How to Determine a Valve Size for a Tub Shower

Old Fixture Method

• 1

  Turn off the water supply to your bathroom.

• 2

  Remove the faucet handles on your bathtub. Use your screwdriver to loosen the handles and remove any caulk or sealant around them. Remove your faucet in the same manner.

• 3

  Use your wrench to remove your old bath faucet. Detach it from all connections, which might include supply pipes and the spout pipe.

• 4

  Measure the inside diameter of the faucet by taking a straight line measurement of the distance between opposite points on the inside walls of one of the openings. Measure the outside diameter by taking a similar measurement between opposite points on the outside walls of the pipe. Write these measurements -- usually less than 1 inch -- down.

• 5

  Measure the circumference of the fitting on the faucet by wrapping the measuring tape around the pipe. Compare this measurement to a pipe size chart to get the nominal size of the faucet. Buy a new, bath/shower combination valve with measurements matching those from Section 1, Step 4 or an identical nominal size.

• 6

  Take your old valve to the hardware store and ask an employee for a new bath/shower valve with the same size openings as an alternative to taking measurements.

Pipe Method

• 7

  Turn off the water supply to your bathroom.

• 8

  Use your measuring tape to determine the outside diameter, inside diameter and outside circumference of your water supply pipes installed in your bathroom.

• 9

  Compare these dimensions to your pipe sizing chart to get the nominal size of the pipes. Nominal sizes run in even fraction intervals, such as half-inch and three-fourths inch.

• 10

  Purchase a new bath/shower valve that matches the nominal size of your pipes; your valve must always match the size of your pipes.