Irrigation Run Time Calculator

Turn GPM and head spacing into a precipitation rate, then efficiency-adjusted sprinkler run-time minutes per zone, per day, and split into cycle-and-soak. No email wall.

Precipitation rate

Sprinkler type

Seeds a starting precipitation rate — you can always override it.

Precipitation rate

Inches of water applied per hour. Auto-filled from flow + spacing, or read it off your nozzle chart. Spray 1.5–3.0, rotary/MP 0.4–0.8, rotor 0.2–0.8 in/hr.

preset

in/hr mm/hr

Don't know your precipitation rate? Calculate it from flow + spacing

Zone flow (GPM)

Total flow of all heads on this zone, in gallons per minute. Optional — only needed to derive the precipitation rate.

Head spacing (S)

Row spacing (L)

ft m

Spacing between heads (S) and between rows (L). Triangular layout multiplies the row dimension (L) by 0.866.

Layout

Square Triangular

How much water

Target weekly depth

Inches of water per week. Most lawns want 1–1.5 in; cool-season turf up to 1.5–2.0 in in mid-summer.

in mm

Irrigation efficiency

How much of the applied water reaches the root zone — folds in uniformity, wind, and evaporation. 70–80% is typical.

When you water

Watering days per week

Cycles per watering

Split each day's run into cycles with a soak between — prevents runoff on clay or slopes.

Run time per watering day

—

Schedule

Schedule	Value

Export

Reference only — verify against the nozzle chart; not a compliance record.

PR = 96.3 × GPM / (S × L) → minutes = (depth ÷ PR × 60) ÷ eff ÷ days ÷ cycles How?

How this is calculated

This tool chains two answers an operator needs at the controller. First it derives a precipitation rate (inches of water applied per hour) from your zone flow and head grid, then it turns a weekly target depth into run-time minutes per day.

Step 1 — precipitation rate (skip if you type the PR):
  L_effective = (triangular) ? L × 0.866 : L     (0.866 to the ROW dimension only)
  PR (in/hr)  = 96.3 × GPM / (S × L_effective)

Step 2 — base run-time for the weekly depth:
  baseMinutes   = (targetDepth / PR) × 60

Step 3 — adjust for irrigation efficiency:
  weeklyMinutes = baseMinutes / (efficiency / 100)

Step 4 — split across watering days:
  minutesPerDay = weeklyMinutes / daysPerWeek

Step 5 — cycle-and-soak (optional, anti-runoff):
  minutesPerCycle = minutesPerDay / cycles

Worked example (Rain Bird): a spray zone at 2.4 GPM on an 11 ft × 12 ft square grid gives PR = 96.3 × 2.4 / (11 × 12) ≈ 1.75 in/hr. For 1.5 in/week at 75% efficiency over 3 days, that is ≈ 68.5 min/week → 22.8 min/day, run as two ~11.4-min cycles with a soak between. The 0.866 triangular factor multiplies the row spacing (L) only — applying it to both S and L is the most common error.

Per-type starting rates (spray 1.5, rotary/MP 0.4, rotor 0.5, drip 0.4 in/hr) come from Smart Irrigation (Hunter) and the Sprinkler System Calculator presets; they pre-fill the editable field but a derived or nozzle-chart value always overrides. Runoff and cycle-and-soak guidance is advisory — soil, slope, and weather vary.

Formula: PR = 96.3 × GPM / (S × L) → minutes = (depth ÷ PR × 60) ÷ eff ÷ days ÷ cycles

See Sources below

Sources

Irrigation Design Tip: Calculating Precipitation Rates. Rain Bird. Retrieved Jun 20, 2026.
How To Calculate Precipitation Rate. Sprinkler Warehouse (Sprinkler School). Retrieved Jun 20, 2026.
Zone Run Time. Sprinkler System Calculator. Retrieved Jun 20, 2026.
Sprinkler Watering Times & Schedule. Smart Irrigation (Hunter). Retrieved Jun 20, 2026.

How we choose and check our formulas