Irrigation Run Time Calculator
How long should you run each zone? Start from the water your plants need for the week, subtract rainfall, then divide by what your system actually delivers. This tool does that math end to end.
Inputs
Result
Planning estimate - not a substitute for a pressure-tested engineering design.
Formula
Gallons = net inches × area (sq ft) × 0.6233 ÷ efficiency. (One inch of water over one square foot is 0.6233 US gallons.) Run time per week (min) = gallons ÷ GPM; per session = weekly minutes ÷ watering days.
Example
1 inch target, 10 GPM zone, 1,000 sq ft, 75% efficiency: 1 × 1000 × 0.6233 ÷ 0.75 ≈ 831 gal/week; 831 ÷ 10 ≈ 83 min/week → about 28 minutes per session over 3 days.
Assumptions
- Even application across the zone; efficiency captures drift, overlap gaps and evaporation.
- Plant targets are weekly references - local ET rates refine them (planned integration).
- Soils that absorb slowly should get the weekly total split into more, shorter sessions.
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
Efficiency is why run times are longer than the raw math suggests: a 75%-efficient spray zone must apply about 1.33 inches to deliver one. Splitting sessions (cycle-and-soak) reduces runoff on slopes and clay.
Next Step
Track what this schedule costs in water with the Water Use Calculator, and set a weekly ceiling with the Water Budget Calculator.
Related Tools
Related Guides
Sources
Frequently Asked Questions
How long should I run sprinklers per session?
From the target inches and your measured application rate. For a typical spray zone at 1.5 in/hr applying 0.33 inch per session, that is about 13 minutes - longer for rotaries and drip. Use the numbers, not a fixed rule.
Why divide watering across several days?
Multiple shorter sessions let soil absorb water instead of shedding it as runoff, especially on slopes and compacted or clay soils.
Will this tool use weather data?
The plan is to integrate reference ET (evapotranspiration) data so targets update with real weather, following the Irrigation Association's ASCE-standardized ET resources.