If you're hosting a dinner party (or any party) and want a few bottles of wine on-hand, this calculator from VinePair tells you how many bottles to buy based on the number of people coming and whether you'll have other drinks available.
The calculator asks you a few simple questions, including how many people are coming to your event, whether you'll also serve cocktails and whether you want more between red or white. The maths that the calculator uses is fairly simple -- it assumes that each bottle of wine is about four standard glasses, and that each person will drink three. They also aim a little higher to make sure you have extra. Click "Show me my wine list", and the calculator will spit out how many bottles of red and white you'll need for your event.
Now that you know the maths, you may not need the actual calculator, but it's a useful tool to have if you're busy planning a party.
The Dinner Party Wine Bottle Calculator [VinePair]