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Pressure Loss: What Size Supply Line Should I Use?

Choosing the right size pipe or hose for your irrigation system is critical. If the line is too small, you will lose too much pressure before the water reaches your sprinklers or drip tape, causing poor performance and uneven watering.

What Is Pressure Loss?

As water moves through a pipe or hose, it rubs against the inside walls and any fittings or bends in the line. This resistance is called friction loss, and it causes a drop in pressure as the water travels.

Two important points about friction loss:

  • In a smaller line, water must move faster to deliver the same flow (GPM), which increases friction.
  • Friction loss is not linear. If you double the flow through the same hose, the pressure loss will increase by more than double.

Why Pipe Size Matters

If your supply line is too small for the flow you are trying to push through it, you may experience:

  • Low pressure at sprinklers or drip lines.
  • Uneven coverage and dry spots in the field.
  • Extra strain on pumps and equipment.

On the other hand, using a line that is larger than necessary increases material cost without much benefit. The goal is to find the right balance between performance and cost.

Main Factors That Affect Pressure Loss

  • Flow rate (GPM): Higher flow increases friction and pressure loss.
  • Pipe or hose size: Smaller diameter = higher water velocity and more friction.
  • Length of the line: The longer the run, the more pressure you lose.
  • Fittings and valves: Elbows, tees, filters, and valves add extra resistance.
  • Elevation changes: Going uphill also reduces pressure at the outlet.

Step 1: Determine Your Required Flow

Before choosing a line size, you need to know how much water your system must carry.

  • List the devices supplied by the line (for example, sprinklers, drip tape, dripline).
  • Find the flow rate per device from the product specifications.
  • Multiply by the number of devices running at the same time to get the total GPM on that line.

Example:

  • 10 sprinklers × 4 GPM each = 40 GPM total.

Step 2: Measure the Distance

Measure or estimate the total length of the supply line from the water source (pump, valve, or main line) to the farthest sprinkler or drip zone connection. This is the distance water must travel while losing pressure.

Step 3: Use a Pressure Loss Calculator

The most accurate way to size your supply line is to use a pressure loss (friction loss) calculator. These calculators let you enter:

  • Flow rate (GPM)
  • Pipe or hose size (diameter)
  • Line length
  • Sometimes pipe material (such as PVC, poly, layflat)

The calculator will estimate how much pressure is lost from the start of the line to the end.

What to Check

  • Start with your available pressure at the source (pump or valve).
  • Subtract the pressure loss from the calculator.
  • Make sure the remaining pressure at the field is at or above the minimum pressure required for your sprinklers or drip system.

Step 4: Adjust Line Size or Flow If Needed

If the calculator shows that the pressure at the end of the line is too low, you have two options:

  • Increase the line size (use a larger diameter pipe or hose).
    – This lowers water velocity and reduces friction loss.
  • Reduce the flow rate by breaking the system into smaller zones:
    • Use valves to divide the field into sections.
    • Run fewer sprinklers or shorter lengths of drip tape at one time.

Often, a combination of slightly larger pipe and smaller zones gives the best performance.

Simple Rules of Thumb (When You Don’t Have a Calculator)

While a pressure loss calculator is always recommended, these general guidelines can help:

  • For flows over 10 GPM, consider at least 1-inch pipe.
  • For flows over 50 GPM, consider at least 2-inch pipe.
  • For flows over 100 GPM, consider at least 3-inch pipe.

For long runs (over 100 feet) or high flows, using a calculator is strongly recommended to avoid under-sizing the line.

Summary

To choose the correct supply line size:

  • Determine the total flow (GPM) of your sprinklers or drip system.
  • Measure the distance from the water source to the field.
  • Use a pressure loss calculator to check pressure at the end of the line.
  • If pressure is too low, increase line size or reduce flow by zoning with valves.

Taking the time to size your supply line correctly will help your irrigation system run efficiently, extend equipment life, and ensure even water distribution across your field.