Input
Example data table
| Scenario | Total zones | Simultaneous zones | Flow per zone | Header length | Velocity limit | Suggested header size |
|---|---|---|---|---|---|---|
| Small drip garden | 6 | 1 | 2.0 gpm | 5 ft | 5.0 ft/s | 3/4" |
| Mixed beds and lawn | 10 | 2 | 5.0 gpm | 8 ft | 5.0 ft/s | 1-1/4" |
| High flow turf | 12 | 3 | 8.0 gpm | 10 ft | 5.0 ft/s | 2" |
Formula used
How to use this calculator
- Select units and enter total zones on the header.
- Enter how many zones can run simultaneously.
- Provide typical flow per zone at design pressure.
- Add manifold length and a fitting allowance factor.
- Set a velocity limit to reduce hammer and noise.
- Enter available supply pressure and minimum required pressure.
- Press Calculate to see size, losses, and downloads.
Design flow and scheduling
Manifold sizing starts with demand. Total design flow equals the number of zones that can run together multiplied by typical zone flow. Overlapping zones increases header flow, velocity, and loss. When supply is limited, stagger irrigation programs or split the system into two manifolds. This calculator highlights the combined flow so you can compare layouts before buying pipe and valves.
Header length and fitting impact
Straight header length rarely tells the full story. Tees, elbows, unions, and adapters add equivalent length and increase friction. A modest allowance helps you plan for real installations without modeling every fitting. Keep runs short, avoid tight turns, and place the manifold close to the source line. Reducing fittings can save more pressure than upsizing one nominal pipe step.
Velocity targets for reliable operation
High velocity can create noise, erosion at fittings, and water hammer when valves close. Many irrigation designers aim for moderate velocities in headers to protect components and maintain stable flow. Use the velocity limit to set your preference, then select the smallest header that stays under it. If you must exceed the limit, consider pressure regulation, slower‑closing valves, or a larger header diameter.
Friction loss and pressure margin
The calculator estimates friction using the Hazen–Williams method and your chosen roughness factor. New smooth plastic typically has a higher C value than older, scaled, or metallic pipe. Loss rises steeply with flow, so small increases in simultaneous zones can reduce pressure at valves. Compare the estimated pressure after the manifold against the minimum required pressure to maintain uniform sprinkler or drip performance.
Field checks and practical recommendations
After selecting a header size, verify pipe specifications, valve port sizes, and connection types. Confirm that the supply line feeding the manifold is not undersized, because upstream loss will compound. Install a flush cap or drain for maintenance, and label valves for troubleshooting. If results show low remaining pressure, prioritize reducing overlap, shortening runs, or splitting the manifold into two sections.
FAQs
Q1: What does “simultaneous zones” mean?
It is the maximum number of zones that can run at the same time. The header must carry their combined flow, so higher overlap usually requires a larger manifold and increases friction loss.
Q2: Which pipe roughness value should I use?
Use a Hazen–Williams C value that matches your pipe condition. Smooth new plastic is often around 150, while older or rougher pipe is lower. If unsure, choose a conservative lower value.
Q3: Why does the calculator add a fitting allowance?
Fittings act like extra pipe length. The allowance approximates added loss from tees, elbows, and adapters without measuring each one. Increase it for complex manifolds and decrease it for very simple layouts.
Q4: Is the recommended valve size the same as the manifold size?
Not always. Valve sizing is usually tied to zone flow and the connected lateral, while the manifold header is sized for combined flow. Use the valve note as a starting point and confirm with manufacturer ratings.
Q5: What if the pressure check fails?
Reduce overlapping zones, shorten the manifold, use fewer fittings, increase header diameter, or split the system into multiple manifolds. Also verify upstream supply line sizing and any filters or regulators that may add loss.
Q6: How accurate are the pipe diameters in the table?
The inside diameters are typical values used for sizing guidance. Actual IDs vary by pipe class, schedule, and brand. For final design, confirm the manufacturer’s dimensions and rerun the calculation if they differ.