Irrigation Pipe Size Calculator
Estimate minimum irrigation-pipe inside diameter from flow and target velocity, plus line volume, fill time and pipe cost allowance.
Default project preview
1.28 in
Required flow area
1.28 in²
Line water volume
10 gal
Line fill time
0.5 min
Pipe length
150 ft
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Design flow
Enter this project assumption in gpm. The current imperial default is 20 gpm.
Maximum design velocity
Enter this project assumption in ft/s. The current imperial default is 5 ft/s.
Pipe length
Enter this project assumption in ft. The current imperial default is 150 ft.
Pipe price
Enter this project assumption in per ft. The current imperial default is 1.5 per ft. Minimum: 0.
How to read the outputs
Required flow area
inside cross-section. Default preview: 1.28 in².
Line water volume
at calculated diameter. Default preview: 10 gal.
Line fill time
at design flow. Default preview: 0.5 min.
Pipe length
entered run. Default preview: 150 ft.
Target velocity
sizing constraint. Default preview: 5 ft/s.
Pipe cost allowance
pipe only. Default preview: $225.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Required flow area = volumetric flow ÷ target velocity. Inside diameter = √(4 × flow area ÷ π). Line volume uses that calculated area and entered length.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Compare preliminary irrigation pipe diameters from zone flow, pipe length and acceptable velocity or pressure loss.
Calculation steps
- 1Determine design flow from simultaneously operating emitters.
- 2Enter straight length and equivalent fitting allowance.
- 3Compare velocity and friction loss by candidate diameter.
- 4Check remaining pressure at the critical outlet.
Worked numerical example
A zone flowing 15 gpm through 200 ft of pipe may have acceptable velocity in one diameter but excessive friction loss; increasing diameter can preserve pressure at sprinklers.
Purchasing and rounding
Purchase pipe and fittings only after confirming material, pressure rating, burial and valve compatibility.
Interpret the result
The result is preliminary hydraulic planning, not a complete irrigation design.
Page-specific assumptions
- Steady flow.
- One pipe material per run.
- Fitting allowance is representative.
Common mistakes
- Sizing from main pressure alone.
- Ignoring elevation change.
- Adding all zones that never run together.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
FAQ
Irrigation Pipe Size Calculator questions
Why use actual inside diameter?+
Nominal pipe sizes can have different inside diameters depending on material and wall thickness.
Does a larger pipe reduce pressure loss?+
Generally yes, because velocity and friction loss decrease for the same flow.
Does this calculator include fittings?+
No. Valves, elbows, filters and elevation add pressure requirements that need a full hydraulic check.