Water Heater Anode Rod Size: Which One Fits?

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Quick answer from a licensed plumber: Most residential tank anodes thread into a 3/4-inch NPT port, and Rheem’s replacement steps use a 1-1/16-inch socket. What differs is the style and the length. A hex-head anode sits in its own port on top of the tank; a combination anode is built into the hot-water outlet. Lengths in the listings we read run from 33 to 44-3/16 inches, so match the rod you remove. The metal (magnesium, aluminum or zinc) is a separate choice; see the anode metal section below. Some heaters, Rheem says, have no accessible anode at all.

Hex-head anode in a Rheem, A. O. Smith or similar tank? Rheem’s genuine 3/4 in. NPT magnesium rod: see the Rheem SP11309T on Amazon

The Two Anode Styles

Replaceable tank anodes come in two styles, and they are not interchangeable:

Look at the top of your tank. If the hot pipe runs into a plain fitting and there is a separate capped port with a hex head under it, you have a hex-head anode. If there is no separate anode port, the anode is probably on the hot outlet. The parts list for your model settles it: A. O. Smith’s electric manual lists both an “ANODE ROD” and a “HEAT TRAP/ANODE ROD COMBINATION,” and says to give the “Model and serial number” when ordering.

Thread, Hex and Diameter

Every anode below whose maker document or listing prints a thread size gives the same one, 3/4-inch NPT:

Rheem’s tool list for the job starts with a “1-1/16 inch socket,” a ratchet and a “24″ cheater bar or pipe,” and it warns that “In most cases, you will need a long cheater bar to break the initial seal on the anode rod.” Turn off the power, or set the gas control to “OFF” as Bradford White’s anode steps begin, and turn off the cold water supply before you drain the tank; afterward Rheem says to “check for leaks at the anode rod threads; and check again 24 hours later.”

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Length: Match the Old Rod

Anode length varies by tank, from the 33-inch A. O. Smith rod to Rheem’s 44-3/16-inch assembly in the table. The safest match is the length of the rod you pull out or the one listed for your model. Bradford White notes that its segmented anodes “may need to be cut to length.” Before you buy a long rigid rod, measure the clearance above the tank; our anode rod replacement guide covers the removal itself. If clearance is tight, see flexible vs rigid anode rods.

Magnesium, Aluminum or Zinc: Which Anode Metal?

Size and style get the rod into the tank. The metal decides how fast it wears and whether it feeds a hot-water smell. The maker documents say more about this than most listings do, so this section sticks to them.

Softened water. Every maker we read says a softener shortens the rod’s life. A. O. Smith: “If you use a water softener, your anode rod will deplete faster than normal.” Rheem says softened water makes the rod’s current demand “drastically higher,” and Bradford White says to inspect the anode more often. Bradford White adds that “Overly softened water will increase conductivity” and gives 10 – 12 grains per gallon of hardness as a common target. For water that is more conductive, its bulletin says “aluminum is a better choice.”

Hard water. The documents we read give no hard-water rule for picking a metal. Rheem says the most important factor in rod life is water chemistry, “the hardness or softness of the water,” and that a magnesium rod’s performance is “dependent on water chemistry”: in low-conductivity water the rod runs at “a very low current level,” which means “not much cathodic protection,” and high conductivity uses the rod up sooner than predicted. Bradford White lists calcium and lime, sulfur, softener use and well versus city water as conditions that change how a rod behaves. So read the old rod. Rheem says to replace it when more than 6 inches of core wire shows at either end. If it is nearly gone at every yearly check, ask the maker about a different metal.

Rotten-egg smell. The rod is part of the cause, not all of it. Rheem calls the smell “a water chemistry condition, rather than a water heater problem.” Sulfate-reducing bacteria, which Rheem says are harmless to health, use hydrogen from the anode reaction to make hydrogen sulfide. Bradford White says a magnesium anode “generates enough hydrogen to create an odor problem” when sulfur and bacteria are present. So a different rod can help, and each maker names one: A. O. Smith its zinc rod, Bradford White its A420 rod (“may minimize the problem”), and Rheem its R-TECH rod, which it says “reduces the overall anode activity.” Bradford White’s parts lists label the A420 an aluminum rod; Rheem’s bulletin does not name the metal in R-TECH.

The makers put the bacteria first. A. O. Smith calls chlorination “the only sure treatment.” Bradford White says the only satisfactory method is to control the bacteria. Rheem says a new rod after a bleach flush “may not be 100% effective,” and that the problem is “more prevalent in softened water.” Rheem also says the source is in the cold supply, such as well water, and only shows once the water is heated. Our rotten-egg smell guide covers the tests. A rod that does not dissolve at all is a third route; see the best powered anode rod page.

Warranty. The warranty notes below are about leaving the tank with no rod. Bradford White says complete removal of the anode, magnesium or aluminum, voids the warranty. None of the maker documents we read says that changing the metal does, and for odor, Bradford White’s manual says “an alternative anode(s) may be purchased from the supplier that installed this water heater.” Still, call the maker listed on the rating plate, or ask your installer, before you fit a different type.

