Know Your Bottle Size

• A standard wine bottle, the most common size, holds 750 milliliters, a little over 25 fluid ounces. A magnum bottle, those bigger bottles most stores carry, holds 1.5 liters, or a little over 50 fluid ounces. A traditional box of wine holds 3 liters, or a little over 100 fluid ounces. Once considered to have inferior quality, boxed wines now include several good-quality brands at economical prices.

Decide on the Pour

• In the language of wine, the "pour" refers to how many ounces will be in a glass. With a traditional 5-ounce pour, a 750 ml bottle holds five glasses. If your toast is at an event where people will be drinking other alcoholic beverages and only toasting with the wine, you might want to consider a smaller three- or four-ounce pour.

Do the Math

• If you go with a standard 5-ounce pour per glass, you'll need eight standard bottles, four magnum bottles or two standard boxes to get the 200 ounces needed for 40 pours. If you go with a 4-ounce pour, seven standard bottles with be enough with a little to spare. Once you know how many ounces will be in each glass and how many ounces are in each type of bottle or box, the math is simple.

Buy an Extra Bottle

• There are a few reasons why you might want to buy an extra bottle or two. The first is that you'll have enough for any unexpected guests that show up. There's also a chance that one of the bottles you purchase could be tainted by oxygen that has seeped in or by extreme changes in temperature. Finally, you may want a bottle to put away for an anniversary of the occasion you're toasting a number of years down the road.