Why poly pipe size matters
A pipe that's too small for its flow forces water through at a higher velocity, and friction loss climbs fast with velocity, not in a straight line. Push the same flow through a pipe one size smaller and friction loss can roughly double or more. That shows up as weaker sprinkler performance at the far end of a zone, a pump working harder than it needs to, and a higher risk of water hammer — the pressure spike that happens when a fast-moving column of water is stopped suddenly by a closing valve.
Rain Bird's published polyethylene (PE) pipe chart flags velocities above 5 feet per second (about 1.5 m/s) for caution. It's a widely used design guideline for irrigation mainlines and submains, not a law of physics, so this calculator lets you set your own target if your system calls for something more conservative, such as 3–4 ft/s on a long run with a lot of elbows.
| Material | Typical range | Design C |
|---|---|---|
| Polyethylene (PE) | 130–150 | 140 |
| PVC | 145–160 | 150 |
| Copper, brass | 120–150 | 130 |
| Welded steel | 80–150 | 100 |
This calculator defaults to C = 140 for polyethylene, the design value Toro Ag lists, and lets you switch to 150 or 130, or enter your own. If you're comparing against a PVC pipe, use the irrigation pipe size calculator instead, since PVC's C-factor and dimensions are both different.
Check the pressure rating separately. The pipe size and SIDR pressure class are two different decisions. Pick the size here for flow and velocity, then choose a SIDR class rated for your system's actual operating pressure. Follow local plumbing and utility rules for burial depth, bedding and backflow prevention.
How this poly pipe size calculator works
The calculator works out the smallest standard SIDR/IPS inside diameter that keeps velocity at or below your target, then applies the Hazen-Williams equation to that pipe's exact inside diameter to get friction loss. If you check a size you already have instead, it skips the sizing step and runs the same two formulas on the diameter you picked.
1. Minimum inside diameter for your target velocity
Required inside diameter
D = √(0.408 × Q ÷ Vmax)
Q is flow in gallons per minute, Vmax is your target velocity in ft/s, and D comes out in inches. The calculator then picks the smallest standard size at or above this value.
2. Velocity in the pipe
Velocity
V = 0.408 × Q ÷ D²
Same variables as above; this version tells you the actual velocity once a specific inside diameter is chosen. Published in Rain Bird's and Toro Ag Irrigation's PE pipe references.
3. Friction loss (Hazen-Williams)
Friction loss per 100 ft
Hf = 0.2083 × (100÷C)1.852 × Q1.852 ÷ D4.8655 × 0.433
C is the Hazen-Williams roughness coefficient you select. The result is in psi per 100 ft of straight pipe; multiply by (length ÷ 100) for the total over your run. This matches the equation and inside-diameter chart published in Rain Bird's PE SDR pipe reference.
Standard SIDR/IPS inside diameters
ASTM D2239 controls the inside diameter of SIDR pipe, so it stays the same across every pressure class at a given nominal size — only the wall thickness changes. The values below are confirmed against both an ASTM D2239 manufacturer data sheet and Rain Bird's published PE pipe chart.
| Nominal size | Inside diameter (in) | Inside diameter (mm) |
|---|---|---|
| 1/2 in | 0.622 | 15.8 |
| 3/4 in | 0.824 | 20.9 |
| 1 in | 1.049 | 26.6 |
| 1 1/4 in | 1.380 | 35.1 |
| 1 1/2 in | 1.610 | 40.9 |
| 2 in | 2.067 | 52.5 |
| 2 1/2 in | 2.469 | 62.7 |
| 3 in | 3.068 | 77.9 |
| 4 in | 4.026 | 102.3 |
Results round to 0.1 ft/s (0.01 m/s) for velocity and 0.1 psi (whole kPa) for friction loss, matching the precision used in the worked example below.
When this calculator isn't the right tool
- PVC pipe. Schedule 40/80 PVC has different inside diameters and a different typical C-factor (150). Use the irrigation pipe size calculator.
- Pipe larger than 4 in, or full mainline layouts. See the mainline size calculator.
- A lateral with several outlets along its length (multiple sprinkler heads or emitters tapping off one line), where flow — and so velocity and loss — drops after each takeoff. See the multiple outlet friction loss calculator.
- Drip tubing runs, which are sized by emitter spacing and flow, not by this kind of mainline velocity target. See the drip line max length calculator.
