TradesHow this works
Irrigation pipe is sized by velocity, not just by whether the water fits. Every pipe has a flow at which the water hits 5 feet per second, the safe design limit, and past that friction loss and water hammer climb fast. So the right pipe is the smallest one that carries your zone's flow while staying under that speed.
Enter the GPM the pipe has to carry and this picks that size, shows the actual velocity, and tells you how much more flow the pipe could take before it maxes out. It also offers the next size up, which runs slower and quieter and leaves room to add heads. It stops at velocity, so on a long run check the pressure at the far head separately.
Common questions
What size pipe do I need for my sprinkler zone?
Size it by flow and velocity. Add up the GPM of all the heads that run at once on the zone, then pick the smallest pipe that carries that flow at or under 5 feet per second. A 1 inch Schedule 40 line carries about 13 GPM at that limit, and a 1-1/4 inch line about 23, so the jump between sizes is large.
Why is 5 feet per second the limit?
Push water faster than that and two things go wrong: friction loss climbs steeply and eats your pressure, and a valve slamming shut sends a water-hammer surge that can crack fittings over time. Five feet per second is the industry ceiling that keeps both in check, so pipe is sized to stay under it, not just to physically pass the water.
Does this account for pressure loss over a long run?
No. This sizes for velocity, which is the first and most common mistake to avoid. On a long or uphill run, friction loss along the pipe can still drop the pressure at the far head below what the heads need, so check that separately with the pipe-friction engine.
Sources: Schedule 40 PVC dimensions: ASTM D1785 (ID = OD - 2 x wall) · Max design velocity 5 ft/s for PVC (Rain Bird, Irrigation Tutorials). Engine version 1.0.0. Risk tier 1.