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Calculate the correct PEX tubing size for your plumbing project based on fixture units, flow rate, and pipe run length.
See fixture unit chart below
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PEX (Cross-linked Polyethylene) is a flexible plastic tubing used for water supply lines in residential and commercial plumbing. It has largely replaced copper and CPVC due to its flexibility, ease of installation, resistance to freezing, and lower cost.
Proper PEX sizing depends on several factors that affect water delivery:
Each fixture has a “weight” based on its water demand. Total fixture units help determine pipe size.
Maximum expected flow rate in gallons per minute (GPM). Different fixtures have different flow requirements.
Longer runs experience more friction loss and may require larger pipe sizes to maintain adequate pressure.
Lower supply pressure may require larger pipes to ensure adequate flow at fixtures.
Ideal velocity is 4-8 fps. Too high causes noise and wear; too low may cause sediment buildup.
| PEX Size | Inner Diameter | Max Flow (GPM) | Max FU | Typical Use |
|---|---|---|---|---|
| 3/8" | 0.36" | 2 GPM | 3 | Single fixture, ice maker |
| 1/2" | 0.475" | 4 GPM | 6 | Individual fixture branches |
| 3/4" | 0.671" | 8 GPM | 16 | Small home main, bathroom groups |
| 1" | 0.862" | 16 GPM | 32 | Main water supply line |
| 1-1/4" | 1.076" | 25 GPM | 50 | Large home, commercial |
| 1-1/2" | 1.278" | 36 GPM | 75 | Commercial main |
Fixture units are used to estimate water demand. Add up all fixtures being served to determine total fixture units.
| Fixture | Cold FU | Hot FU | Total FU | Flow Rate (GPM) |
|---|---|---|---|---|
| Lavatory (bathroom sink) | 0.5 | 0.5 | 1.0 | 1.5 |
| Kitchen sink | 0.75 | 0.75 | 1.5 | 2.2 |
| Shower | 1.0 | 1.0 | 2.0 | 2.5 |
| Bathtub | 1.0 | 1.0 | 2.0 | 4.0 |
| Toilet (tank) | 2.5 | — | 2.5 | 3.0 |
| Dishwasher | — | 1.5 | 1.5 | 2.0 |
| Washing machine | 1.0 | 1.0 | 2.0 | 4.0 |
| Hose bib/outdoor faucet | 2.5 | — | 2.5 | 5.0 |
Engel (Peroxide) Method
Silane (Moisture Cure) Method
Electron Beam Method
Home run (manifold) systems use individual lines to each fixture, allowing shutoff control but using more pipe. Trunk and branch uses less pipe but shares lines.
PEX degrades in sunlight. Never leave exposed outdoors. Cover any exposed sections within 30 days of installation.
PEX expands and contracts with temperature. Allow slack in runs and do not secure too tightly. Use proper PEX supports every 32” horizontally, 4' vertically.
Minimum bend radius is 8× the outer diameter. For 1/2” PEX, minimum bend is 4”. Use bend supports for tighter turns.
Typically 1/2” PEX is used for individual fixtures (sink, toilet, shower). A 3/4” line can serve as a branch to multiple bathrooms, with 1/2” lines breaking off to each fixture.
No. PEX should not be connected directly to a water heater. Use at least 18” of copper or CPVC from the water heater before transitioning to PEX.
3/4” is typically recommended for the main supply line to handle multiple fixtures simultaneously. 1/2” is fine for individual fixture runs.
PEX is rated for 50+ years of service life when properly installed. It typically outperforms copper in corrosive water conditions.
PEX should not be exposed to UV light (sunlight). It can be buried underground below the frost line, but any above-ground sections must be protected.