- Your whole system's available pressure budget, once you know friction loss for each section. See the available irrigation pressure calculator.
Poly pipe sizing example
A mainline needs to carry 20 gpm. The designer keeps the default 5 ft/s velocity target and the default C = 140, and the run is 300 ft.
- Required inside diameter: √(0.408 × 20 ÷ 5) = √1.632 ≈ 1.28 in.
- Smallest standard size at or above that: 1 1/4 in SIDR/IPS, inside diameter 1.380 in.
- Velocity at that size: 0.408 × 20 ÷ 1.380² ≈ 4.3 ft/s, within the 5.0 ft/s target.
- Friction loss per 100 ft: using C = 140 and D = 1.380 in, Hf ≈ 2.6 psi per 100 ft, matching Rain Bird's published chart for 20 gpm through 1 1/4-inch PE pipe.
- Total loss over 300 ft: 2.6 × (300÷100) ≈ 7.8 psi.
The same calculation in metric
A submain needs to carry 60 L/min. The designer keeps the default 1.5 m/s velocity target and C = 140, over a 50 m run.
- Converted to US units for the formula: 60 L/min ÷ 3.785 ≈ 15.85 gpm; 1.5 m/s ÷ 0.3048 ≈ 4.92 ft/s.
- Required inside diameter: √(0.408 × 15.85 ÷ 4.92) ≈ 1.15 in → 1 1/4 in SIDR/IPS (35.1 mm).
- Velocity: 0.408 × 15.85 ÷ 1.380² ≈ 3.40 ft/s ≈ 1.04 m/s, within the 1.5 m/s target.
- Friction loss: ≈ 1.69 psi per 100 ft ≈ 38 kPa per 100 m.
- Total loss over 50 m: 38 × (50÷100) ≈ 19 kPa.
Both examples land on the same nominal size for a similar-sized job, which makes sense: 20 gpm and 60 L/min (about 15.85 gpm) are close enough in flow that the same 1 1/4-inch pipe keeps velocity under a 5 ft/s (1.5 m/s) target in both.
Using the result
Finding your flow rate
For a single valve or zone, a bucket test (time how long a known volume takes to fill) is the most direct measurement. For a shared mainline feeding several zones, add up the flow of whatever zones can run at the same time; the irrigation system GPM calculator walks through that math. Use the largest realistic combined flow the pipe will ever carry, not just what one zone needs.
Buying and connecting the pipe
Once you know the size, choose a SIDR pressure class rated above your system's operating pressure — that choice doesn't change the inside diameter or the numbers above. Because SIDR pipe is sized by a controlled inside diameter rather than a controlled outside diameter, it's normally joined with barbed insert fittings and clamps rather than the OD-based slip fittings made for Schedule 40 PVC; check that any fitting is rated for the SIDR class and nominal size you're buying.
Common mistakes
- Sizing to a hose bibb or short branch instead of the mainline's total flow. Size for the largest combined flow the pipe carries, not the smallest fitting on it.
- Assuming a heavier SIDR class carries more flow. It doesn't; SIDR's inside diameter is fixed by nominal size, so only pipe size changes velocity and friction loss.
- Using PVC's C-factor for poly pipe, or the reverse. The two materials' typical design values differ (140 vs. 150), which shifts the friction-loss result noticeably over a long run.
- Ignoring fittings, valves and elevation change. This calculator covers straight-pipe friction only; add those separately for a full pressure budget.
What changes velocity and friction loss
| Factor | Effect | What to adjust |
|---|---|---|
| Pipe length | Friction loss scales directly with length. | Enter your actual run length for a total, not just per-100 loss. |
| Pipe size | The single biggest lever; friction loss falls sharply with each size up. | Try the next size up or down to compare. |
| C-factor / pipe age | Interior scale or biofilm buildup lowers C over time, raising loss. | Use 130 for older pipe, 150 for new pipe you're confident in. |
| Multiple takeoffs along the run | Flow drops after each branch, so a single flow figure overstates loss past the first takeoff. | Use the multiple outlet friction loss calculator for laterals with several outlets. |
| Elevation change | Uphill runs add pressure loss beyond friction; downhill runs recover some. | Add about 0.433 psi per foot of rise, or use the available irrigation pressure calculator. |
| Fittings and valves | Each elbow, tee or valve adds a small extra loss beyond straight pipe. | See the valve pressure loss calculator for fitting losses on long runs. |
| SIDR pressure class | Doesn't change inside diameter, so it doesn't change velocity or friction loss. | Pick the class based on your system's operating pressure, independent of the size chosen here. |
Frequently asked questions
What size poly pipe do I need for my flow rate?
