Measure the dimensions of the room before shopping for carpet padding. Note the square footage by multiplying the width by the length of the room. Add on at least 4 feet of padding to ensure you have enough for awkward corners.
Check your carpet's warranty. By choosing a type of padding not recommended by the manufacturer, you could void the warranty. Call the manufacturer if there is no carpet padding recommendation to find out what would work best for your carpet.
Check your subflooring for any cracks. Cracks indicate that an old carpet padding had a moisture barrier, which locked in the moisture and caused damage to the subflooring. If there are no cracks, your carpet padding selection widens.
Decide if you need a moisture barrier. Homes that are exposed to flooding or house pets may need a moisture barrier on the carpet padding to prevent it from being ruined by an accident.
Shop around at different carpet retailers. The majority of carpet retailers have few carpet padding options, because they buy it in bulk. However, there are five main kinds of carpet padding: urethane foam, bonded urethane, waffle rubber, fiber cushion and flat rubber. Each offer different benefits. Read more about the benefits of each type of carpet padding in The Carpet Buyer's Handbook (see References).