IrrigationForge

GPM to PSI Calculator

GPM and PSI measure different things, so they convert through a pipe: flow through a given pipe size and length costs pressure. This tool uses the Hazen-Williams equation to show the pressure you give up as friction loss.

Inputs

Result

US customary ⇄ metric
Friction loss-ft of head
Friction loss-PSI

Planning estimate - not a substitute for a pressure-tested engineering design.

Formula

Hazen-Williams: hf (ft) = 4.52 × L × Q1.85 ÷ (C1.85 × d4.87), where Q is GPM, d is inside diameter in inches, L is length in feet, C is the material roughness coefficient. PSI = ft of head ÷ 2.31.

Example

10 GPM through 100 ft of 1-inch PVC (C=150): hf ≈ 7.7 ft of head ≈ 3.4 PSI lost. The same flow in 3/4-inch pipe would cost roughly 4× more.

Assumptions

  • Steady, fully turbulent water flow in a full pipe.
  • Fittings, valves and elevation changes are not included - add fittings as equivalent pipe length.
  • C coefficients are typical published values.

Limitations

This tool is a planning aid for DIY design and pre-design work. It does not replace an on-site pressure and flow test, a catch-cup uniformity test, or local code and backflow requirements. For steep slopes, long pipe runs, large elevation changes or commercial-scale systems, have a qualified irrigation designer verify the plan before installation.

What This Means

Friction loss is why a zone that works on paper can underperform in the yard. Keep total losses (pipe + fittings + valves + elevation) well below the difference between static and required working pressure.

Next Step

Check velocity in the same pipe with the Pipe Flow Calculator, and re-measure your source with the GPM Calculator.

Related Tools

Pipe Flow Calculator GPM Calculator Irrigation Flow Calculator

Related Guides

Pipe Friction Loss Table (PSI per 100 ft)Reference table of Hazen-Williams friction loss in PSI per 100 ft for Sch 40 PVC pipe at 5-25 GPM, computed from the standard equation.

Sources

Frequently Asked Questions

Can you convert GPM to PSI directly?

Not without a pipe. GPM is a flow rate, PSI is pressure. The conversion only exists as friction loss through a specific pipe at a specific flow - which is exactly what this calculator does.

What friction loss is acceptable?

A common planning target is keeping loss in the main line under 5% of static pressure, and designing zones so total dynamic losses stay within the manufacturer's working-pressure range for your heads.