gathering after work to a formal reception, there's a certain amount of math to be done. Like balancing a checkbook, this tallying, while not complex, has humbling consequences when wrong.

First, decide which cocktails or mixed drinks you'd like to serve. There's no need to mix more than two or three types, especially while you're still polishing your mixing skills. If you decide to serve only a few drink recipes, it's quite acceptable for them all to have the same primary liquor.

A successful cocktail party rarely lasts longer than three hours: an hour for everyone to be late, an hour for everyone to sound smart, and an hour for everyone to leave. Evening gatherings should start earlier rather than later, so if new friends are made people can continue the evening over dinner or a movie. If you're planning to serve dinner after cocktails, drink time need only last an hour or so. If you're not planning a meal, at least serve hors d'oeuvres. If you have snacks like nuts or pretzels, plan on refilling their bowls at least every 20 minutes. Always reserve some special snacks for those chosen few you'd like to invite to stay after the crowd thins - when you can really relax. Plan on about four to six snacks per person.

Bottled spirits will be your greatest expense at any cocktail party. A 750-milliliter bottle of a primary liquor will make about 18 drinks, with a jigger each of the liquor. Buy the best you can afford.

Most human bodies can process one serving of alcohol per hour. For guests particularly wanting to unwind, plan on serving two drinks in the first hour and one every 45 minutes after that. Of course, you'll also need to budget for those guests who will be driving. A typical party of 30 will consist of about 15 drivers and 15 talkers. Over three hours, plan to serve four drinks to each talker, and two drinks to each driver. Although that's 90 drinks, plan on 100 servings - a slight buffer for unannounced guests.

Depending on the occasion and number of guests, you will need between 20 and 100 6-ounce glasses. Close friends at casual gatherings probably won't mind washing their own glass as they go - a small price for an evening's entertainment. However, a formal gathering may dictate buying or renting additional glasses.

Renting glassware costs about 45 cents per item. For a group of friends or family, you'll need only one glass per person and a few extra to cover breakage. If these guests are slow drinkers and you want to impress them, two glasses per person will suffice, because you can employ a friend or significant other to help clean as you go. For formal gatherings, budget one glass for every drink you intend to serve. A typical crowd of 30, for instance, will require about US$45 in rented glassware. Paying a trusted neighbor kid or helper about $10 an hour to wash glassware might be a cheaper alternative, assuming your kitchen or bar space would comfortably allow such work without intruding on the party.

When it comes to doing the math for the amount of ice needed, always round up. Running out of ice will end a party faster than running out of distilled spirits: Guests will volunteer to buy more liquor, but they'll leave before being drafted to get an awkward bag of sweaty ice. As a general rule, have one pound of ice per person for every three hours. Don't kid yourself into thinking you can make all the ice. Arrange to have a few close friends or people who tend to arrive early buy ice on their way to the party.

For citrus, remember that the typical lemon or lime contains about an ounce of juice, while an orange contains a little more. Most recipes call for less than half an ounce of citrus juice, so safely plan on making two drinks from one piece of fruit.

Have alternative beverages, such as soda water (flavored and plain), bottled juices, and beer on hand. Plan for no more than two of these per guest, or the cocktails will appear secondary.

I find the easiest way to handle all this math is by taking the time to properly figure it once on paper for my two most common party sizes: 10 and 30. From there, I can roughly add or subtract guests as needed, always rounding up - just like I do with my checkbook.