PEX Pipe Flow Rate & Velocity Calculator
Calculation Summary:
Water Velocity: 0 FPS
Pressure Drop: 0 PSI
Status: —
*Calculations based on Hazen-Williams formula for SDR-9 PEX tubing at 60°F.Understanding PEX Flow Rates and Velocity
When designing a residential or commercial plumbing system using PEX (Cross-linked Polyethylene), understanding the relationship between pipe size, flow rate (GPM), and water velocity (FPS) is critical. Improperly sized pipes can lead to noisy systems, "water hammer" effects, and premature erosion of the pipe walls.
Recommended Water Velocity for PEX
For most domestic hot and cold water systems, the following velocity guidelines apply:
- Cold Water: Maximum velocity of 8 feet per second (FPS).
- Hot Water: Maximum velocity of 5 feet per second (FPS) to reduce potential erosion and noise.
- Ideal Range: Most engineers aim for a velocity between 2 and 5 FPS for a balance of efficiency and system longevity.
PEX Inner Diameter (ID) vs. Nominal Size
It is important to note that PEX is sized by Outside Diameter (OD) but has a thicker wall (SDR-9) than copper pipe. Consequently, the actual Inner Diameter (ID) of PEX is smaller than that of the same nominal size copper pipe. For example, a 1/2″ nominal PEX pipe has an approximate ID of 0.475″, whereas 1/2″ Type L copper has an ID of roughly 0.545″. This difference is why calculating flow specifically for PEX is essential.
How Pressure Drop Affects Your System
Pressure drop is the loss of water pressure caused by friction between the flowing water and the inner walls of the PEX pipe. As the flow rate (GPM) increases or the pipe diameter decreases, the friction increases, leading to a higher pressure drop. If your pressure drop is too high, you may experience low water pressure at fixtures located furthest from the source.
Example Calculation
If you are running 5 GPM through a 100-foot run of 3/4″ PEX:
- ID: 0.671 inches.
- Velocity: Approximately 4.54 FPS (within safe limits).
- Pressure Loss: Approximately 7.5 PSI loss over the 100ft run.