Pick the smallest pipe whose inside diameter keeps velocity at or below your target, typically 5 feet per second (about 1.5 m/s) for irrigation mainlines, per Rain Bird's published PE pipe chart. This calculator does that automatically: enter your flow rate and it works up from a 1/2-inch to a 4-inch SIDR/IPS pipe until it finds the first size that meets your velocity target, then shows the resulting friction loss.
What's the difference between SIDR and IPS poly pipe?
IPS (Iron Pipe Size) describes an outside diameter that matches steel pipe, which is what fittings are sized to. SIDR (Standard Inside Dimension Ratio) describes pipe by a fixed inside diameter, set by ASTM D2239. Common irrigation-grade poly pipe combines both: it uses the IPS outside diameter for fittings, while ASTM D2239 holds the inside diameter constant for a given nominal size, which is what this calculator uses for velocity and friction loss.
Does a pipe's SIDR pressure class change its inside diameter?
No, and that's the point of SIDR pipe. ASTM D2239 controls the inside diameter, so a 1-inch SIDR 15 pipe (125 psi rated) and a 1-inch SIDR 11.5 pipe (160 psi rated) share the same 1.049-inch inside diameter; only the wall thickness and outside diameter change. That means you can pick the pipe size for flow and velocity first, then choose whichever SIDR pressure class your system's operating pressure requires, without it changing the hydraulics.
What Hazen-Williams C-factor should I use for polyethylene pipe?
Toro Ag Irrigation's published C-factor reference lists a design value of 140 for polyethylene pipe, within a range of about 130 to 150 depending on age and condition. This calculator defaults to 140. Use 150 to model new, unworn pipe, or 130 for older pipe when you want a more conservative, higher friction-loss estimate.
What's a safe velocity for poly pipe in an irrigation system?
Rain Bird's published PE pipe chart flags velocities above 5 feet per second (about 1.5 m/s) for caution, since higher velocities raise friction loss quickly and increase the risk of water hammer when valves close. It's a design guideline, not a hard limit, so this calculator lets you enter your own target if your system calls for something different.
Can I use this calculator for PVC pipe?
Not accurately. Schedule 40 or Schedule 80 PVC has a different wall thickness, and so a different inside diameter, at the same nominal size, and PVC's typical Hazen-Williams C-factor is 150, not the 140 used for polyethylene here. Use the irrigation pipe size calculator for PVC mainlines instead.
See the irrigation pipe size calculator.
How do I find my system's flow rate in GPM?
Time how long it takes to fill a container of known volume from the pipe or valve, then divide the volume by the time; the bucket test calculator does that math for you. If you're sizing a mainline that feeds several zones, add up the flow of the sprinklers or emitters that run at the same time instead, which the irrigation system GPM calculator can help with.
See the bucket test calculator and the irrigation system GPM calculator.
Why does my friction loss number differ from a table I found elsewhere?
Friction loss is sensitive to the exact inside diameter and C-factor used, and different sources round these differently or use PVC's C-factor of 150 instead of polyethylene's 140. This calculator uses the ASTM D2239 controlled inside diameters and whichever C-factor you select, so results will match another source only if it uses the same diameter and C-factor.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Inside diameters verified against ASTM D2239 and a manufacturer's SIDR/IPS data sheet.
- Velocity and friction-loss formulas checked against Rain Bird's and Toro Ag Irrigation's published PE pipe references.
- Both worked examples matched by hand and against the calculator's output.
Sources
- ASTM International. D2239-22, Standard Specification for Polyethylene (PE) Plastic Pipe (SIDR-PR) Based on Controlled Inside Diameter.
- Advanced Drainage Systems (ADS). ADS PolyFlex Standard Grade Water Service Pipe (SIDR IPS) Specification.
- Rain Bird Corporation. Polyethylene (PE) SDR Pressure Rated Tube (velocity and friction-loss chart).
- Toro Ag Irrigation. Hazen-Williams Equation (roughness coefficient reference).