Irrigation Water Budget and Runtime Calculator

Convert a stated irrigation depth into water volume and runtime.

Inputs

Preset: 1,000 ft² irrigation

An example 1 in target with 0.25 in effective rain, 75% assumed application efficiency and 10 GPM measured flow. Set the water target for local weather, soil and plants; this is not a universal watering schedule.

Minutes - Project area (ft²), Target water depth (in), Effective rainfall (in), Application efficiency (0-1), Measured flow (GPM)
Enter project area in ft².
Enter target water depth in in.
Enter effective rainfall in in.
Enter the fraction of applied water counted toward net demand, greater than 0 and no more than 1.
Enter measured flow in GPM.
Formula, inputs and conditions

Formula: Project area*max(0,Target water depth-Effective rainfall)*(144/231)/Application efficiency/Measured flow

Decimal results are displayed to two places when needed.

Inputs
Area, target and effective-rain depths, application efficiency, and measured flow in GPM.
Outputs
Gross applied U.S. gallons after the efficiency adjustment, and runtime in minutes.
Conditions and scope
The target and effective rainfall need local or user evidence; no universal weekly watering prescription is given. Do not apply efficiency corrections twice. The calculator does not design pipe size or pressure. Results are planning estimates, not engineering, permit, code or contractor-price decisions.

Water depth becomes a volume and a runtime

An irrigation budget first subtracts effective rainfall from the target water depth, never going below zero. Area times that net depth gives the water volume. Application efficiency increases the gross volume needed, and measured system flow converts volume into runtime.

Measure and calculate

  1. Measure the irrigated area and choose a locally appropriate target water depth.
  2. Enter effective rainfall and a measured or justified application-efficiency fraction.
  3. Use the flow of the operating zone for runtime, or volume mode to inspect water demand.
Calculation method

Minutes = area × maximum(0, target depth − effective rain) × (144 ÷ 231) ÷ efficiency ÷ GPM.

A worked example

Inputs
The area is 1,000 ft², target depth 1 in, effective rain 0.25 in, efficiency 75% and zone flow 10 GPM.
Calculation
Gross volume: 1,000 × 0.75 × 144 ÷ 231 ÷ 0.75 = 623.3766 US gallons. Runtime: 623.3766 ÷ 10 = 62.3377 minutes.
Result
62.34 min

The estimate is 62.34 minutes under these assumptions. It is not a universal lawn schedule, and it does not choose how to split watering into cycles.

Read the result

Output interpretation and next steps
Output or checkWhat it means for your project
Net depth: inSubtract effective rainfall from the chosen target, with zero as the lower bound. This is the depth still needed under the water-budget assumptions.
Gross irrigation volume: US galThe tool divides the area-scaled net volume by application efficiency. This output includes the modeled efficiency loss; it is larger than net volume when efficiency is below 1.
Runtime: minGross volume divided by the operating zone’s GPM gives runtime. The estimate does not choose a local target or split watering into cycles.

Common questions

What if rainfall exceeds the target?

The modeled net irrigation demand is zero. This tool does not return a negative watering time.

Can I enter water pressure instead of flow?

No. Runtime needs volume per minute for the operating zone. Pressure and flow are different measurements.