Plumbing Pipe Sizing Guide: Water Supply and Drain Sizing

Water Supply Pipe Sizing

Supply pipes need to maintain adequate pressure and flow at all fixtures simultaneously. Key principle: trunk lines are larger than branch lines.

Main service line (street to house): 3/4" to 1" minimum (1" preferred)
Main trunk lines: 3/4"
Branch lines to bathrooms/kitchen: 1/2"
Individual fixture supply: 3/8" to 1/2"

Drain and Vent Sizing

Drains use gravity flow — sizing ensures adequate velocity for self-cleaning. Vents allow air in to replace water flowing out (prevents siphoning and odors).

Toilet: 3" drain, 2" vent minimum (3" better)
Shower/tub: 2" drain, 1.5" vent
Lavatory sink: 1.5" drain, 1.5" vent
Kitchen sink: 1.5" to 2" drain
Main stack: 3" to 4" (all drains connect to this)

PEX vs. Copper vs. CPVC for Supply

PEX: Flexible, freeze-resistant, easiest DIY installation. Requires special crimping or expansion tools. Currently the most popular new installation choice.
Copper: Traditional standard, very long lifespan, high cost, requires soldering (sweat fitting). Still preferred by some plumbers for quality installations.
CPVC: Cheaper than copper, uses glue fittings, less flexible than PEX. Common in commercial applications and some regions.

Frequently Asked Questions

What is water hammer and how do I fix it?

Water hammer is the banging sound when a fast-closing valve stops water suddenly. The momentum of moving water has nowhere to go and creates pressure waves. Solutions: install water hammer arrestors at washing machine and dishwasher connections, reduce water pressure if above 80 PSI, or secure loose pipes that amplify the sound.

How do I know if my water pressure is too high?

Buy a simple pressure gauge (under $20) that attaches to a hose bib. Normal residential pressure: 40–80 PSI. Above 80 PSI causes premature fixture and appliance failure, pipe stress, and water hammer. Install a pressure-reducing valve (PRV) if your pressure consistently exceeds 80 PSI.