You can consult your owner's manual, the back of the coolant jug, or the charts that coolant manufacturers supply to find out the number of quarts that your cooling system holds. Many coolants are now pre-diluted with water, but if you're buying straight coolant, divide that number by two and buy that amount of coolant. Adding an equal amount of water to straight coolant gives you a 50/50 water/coolant mixture, which is fine for everything but extremely cold weather.
Remember: Some auto manufacturers make coolant formulated specifically for their vehicles. At least one coolant manufacturer advertises its coolant as being safe for all vehicles. Whatever you buy, if your vehicle has an aluminum engine, make sure that the coolant is marked "Safe for use on aluminum engines." If you're adding coolant, always use the same brand and type as the coolant already in the engine.
From Auto Repair for Dummies, copyright © 2009 by Wiley Publishing, Inc., Indianapolis, Indiana. Used by arrangement with John Wiley & Sons, Inc.