To measure your P trap first locate it. It will be a U-shaped pipe connected to the straight pipe beneath your drain. Using a pipe wrench, unscrew the P trap's connector and twist the P trap until you can slip a measuring tape across its opening.
Then, make a note of this measurement. A P trap, whether installed in your kitchen or in your bathroom, is there to keep sewer gases from backing up out of your toilet, sink or bathtub. At times, you will need to measure yours to get the right replacement.