Pipe Sizing & Flow Rates
Updated February 20, 2026
Undersized pipes mean low pressure, slow filling, and poor shower performance. Oversized pipes waste material and money. Getting the sizing right ensures every fixture performs well, even when multiple fixtures run simultaneously. This guide covers supply and drain pipe sizing for residential applications.
Overview
Undersized pipes mean low pressure, slow filling, and poor shower performance. Oversized pipes waste material and money. Getting the sizing right ensures every fixture performs well, even when multiple fixtures run simultaneously. This guide covers supply and drain pipe sizing for residential applications.
What to Know
Buying Tips
- When in doubt, size up. A 3/4 inch line costs marginally more than 1/2 inch but delivers significantly more flow.
- Long runs need larger pipe. Friction loss increases with distance -- a 50-foot 1/2 inch run loses more pressure than a 10-foot run.
- Home-run PEX manifold systems deliver better pressure to each fixture because each line is dedicated.
- If adding a bathroom or fixture, verify the existing main line can handle the additional demand.
Common Mistakes
- Running 1/2 inch to a shower on a long run. The pressure drop makes for a weak shower.
- Using 1-1/2 inch drain for a kitchen sink with a disposal. Code requires 2 inch.
- Not accounting for simultaneous use. The system must handle peak demand, not just one fixture at a time.
- Reducing pipe size at transitions. Reductions create bottlenecks that limit flow to everything downstream.
Bottom Line
3/4 inch supply mains, 1/2 inch branches, 2 inch drains for kitchens and baths, 3 inch for toilets. Size up for long runs. The cost difference is small but the performance difference is significant.