Water Hammer Arrestor Sizing Calculator
Enter the fixtures served by one hot or cold branch to total the fixture units, choose the PDI size letter, adjust for pressure above 65 psi, and see how many 20-foot placement sections the branch needs.
Pressure-adjusted arrestor sizeA
- Calculated fixture-unit load
- 6.5
- Rounded sizing load
- 7
- Base arrestor size
- A
- Fixture-unit headroom
- 4
- Placement sections
- 1
- Placement guidance
- Place one arrestor at the branch end within 6 feet of the last quick-closing fixture.
Fixture-unit breakdown
| Fixture | Count | Unit weight | Fixture units |
|---|---|---|---|
| Tank-type toilets | 1 | 2.5 | 2.5 |
| Lavatories | 1 | 1 | 1 |
| Clothes washers | 1 | 3 | 3 |
How to use this calculator
- Choose whether the fixtures are private-use or public-use fixtures.
- Choose the hot or cold branch because the table weights differ by supply.
- Enter flowing pressure, branch length, and the fixture counts served by that branch.
- Use the final size and placement guidance with the arrestor manufacturer and local code.
How the arrestor size is calculated
This calculator follows the fixture-unit method shown in the Sioux Chief HydraRester sizing guide. Each fixture has a weight for the selected installation type and branch supply. The tool multiplies every entered fixture count by that table weight, excludes combinations that are not listed for the selected hot or cold branch, and adds the remaining loads.
The load formula is:
raw load = Σ fixture count × fixture-unit weight
If the raw load has a fraction, the sizing load is rounded up to the next whole fixture unit. The rounded load is mapped to the published size bands: AA for 1-4 fixture units, A for 5-11, B for 12-32, C for 33-60, D for 61-113, E for 114-154, and F for 155-300.
Flowing pressure changes the result. At 65 psi or less, the base size is used. Above 65 psi, the guide calls for the next larger arrestor size. A branch that already lands on size F and also exceeds 65 psi is outside the published upsize table, so the result is flagged for manufacturer or engineering review.
What moves the answer
Flush-valve water closets, public restroom fixtures, and clothes washers usually move the total fastest because their fixture-unit weights are high or because their valves close quickly. Hot and cold branches can land on different answers. For example, a private shower counts as 1 fixture unit on the cold branch and 2 on the hot branch, while a dishwasher contributes only to a private hot branch in the source table.
Branch length does not change the PDI size letter directly. It changes placement. A branch up to 20 feet gets one arrestor at the branch end within 6 feet of the last fixture. A longer branch must be divided into 20-foot sections, and each section should be sized from the fixtures actually served in that section.
What this calculator leaves out
The result is a table-based selection, not a transient pressure model. It does not calculate pipe velocity, valve closing time, pipe material, elevation, pressure-wave reflection, simultaneous operation, or special project specifications. It also does not combine individual bathroom fixtures with bathroom-group values, because doing both would double-count the same fixtures.
Worked example
Suppose a private cold branch serves one tank-type toilet, one lavatory, one shower, one clothes washer, and one ice maker at 72 psi, with a 42-foot branch length. The fixture-unit weights are 2.5, 1, 1, 3, and 1, for a raw load of 8.5 fixture units. Rounded up, the sizing load is 9 fixture units, which maps to base size A.
Because the flowing pressure is above 65 psi, the final selected size advances one class to B. Size B carries up to 32 fixture units, leaving 23 fixture units of headroom against the rounded load. A 42-foot branch produces three placement sections, so each 20-foot section should be checked with its actual fixture inventory.
Common questions
What size water hammer arrestor do I need?
For a multi-fixture branch, add the fixture-unit load for the fixtures served by that hot or cold branch, round up to a whole fixture unit, and select the PDI size band. This calculator also advances the size one class when flowing pressure is above 65 psi.
What are fixture units in water hammer arrestor sizing?
Fixture units are table weights that compare the load created by different plumbing fixtures. In this method, the fixture count is multiplied by the listed fixture-unit weight for the selected occupancy and supply branch.
Why are hot- and cold-water totals different?
The source table gives separate cold-water and hot-water weights for many fixtures. Toilets and urinals generally affect the cold branch only, while dishwashers are listed for private hot-water branches only.
Why does pressure above 65 psi require a larger arrestor?
The cited sizing guide says to use the next larger water hammer arrestor when flow pressure at the fixture exceeds 65 psig. Pressure exactly 65 psi does not trigger that increase in this calculator.
Where should a water hammer arrestor be installed?
For a branch 20 feet or shorter, place the arrestor at the branch end within 6 feet of the last fixture served. For longer branches, divide the run into 20-foot sections and place an arrestor near the last fixture in each section.
Does a washing machine or dishwasher need a water hammer arrestor?
Solenoid valves can close quickly, so washing machines and dishwashers are common places to check. In the table used here, a private clothes washer contributes to both hot and cold branches, while a dishwasher contributes to the private hot-water branch.