P traps in bathrooms, if you need to replace one, are typically 1 1/2 inches to 2 inches in diameter. P traps can be used in both kitchens and bathrooms. Because they can become clogged with materials that pass through them, they may eventually need to be replaced. When this happens you will need to know what size P trap to buy to replace the old one.
To determine the size of a P trap you require for a replacement, measure across the widest part of the pipe your trap will be attached to. Your trap will need to be the same size as this pipe.