What size anode rod for Rheem water tank?

1. If you got an owners manual check that. Check on the tank to see if there's anything marked anywhere.

2. Check Rheem web site and input the serial number of your water heater to see if it'll identify that part. Or contact them by phone or email with your thank info to see if they can tell you.

3. If all else fails go part way through the replacement procedure to pull the existing rod just a few inches out of the tank so that you can measure the diameter of the existing rod ideally with a caliper. Tighten the rod back down until you can buy the replacement. Hopefully existing rod hasn't corroded too much and can still be measured.

It is possible that all 3 sizes will fit your tank and the manufacturer simply uses the fatter rods (which thus contain more metal and provide longer protection) to determine the warranty sticker and price tag which will be slapped on that tank. eg the skinny rod goes on a 6 year tank, the medium rod on a 9 year and the fat one on a 12 year tank. Just guessing.