Pipe flow · calculator
PEX Pipe Flow Rate & Sizing Calculator
Calculate water flow rate, fluid velocity, and friction head loss through PEX-A and PEX-B piping networks.
Method record
What this calculator measures
PEX flow calculations use the Hazen-Williams friction loss formula with a roughness coefficient C = 150 for smooth cross-linked polyethylene. Fluid velocity must remain below 8 ft/s for cold water and below 5 ft/s for hot water distribution.
Q = [ΔP × C^1.85 × d^4.87 ÷ (4.52 × L)]^(1/1.85) Use it correctly
Calculation steps
- Use actual inside diameter (0.475 in for 1/2-inch, 0.671 in for 3/4-inch PEX).
- Enter the total equivalent length including fitting allowances.
- Verify that calculated fluid velocity does not exceed 8.0 ft/s (cold) or 5.0 ft/s (hot).
Applicability
Use the model within its limits
PEX crimp fittings (PEX-B) reduce internal diameter at every fitting. Factor fitting equivalent lengths into the total run length.
What this result represents
- System
- Pipe flow
- Formula
- Q = [ΔP × C^1.85 × d^4.87 ÷ (4.52 × L)]^(1/1.85)
- Review
- Verify safety-sensitive work with qualified guidance.
Worked Field Example
Worked Example: Sizing a 3/4" PEX Run to a Master Shower Fixture
Field Scenario: Evaluating whether a 60-foot run of 3/4" PEX-A tubing (d = 0.671 in) can deliver 8.0 GPM to a luxury shower fixture without exceeding maximum velocity limits or suffering excessive friction drop.
Given Input Parameters
Step-by-Step Mathematical Solution
- Calculate Mean Velocity: V = (0.4085 × 8.0) / (0.671)² = 3.268 / 0.4502 ≈ 7.26 ft/s.
- Verify Velocity Code Limit: 7.26 ft/s satisfies the 8.0 ft/s maximum velocity limit for cold water distribution under the Uniform Plumbing Code (UPC).
- Calculate Friction Head Loss: ΔP = [4.52 × 60 × (8.0)^1.85] / [(150)^1.85 × (0.671)^4.87] ≈ 12,771 / 1,545.8 ≈ 8.26 PSI (19.06 ft head loss).
7.26 ft/s velocity and 8.26 PSI friction loss across 60 ft; system safely satisfies residential plumbing code criteria.
Professional Guidance: For hot water runs above 140°F, sizing should be upgraded to 1.0" PEX to keep fluid velocity strictly below 5.0 ft/s.
Quick Answer & Key Principles
How Many GPM Can 1/2-Inch and 3/4-Inch PEX Pipe Support?
Direct Answer: A standard 1/2-inch PEX line delivers 2.7 GPM at 5 ft/s (recommended continuous limit) and up to 4.4 GPM at 8 ft/s (maximum intermittent code limit). A 3/4-inch PEX line delivers 5.5 GPM at 5 ft/s and up to 8.8 GPM at 8 ft/s.
GPM = (Velocity × d²) ÷ 0.4085 - 1/2" PEX (0.475" ID): 2.7 GPM continuous / 4.4 GPM intermittent (ideal for single fixtures: shower, sink, toilet).
- 3/4" PEX (0.671" ID): 5.5 GPM continuous / 8.8 GPM intermittent (ideal for bathroom groups and water heaters).
- 1.0" PEX (0.862" ID): 9.1 GPM continuous / 14.5 GPM intermittent (ideal for main service trunks).
Engineering Standard Reference
Standard PEX Tubing Water Flow Capacity & Sizing Matrix
Flow capacity (GPM) and pressure drops for ASTM F876/F877 CTS SDR-9 PEX tubing across standard residential design velocities (5 ft/s standard and 8 ft/s maximum limit):
| Nominal Size | Tubing Standard | Actual Inside Diameter | Flow Area | Max GPM @ 5 ft/s | Max GPM @ 8 ft/s | PSI Drop / 100 ft @ 5 ft/s |
|---|---|---|---|---|---|---|
| 3/8" PEX | ASTM F876 CTS SDR-9 | 0.350 in | 0.096 sq in | 1.5 GPM | 2.4 GPM | 14.2 PSI |
| 1/2" PEX | ASTM F876 CTS SDR-9 | 0.475 in | 0.177 sq in | 2.7 GPM | 4.4 GPM | 9.8 PSI |
| 3/4" PEX | ASTM F876 CTS SDR-9 | 0.671 in | 0.354 sq in | 5.5 GPM | 8.8 GPM | 5.9 PSI |
| 1.0" PEX | ASTM F876 CTS SDR-9 | 0.862 in | 0.584 sq in | 9.1 GPM | 14.5 GPM | 4.1 PSI |
| 1-1/4" PEX | ASTM F876 CTS SDR-9 | 1.054 in | 0.872 sq in | 13.6 GPM | 21.8 GPM | 3.2 PSI |
Engineering Note: PEX tubing has thicker walls than copper or PVC, resulting in smaller internal diameter for the same nominal trade size. Use expansion fittings (PEX-A) or crimp fittings (PEX-B) accounting for minor fitting restrictions.
Engineering FAQs
Frequently Asked Questions
Why does PEX pipe deliver less flow (GPM) than copper of the same nominal size?
PEX tubing is manufactured under the Copper Tube Size (CTS) standard with a fixed Standard Dimension Ratio (SDR-9). Because the wall thickness must be heavy enough to handle operating pressures, the inside diameter of 3/4" PEX is only 0.671", compared to 0.785" for 3/4" Type L copper. Because cross-sectional flow area scales with the square of diameter (d²), 3/4" PEX delivers 27% less flow area than copper of the same trade size.
What is the hydraulic difference between PEX-A, PEX-B, and PEX-C?
The raw tubing dimensions under ASTM F876 are identical across all three types. However, fitting geometry creates substantial flow differences. PEX-A allows cold-expansion fittings (ASTM F1960) that maintain the pipe's full internal diameter. PEX-B uses crimp insert fittings (ASTM F1807) that sit inside the tubing, necking down the flow diameter by up to 30% at every connection.
What is the maximum recommended water velocity in PEX piping?
Plumbing engineering standards (including PPI and UPC guidelines) specify a maximum continuous velocity of 8.0 ft/s for cold water lines to prevent flow noise and erosion, and 5.0 ft/s for hot water lines (above 140°F) to prevent premature thermal-oxidative degradation.