When to Replace It

Bradford White’s power-vent service manual gives a clear line: if depletion reaches “½ of the original anode diameter (approximately ¾″ diameter), replacement is recommended,” and “If any of the steel core of the anode is exposed, replacement is recommended.” A. O. Smith recommends you “remove and inspect anode rod after first six months of operation and at least annually thereafter.”

Two warranty notes. A. O. Smith: “an anode rod must be installed in the water heater at all times or the warranty is void.” Bradford White, in its light-duty commercial electric installation manual: “Removal of any anode, except for inspection and/or replacement, will nullify the warranty.” Both makers mention an alternative anode for smelly hot water; A. O. Smith suggests a special zinc anode and says to contact a qualified person. For the smell itself, see our rotten-egg smell guide.

What to Buy

We found one genuine maker anode shipped and sold by Amazon when we checked:

• Hex-head tank with room above it: Rheem SP11309T magnesium anode, 39-3/8 in., sold by Amazon. Its listing gives a 3/4 in. NPT fitting and names Rheem, Ruud, State, A. O. Smith and Bradford White among the brands it fits. Check your old rod’s length first.

We did not find a genuine combination (outlet) anode or the A. O. Smith and Bradford White hex anodes shipped or sold by Amazon when we checked; order those by model number through the maker’s parts channel.

Parts You’ll Also Need

For a hex-head anode:

• TEKTON 1/2 in. drive 1-1/16 in. 6-point socket, sold by Amazon, for the socket size Rheem lists.
• Oatey water-line thread seal tape (not linked: its third-party seller on Amazon had 3 left on September 29, 2026). Rheem says to prepare the new anode with “pipe dope or Teflon tape.”

When to Call a Licensed Plumber

• The anode is on the hot outlet. That means disconnecting the water piping at the top of the tank.
• The hex will not break loose with a cheater bar, or the tank turns when you pull on it.
• You have a gas heater and are not comfortable shutting it down and relighting it by its label.
• Rheem says your model’s anode is not accessible.

FAQ

What size socket fits a water heater anode rod?

Rheem’s instructions call for a 1-1/16-inch socket, and a Rheem anode listing gives a 1-1/16-inch hex head.

What thread is a water heater anode rod?

Every anode in the maker documents and listings we read that prints a thread size is 3/4-inch NPT, including Rheem’s SP11309T and Bradford White’s hex anodes.

How long is a water heater anode rod?

It varies by tank. The ones we found run from 33 inches (an A. O. Smith rod) to 44-3/16 inches (a Rheem assembly). Match the rod you remove or the part listed for your model.

What is a combination anode rod?

An anode built into the hot-water outlet nipple. A. O. Smith lists a heat trap/anode rod combination, and Bradford White lists hot water outlet anodes for its residential electric models.

When should an anode rod be replaced?

Bradford White’s power-vent service manual recommends replacement when depletion reaches half of the original anode diameter, or when any steel core shows. A. O. Smith recommends inspecting it after six months and at least once a year.

Is an aluminum or magnesium anode rod better?

Neither wins everywhere. Rheem calls magnesium the standard rod, and Bradford White says it protects the tank best. Bradford White says aluminum is the better choice where softened water or low pH makes the water more conductive.

Does a water softener use up an anode rod faster?

Yes, per the makers. A. O. Smith says the rod will deplete faster than normal with a softener, and Rheem and Bradford White say the same. Inspect it more often than once a year.

Will a different anode rod stop the rotten egg smell?

It may reduce it, but the makers say it does not fix the cause. A. O. Smith calls chlorination the only sure treatment, and Rheem lists chlorination ahead of a low-activity rod.

Why Trust Plumbing Picks

Plumbing Picks is written by a licensed Georgia journeyman plumber. Every part number, size, rating and fit on this page comes from the manufacturer pages and documents listed below, which we opened and read, or, where the page says so, from an Amazon listing; none of it comes from hands-on testing. Each linked product was shipped or sold by Amazon, new and in stock, when we checked on September 24, 2026. Where a link goes to Amazon we may earn a commission at no extra cost to you. Nobody pays to be included here. More on who writes this and how we pick.

Sources

• Rheem Technical Service Bulletin 1232: Anode Rod Replacement Instructions
• A. O. Smith: Residential Electric Water Heater Installation Instructions and Use & Care Guide (March 2021)
• Bradford White: Residential Gas Replacement Parts List 238-51515-00K (RG2MH models)
• Bradford White: Residential Electric Replacement Parts List 238-51382-00AA (July 2025)
• Bradford White: Power Vent Service Manual 238-51540-00E
• Bradford White: Installation and Operation Instruction Manual 238-54649-00F
• Rheem Technical Service Bulletins 1201 (Anode Rods, Cathodic Protection and the Porcelain Lining) and 1206 (Smelly Water and the Rotten Egg Odor)
• Bradford White For The Pro: Water Heater Anode Rods
• Bradford White For The Pro: Hydrogen Sulfide Odor / Rotten Egg Smell and Chlorinating Water Heaters

Bottom Line

Find the style first: a hex head on top or an anode on the hot outlet. Then match the 3/4-inch NPT thread, the 1-1/16-inch hex and the old rod’s length. Replace it at half its original diameter or when the steel core shows, and check whether your model’s anode is reachable before you buy. Keep the same metal as the factory rod unless softened water or a hot-water smell gives you a reason to change it.