Irrigation Calculator
Calculate the water a garden needs and how long to run an irrigation zone.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
About this calculator
Enter the supplemental water depth needed after rainfall. The estimate converts that depth and area to gallons, adjusts for the entered system efficiency, and estimates runtime from flow rate. It does not account for soil, slope, plant type or local watering rules.
Worked example
calculated with the default values below
Irrigated area 1,000 sq ft · Water need per week 1 in · System type Drip irrigation · Zone flow rate 10 gal/min · System efficiency 90 % · Sessions per week 3 · Effective rainfall this week 0 in Net supplemental water need per week 623.0 gal · Water to pump (accounting for efficiency) 692.2 gal · Minutes per session 23.1 min · System run time per week 1.2 h · Note Drip runs long and slow — split into several sessions so water soaks in instead of running off
Method and assumptions
net depth = max(0, weekly need − effective rainfall); net gallons = area × net depth × 0.623; pumped gallons = net gallons ÷ efficiency Rainfall is only deducted at the effective amount you enter; runtime uses entered flow and efficiency. Enter the supplemental water depth needed after rainfall. The estimate converts that depth and area to gallons, adjusts for the entered system efficiency, and estimates runtime from flow rate. It does not account for soil, slope, plant type or local watering rules. Use the cited guidance and your own measurements, bills or product specifications to choose inputs. Sample defaults are not current local data.
Data sources
Use the cited guidance and your own measurements, bills or product specifications to choose inputs. Sample defaults are not current local data.
Disclaimer
These are general gardening estimates. Planting windows, growth, water and nutrient needs vary by place and plant. Check local extension guidance and product labels before acting on an estimate.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
net depth = max(0, weekly need − effective rainfall); net gallons = area × net depth × 0.623; pumped gallons = net gallons ÷ efficiency
- G_p
- Pumped water per week (gal)
- A
- Irrigated area (sq ft)
- W
- Target water per week (in)
- e
- System efficiency (%)
- Q
- Flow rate (gpm)
- s
- Sessions per week
Rainfall is only deducted at the effective amount you enter; runtime uses entered flow and efficiency.
Frequently asked questions
Does the rainfall field subtract all rain from watering needs?
It subtracts only the effective rainfall amount you enter. The calculator cannot estimate how much rain your soil or plants retain, so use a local or measured effective amount.
Which inputs and limits apply to Irrigation Calculator?
Use the units shown beside each field. These are form limits, not recommended settings. Irrigated area: 1–500,000 sq ft; Water need per week: 0.1–4 in; Zone flow rate: 0.1–500 gal/min; System efficiency: 30–100 %; Sessions per week: 1–7; Effective rainfall this week: 0–4 in net depth = max(0, weekly need − effective rainfall); net gallons = area × net depth × 0.623; pumped gallons = net gallons ÷ efficiency Rainfall is only deducted at the effective amount you enter; runtime uses entered flow and efficiency. Enter the supplemental water depth needed after rainfall. The estimate converts that depth and area to gallons, adjusts for the entered system efficiency, and estimates runtime from flow rate. It does not account for soil, slope, plant type or local watering rules. Use the cited guidance and your own measurements, bills or product specifications to choose inputs. Sample defaults are not current local data.
What is a complete worked example for Irrigated area = 1,000 sq ft; Water need per week = 1 in?
Inputs: Irrigated area = 1,000 sq ft; Water need per week = 1 in; System type = Drip irrigation; Zone flow rate = 10 gal/min; System efficiency = 90 %; Sessions per week = 3; Effective rainfall this week = 0 in. Calculation: net depth = max(0, weekly need − effective rainfall); net gallons = area × net depth × 0.623; pumped gallons = net gallons ÷ efficiency. Results: Net supplemental water need per week = 623.0 gal; Water to pump (accounting for efficiency) = 692.2 gal; Minutes per session = 23.1 min; System run time per week = 1.2 h; Note = Drip runs long and slow — split into several sessions so water soaks in instead of running off.