PEX Flow Rate Calculator
Enter pipe size, pressure, length, fittings, and demand data. The result appears above this form after submission.
Formula Used
The main calculation uses the Hazen-Williams equation for water flow in pressure pipe.
h = 4.52 × L × Q^1.85 ÷ (C^1.85 × d^4.87)
Solving for flow gives:
Q = ((h × C^1.85 × d^4.87) ÷ (4.52 × L))^(1 ÷ 1.85)
- Q is flow in gallons per minute.
- h is available friction head in feet of water.
- L is straight pipe plus equivalent fittings length in feet.
- C is the Hazen-Williams roughness coefficient.
- d is inside diameter in inches.
Velocity is calculated with V = 0.4085 × Q ÷ d². Demand is estimated from fixtures, fixture flow, and simultaneous use.
How to Use This Calculator
- Select the input unit system.
- Choose a nominal PEX size or enter the actual inside diameter.
- Enter straight pipe length and equivalent fitting length.
- Add the pressure drop available for this pipe run.
- Enter elevation change if the outlet is higher or lower.
- Adjust C factor, velocity limit, demand, and safety factor.
- Press the calculate button and review the result cards.
- Use the chart, CSV button, or PDF button for reporting.
Example Data Table
| Nominal Size | Inside Diameter | Length | Pressure Drop | Estimated Flow | Typical Use |
|---|---|---|---|---|---|
| 1/2 in | 0.475 in | 80 ft | 10 psi | About 4.1 GPM | Single fixture branch |
| 3/4 in | 0.671 in | 100 ft | 12 psi | About 10.7 GPM | Small group branch |
| 1 in | 0.862 in | 120 ft | 15 psi | About 20.3 GPM | Main distribution line |
Understanding PEX Flow Rate
PEX tubing is common in modern water systems. It bends easily. It also resists scale better than many metal pipes. Flow still depends on physics. Diameter, length, fittings, pressure, and elevation all matter. A small tube can carry useful water. Yet it may lose pressure fast when the run is long.
Why Pressure Drop Matters
Water moves because pressure energy is available. Each foot of tubing consumes part of that energy. Elbows, tees, valves, manifolds, and adapters act like extra pipe. This calculator adds their equivalent length. It then estimates the flow that can pass through the total run.
Velocity and Comfort
High velocity can create noise. It can also increase wear at fittings. Many designers keep domestic water velocity within a practical limit. The tool compares calculated velocity with your chosen limit. This helps you avoid a pipe that looks large enough, but runs too fast.
Using Diameter Carefully
Nominal PEX size is not always the true inside diameter. Brand, wall type, standard, and fittings can change the real bore. Use the custom diameter field when you know the exact value. A small diameter change can create a large flow change.
Demand Planning
The calculator also estimates served demand. Enter the number of fixtures, average fixture demand, and simultaneous use. This creates a simple demand load. The recommended capacity includes your safety factor. The margin shows whether the run is likely comfortable or limited.
Best Use
Use this result for planning and comparison. It is not a substitute for local plumbing code. Check manufacturer tables for final sizing. Real systems may include valves, filters, heaters, pressure regulators, and mixed pipe materials. These items can add extra loss. Good design leaves reserve capacity for aging, future fixtures, and short pressure swings.
Reading the Output
Start with the flow estimate. Then check velocity. Next compare recommended capacity with demand. A positive margin suggests the pipe has reserve. A negative margin suggests a larger tube, shorter path, fewer fittings, or more pressure may be needed. Review the chart to see how strongly pressure changes affect output. Always confirm final choices with a qualified professional before starting installation work.
FAQs
1. What is PEX flow rate?
PEX flow rate is the volume of water moving through PEX tubing over time. This calculator reports it mainly in gallons per minute and liters per minute.
2. Does a larger PEX size always increase flow?
Yes, if pressure and length stay similar. Larger inside diameter lowers friction loss. That allows more water to move at a lower velocity.
3. Why does equivalent fitting length matter?
Fittings disturb water flow. Each elbow, tee, valve, or adapter adds resistance. Equivalent length converts that resistance into extra pipe length for easier calculation.
4. What C factor should I use for PEX?
New smooth plastic tubing is often estimated near 150. Older systems, restrictive fittings, or mixed materials may need a lower value for a safer result.
5. What velocity is acceptable in PEX tubing?
Many domestic designs target about 5 to 8 feet per second. Lower velocity can reduce noise, pressure drop, and stress on fittings.
6. Can this replace plumbing code sizing?
No. Use it for planning, estimates, and comparisons. Final pipe sizing should follow local codes, fixture unit rules, and manufacturer guidance.
7. Why is my capacity margin negative?
A negative margin means estimated demand exceeds recommended capacity. Try a larger pipe, shorter path, lower fitting loss, or higher allowed pressure drop.
8. Why enter the actual inside diameter?
Nominal size is a label. Real inside diameter can vary by product and standard. The actual value gives a more accurate calculation.