Measuring for a bathtub drain may seem to be a daunting task but there are a few short steps you can take to ensure that you get the right fit. Knowing where and how to access the plumbing to be able to measure it is half the battle. Once you locate this area, a few small measurements are all it takes to get you fitted for a new or replacement bathtub drain.
Measure the drain hole in the bathtub with a tape measure and write that measurement down. This will be the width measurement of your drain pipe.
Remove the plumbing access panel located on the back of the wall that holds the faucet fixtures. You will want to take care removing this, as you don’t want to have to replace more damaged wall if you can help it. Locate the plumbing that is installed for the drain pipe and overflow pipe.
Remove the old drain pipe, if it is still in place, by loosening the connecting bolts from the plumbing pipe and from the strainer device connected to the tub. Use a large adjustable wrench or a Channel-lock pliers for this. Use a lubricant such as WD-40 if you have difficulties loosening the bolts. Measure the old drain pipe to get a horizontal and vertical measurement. This will give you your length and width of the drain you need.
Position yourself level to the floor to take some measurements if you don’t have the old drain still attached. Measure straight down from the bathtub drain hole to the level where the piping branches off towards the drain (the plumbing pipe splits in two directions, the one for the overflow tank will go straight up, the other will branch towards the drain). Write down the measurement for the vertical distance between the drain hole and the level where the piping branches off toward the drain. Measure from the piping section where it branches out toward the drain over to the drain hole and write that horizontal distance measurement down. That will give you the length and width that is needed for your drain. Your local hardware or home improvement store should carry a bathtub drain that fits your needs.
Connect your new bathtub drain with a large adjustable wrench or Channel-lock pliers. Make sure that it is on tight so it does not leak. If the drain fits properly--the connection between the tub and the main piping line up properly, as well as the new drain pipe connecting snugly to the tub--apply plumbers putty to the drain and secure into place to ensure that it does not leak. Secure the strainer in place. Run some water through the drain before you replace the plumbing access panel to see whether everything is sealed properly and is not leaking. If it does not fit properly or it leaks, you have the wrong size. Measure again and exchange the drain until everything lines up and there are no leaks.