Water Hammer Arrestor Sizing Calculator
Banging pipes are fixed with a small shock absorber plumbed into the line, and they come in set sizes. Tell this page which fixtures the pipe feeds, how long the run is and what your water pressure is, and it works out which size to buy and where to fit it. Nothing uploaded.
Pressure and placement
How the total was reached
PDI rounds a half fixture unit up to the next whole number, so 11.5 is entered in the table as 12.
The PDI size ladder
Table V of PDI-WH 201. The same six letters appear in every manufacturer's catalogue, so a "C" from one maker interchanges with a "C" from another.
Take it to the merchant
Copies the selection with the fixture count and the assumptions attached, so a note on a plan carries its own reasoning.
Learn more: sizing a water hammer arrestor from fixture units
Why the answer is a letter and not a pipe size
Water hammer is the pressure spike you get when a fast-closing valve stops moving water dead. A washing machine or dishwasher solenoid shuts almost instantly, and the column of water behind it has nowhere to put its momentum.
An arrestor gives it somewhere to go. Inside is a sealed chamber of gas behind a piston or bellows, and the spike compresses it instead of slamming the pipe. Because the job is absorbing energy rather than carrying flow, the device is rated by how much shock it can take, not by the pipe it screws into.
The rating scale comes from the Plumbing and Drainage Institute. Its standard PDI-WH 201, Water Hammer Arresters, sets out six certified sizes in Table V, each covering a band of fixture units: A takes 1 to 11, B takes 12 to 32, C takes 33 to 60, D takes 61 to 113, E takes 114 to 154, and F takes 155 to 330. Those are the bands this page selects from.
That is why the sizes interchange between brands. A Watts Series 15M2 in size A and a Smith Hydrotrol figure 5005 are both PDI "A" units rated 1 to 11 fixture units, even though the Watts takes a half-inch connection and the Smith a three-quarter-inch one.
Working a real branch by hand
Table IV of the standard gives each fixture a weight, split four ways: public or private use, cold or hot. This page uses those published numbers unchanged, and it hides any fixture PDI leaves blank for the combination you picked.
The standard's own first example is a public toilet room. Two flush-valve water closets at 8 fixture units each is 16, and four lavatories at 1.5 each is 6, giving 22 on the cold branch. Twenty-two lands in the 12 to 32 band, so the answer is a B.
The hot branch of that same room is only the four lavatories, at 1.5 each on the hot side, so 6 fixture units and an A. Sizing the two pipes together is the most common way people end up buying an arrestor a size bigger than they need.
A house is smaller but works identically. A flush-tank toilet is 2.5 on the cold side in private use, a basin is 1, a bath is 1.5 and a separate shower is 1. A bathroom with two basins therefore totals 2.5 plus 2 plus 1.5 plus 1, or 7 fixture units, and takes an A.
One rounding rule matters. PDI says a half-unit fraction rounds up to the next whole number, so a branch totalling 11.5 is treated as 12, which pushes it out of A and into B.
The 55 psi rule that is not the 55 psi rule
Search for arrestor sizing and you will be told to go up a size above 55 psi. The standard does not say that, and the difference changes the answer for anyone on a normal municipal supply.
What PDI actually prints under Table V is two separate notes. First, that "ideally the flow pressure in branch lines serving fixtures should never exceed 55 p.s.i.g.", and that pressure reducing valves should be installed to hold it there. Second, and only then, that "when, however, the flow pressure exceeds 65 p.s.i.g., the next larger size water hammer arrester should be selected."
So 55 psi is a target for the system, and the answer to missing it is a regulator. Sixty-five psi is the point where the arrestor selection itself changes. This page applies the bump above 65 and tells you when you are in the gap between the two.
The manufacturers who reprint the chart agree. The Smith Hydrotrol sizing sheet carries the note "when water pressure in line exceeds 65 psi, specify the next larger Hydrotrol" directly under its fixture unit table.
There is a ceiling on the correction too. PDI's pressure-based tables run out at 85 psig, so once a branch is running higher than that there is no larger size to specify and the pressure has to come down instead.
Twenty feet, and why two arrestors are rarely the same size
A single arrestor only protects what it can reach. PDI limits one unit to branch lines up to 20 feet measured from the start of the horizontal branch to the last fixture supply, which it calls Rule 1.
Past 20 feet, Rule 2 applies and the branch takes two arrestors, usually with the supply brought in near the middle and one unit on each leg. The sizing rule for a pair is the part people get wrong: the combined fixture unit ratings of the two units have to meet or beat the branch demand, so neither one has to cover the whole branch alone.
An 80 fixture unit branch is the clearest case. Two B units reach only 64 between them, so the standard specifies one C and one B, which together cover 92. A 44 fixture unit branch takes two B units, because two A units reach only 22. This page searches for the smallest combination that clears the demand, which reproduces every pairing PDI publishes.
Where it goes, and what this page does not cover
Placement is fixed by the standard and it is counterintuitive. The preferred spot is at the end of the branch line, between the last two fixtures served, not next to the appliance and not back at the riser.
The exception is a long run feeding one remote item of equipment. There the arrestor goes as close as possible to the valve that slams, so the shock is caught before it gets into the rest of the system.
That second case is also outside what this calculator does. PDI sizes single-fixture and equipment branch lines from Tables VI and VI-A instead, using pipe diameter and length of run rather than fixture units, and this page implements the fixture unit method only.
Two more limits are worth naming. Local code takes precedence over the PDI table wherever the two disagree, and the fixture unit weights in Table IV are based on 1.66 gallon-per-flush water closets and 1.06 gallon-per-flush urinals, so a job full of low-flow fittings is being sized a little conservatively. The standard also spells the device "arrester" where most catalogues and merchants spell it "arrestor", and they are the same thing.
If pressure is the underlying problem rather than shock, a pipe flow and pressure drop calculator is the better starting point, and duct sizing covers the equivalent problem on the air side of the house.
FAQ
What size water hammer arrestor do I need?
Add up the fixture units on the branch, then read the letter off the PDI table: A covers 1 to 11 fixture units, B covers 12 to 32, C covers 33 to 60, D covers 61 to 113, E covers 114 to 154, and F covers 155 to 330. A typical house bathroom on the cold side, with a flush tank toilet at 2.5, two basins at 1 each, a bath at 1.5 and a separate shower at 1, comes to 7 fixture units, which is an A. Size the cold and hot branches separately, because the same fixtures carry different weights on each.
Do I need a bigger water hammer arrestor if my water pressure is high?
Only above 65 psi. PDI-WH 201 says flow pressure in a branch serving fixtures should ideally never exceed 55 psig and that a pressure reducing valve is the answer if it does, then separately says that when flow pressure exceeds 65 psig the next larger size arrestor should be selected. Those are two different rules and plenty of secondary pages merge them into one 55 psi bump. Past 85 psig the PDI tables stop, so there is no larger size to specify and the pressure itself has to come down.
How many water hammer arrestors does one branch line need?
One, unless the branch runs more than 20 feet from the start of the horizontal branch to the last fixture supply. Past that length PDI asks for two, and their combined fixture unit ratings have to meet or beat the demand rather than each one covering it alone. A 44 fixture unit branch takes two B units, because one B covers 32 and two cover 64. An 80 fixture unit branch takes a C and a B, since 60 plus 32 is 92 and two B units would only reach 64.