Replacing a water heater requires planning. A system often is not replaced until it no longer heats water or until water is found on the floor near the heater. One part of the planning that is sometimes neglected is the drain pan. The drain pan sits under the water heater and should have a pipe attached to divert water into a drain should the tank develop a leak. Sizing a water heater drain pan is a simple task that helps ensure you purchase the proper size for your unit.
Measure the circumference of the water heater you plan to install. The existing water heater might be smaller than the new one.
Place a tape measure on the top outside edge of the water heater and measure across to the opposite side. Record the measurement.
Find a drain pan that is 2 inches wider than the new water heater. If the new water heater is 32 inches in diameter, purchase a drain pan that is at least 34 inches across.
Pick a drain pan that is between 1½ and 2½ inches deep and has a hole in the side for attaching a drain pipe.