PDI-WH 201 is the Plumbing & Drainage Institute standard that rates water hammer arrestors by the fixture-unit load they can absorb. It assigns a size letter from A to F, where A handles 1–11 fixture units and F handles up to 330.

What size arrestor does a single washing machine need?

A clothes washer is about 4 fixture units, which falls inside Size A (1–11 units). Almost every single-appliance install for a washer, dishwasher, or ice maker uses the common Size A mini arrestor.

Where should the arrestor be installed?

Install it on the supply branch as close as practical to the quick-closing or solenoid valve that causes the shock. Mounting it far away lets the pressure spike build before the arrestor can absorb it.

Do arrestors have a check digit or expire?

There is no checksum involved in sizing — it is purely a fixture-unit lookup. Modern piston-type arrestors are sealed and maintenance-free, unlike old air-chamber stubs that waterlog over time.

Does a larger arrestor ever hurt?

Oversizing is safe but wastes money and space. The goal is to meet or slightly exceed the protected fixture-unit load, then verify against the specific manufacturer's published PDI rating.

Water Hammer Arrestor Sizing Calculator

Water hammer is the bang you hear when a fast-closing valve — a washing machine solenoid, a dishwasher, an ice maker, or a flush valve — stops moving water abruptly and sends a pressure spike back up the pipe. A water hammer arrestor absorbs that spike. This tool sizes one per the industry standard PDI-WH 201.

How it works

Each protected fixture contributes a standard number of Water Supply Fixture Units (WSFU). The tool sums them and looks up the smallest arrestor whose rated range covers the total:

Size   Fixture units   Typical connection
 A        1 – 11        1/2" nominal
 B       12 – 32        3/4" nominal
 C       33 – 60        1"   nominal
 D       61 – 113       1-1/4" nominal
 E      114 – 154       1-1/2" nominal
 F      155 – 330       2"   nominal

A single washer (4 units) or dishwasher (1.4 units) lands comfortably in Size A. A whole branch feeding many flush-valve water closets can climb into Size D, E, or F.

Example and tips

A laundry box feeding one clothes washer (4 units) plus a nearby utility sink (1 unit) totals 5 fixture units — still Size A. By contrast, a commercial restroom branch with six flush-valve closets at 6 units each is 36 units, requiring Size C. Always mount the arrestor near the offending quick-closing valve, keep the access reachable, and confirm the figure against the specific arrestor maker’s PDI rating, which is the authoritative number for that product.