How to Determine What Size Valve on a Plumbing Shower Fixture

Measure the size of the water pipe in the bathroom shower. Water pipes are generally two sizes: 1/2-inch and 3/4-inch in diameter. To measure the size of your pipes, wrap a string around the pipe. When the ends meet, mark the long end of the string with a ruler. Measure the length from the end of the string to where it’s marked to determine the pipe’s circumference and find the size of the water supply pipe.

Take the string with you when shopping for a shower valve. Many home improvement and plumbing supply stores have a chart to calculate the circumference and estimate the diameter of the pipe. If a chart is not available, use the string to compare the size of the valve to the size of the water pipe.

Select the size and type of the water valve. If the water supply pipes are 1/2-inch but more water pressure is preferred, a 1/2-inch thermostatic valve is recommended. A thermostatic valve controls the water temperature and produces more gallons of water per minute. If the size of the water supply pipes are 3/4-inch, select a 3/4-inch valve.