How Long Should I Irrigate?

What this calculator determines

Estimated irrigation runtime to apply a target water depth over a given area, or the application rate from emitter spacing and flow.

Formula or calculation method

Area and depth mode

• Net gallons = acres × target depth (inches) × 27,154
• Net gallons = square feet × target depth (inches) × 0.623
• Gross gallons = net gallons ÷ (efficiency % ÷ 100)
• Runtime (hours) = gross gallons ÷ system flow (gallons per hour)

Emitter layout mode

• Application rate (inches per hour) = 96.3 × GPM per emitter ÷ (emitter spacing × row spacing)

The constants 27,154 and 0.623 convert depth over area to gallons. The emitter constant 96.3 is defined in the calculator engine.

Definitions of inputs

• Target depth — Water you want to apply, in inches.
• Acres or square feet — Irrigated area (enter one).
• System flow — Total delivery rate of the irrigation system in gallons per hour.
• Efficiency % — Accounts for non-uniform distribution or line losses (1–100).
• GPM per emitter — Flow rate of a single emitter in gallons per minute.
• Emitter spacing — Distance between emitters along the row, in feet.
• Row spacing — Distance between tree rows, in feet.

Worked example

Example only — not a recommendation.

A 2-acre block needs 0.5 inches of water. A drip system delivers 120 gallons per hour at 90% efficiency:

• Net gallons = 2 × 0.5 × 27,154 = 27,154 gallons
• Gross gallons = 27,154 ÷ 0.90 = 30,171 gallons
• Runtime = 30,171 ÷ 120 ≈ 251 hours (about 10.5 days at 24 h/day, or proportionally less per day)

How to interpret the result

Use the runtime as a starting set length. Confirm with soil moisture probes, tensiometers, or shovel checks at root depth. Adjust for heat, mulch, tree size, and soil type.

References

Standard gallon-per-acre-inch conversion factors used in the engine (27,154 gal/acre-inch; 0.623 gal/sq ft-inch).