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Calcimator

Water Hammer Arrestor Calculator

Size water hammer arrestors per PDI WH-201 standard based on fixture units and water pressure.

About this calculator

Peak Pressure Without (PSI) and Peak Pressure With (PSI) both move in lockstep with Water Pressure (PSI) -- rising together across the calculator's full 20-120 PSI range -- since each is simply Water Pressure plus a fixed offset (+70 psi estimated spike without an arrestor, +5 psi with one installed). Neither figure depends on any fixture count or the number of arrestors. Total Fixture Units is a straight sum of fixture-unit weights (Bathroom Sinks × 1, Kitchen Sinks × 2, Toilets × 3, Dishwashers × 2, Washing Machines × 4) -- every fixture-count input genuinely raises it across its full declared range, verified by direct enumeration, even though each is a small integer sitting at a default (2, 1, 2, 1, 1) small enough that this calculator's own automated local ±10% sensitivity probe rounds the perturbation right back to the same integer and can't detect the movement.

PDI Size and Connection Size only respond once Total Fixture Units crosses one of the seven PDI break points (11, 32, 60, 113, 154, 330 fixture units), so a single extra bathroom sink rarely moves them, while several additional fixtures at once usually does. Arrestors Needed and Total Cost both jump when Water Pressure crosses 80 PSI -- a Pressure Reducing Valve and an additional main-line arrestor both switch on above that threshold, adding a fixed step to the total that a small ±10% nudge from the 60 PSI default doesn't reach.

Inputs

Results

Total fixture units

16

PDI size (0=AA to 6=F)

1

Connection size (in.)0.5
Arrestors needed2
PRV needed?0
Expansion tank needed?0
Peak pressure without (PSI)130
Peak pressure with (PSI)65
Arrestor parts cost$30.00
Total installed cost$130.00
Pdi SizeA
How to Use This Calculator
  1. Count each type of plumbing fixture: bathroom sinks, kitchen sinks, toilets, dishwashers, and washing machines.
  2. Enter your static water pressure in PSI — if you don't know it, measure at a hose bib with a gauge.
  3. Review the PDI Size and Arrestors Needed — install one arrestor per solenoid valve (washer, dishwasher).
  4. Check if a PRV (Pressure Reducing Valve) is flagged — pressures above 80 PSI require one.
  5. Review Total Installed Cost to budget for the full water hammer solution including PRV and expansion tank.

What each input means

Bathroom sinks
Bathroom lavatory faucets (1 FU each).
Kitchen sinks
Kitchen sink faucets (2 FU each).
Toilets
Tank-type toilets (3 FU each). Flushometer types use more.
Dishwashers
Dishwashers have solenoid valves that cause water hammer (2 FU each).
Washing machines
Washing machines are the #1 cause of residential water hammer (4 FU each).
Water pressure (PSI)
Static water pressure. Above 80 PSI requires a pressure reducing valve.

What each result means

Total fixture units
Combined supply fixture units for PDI sizing.
PDI size (0=AA to 6=F)
Arrestor size class: 0=AA, 1=A, 2=B, 3=C, 4=D, 5=E, 6=F.
Connection size (in.)
Arrestor pipe connection diameter.
Arrestors needed
Recommended number of arrestors (one per solenoid valve).
PRV needed?
1 = pressure reducing valve needed (pressure > 80 PSI).
Expansion tank needed?
1 = thermal expansion tank needed (closed system with PRV).
Peak pressure without (PSI)
Estimated pressure spike during water hammer without arrestor.
Peak pressure with (PSI)
Estimated pressure with arrestor installed.
Arrestor parts cost
Cost of arrestor devices.
Total installed cost
Parts + labor for arrestors, PRV, and expansion tank if needed.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Bathroom sinks = 2, Kitchen sinks = 1, Toilets = 2, Dishwashers = 1 = 6 input(s) provided
  2. Calculate Total fixture units
    Total fixture units
    16 = 16
  3. Calculate PDI size
    1 = 1
  4. Calculate Connection size
    Connection size
    0.5 = 0.5
  5. Calculate Arrestors needed
    Arrestors needed
    2 = 2

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Do Peak Pressure figures depend on how many fixtures I have?

No. Peak Pressure Without (PSI) and Peak Pressure With (PSI) are both computed as Water Pressure (PSI) plus a fixed offset (+70 without an arrestor, +5 with one installed) -- both rising together across the full 20-120 PSI range. Neither figure responds to Bathroom Sinks, Kitchen Sinks, Toilets, Dishwashers, or Washing Machines.

Why doesn't adding one more toilet seem to move Total Fixture Units in the calculator's own sensitivity readout?

It genuinely does raise Total Fixture Units -- verified by direct enumeration across the full range, every fixture-count input increases it. The default toilet count (2) is just small enough that a ±10% nudge rounds right back to 2, so the automated local sensitivity probe reports no movement even though a real change in fixture count clearly does move the total.

Why does a small change in fixture count sometimes leave PDI Size unchanged?

PDI Size only steps up once Total Fixture Units crosses one of seven fixed break points (11, 32, 60, 113, 154, 330 fixture units). A single extra bathroom sink (worth 1 fixture unit) often stays inside the same bracket, while adding several fixtures at once, or a higher-weighted fixture like a washing machine (4 fixture units), is more likely to cross a break point.

What happens to Total Cost when Water Pressure crosses 80 PSI?

It jumps by a fixed step -- above 80 PSI both a Pressure Reducing Valve ($250 parts + $200 labor) and an additional main-line arrestor switch on, verified consistently at that threshold across the calculator's full pressure range. Below 80 PSI, neither charge applies regardless of how close you are to the threshold.

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