Turn rainfall into smart irrigation reductions for gardens. Adjust schedules using effective rain and efficiency. Avoid overwatering, keep soil balanced, and grow healthier plants.
| Rain (mm) | Effective (%) | Target (mm) | Area (m²) | Efficiency (%) | Rate (mm/hr) | Gross Need (mm) | Apply (L) | Saved (L) |
|---|---|---|---|---|---|---|---|---|
| 12 | 80 | 25 | 60 | 70 | 20 | 22.29 | 1,337 | 806 |
| 5 | 60 | 18 | 35 | 85 | 12 | 16.24 | 568 | 173 |
| 0 | 80 | 20 | 40 | 75 | 15 | 26.67 | 1,067 | 0 |
Rain that reaches the root zone can replace part of a planned irrigation depth. This calculator separates measured rainfall from effective rainfall, representing the portion that infiltrates and stays available. Runoff, canopy interception, and puddling reduce usefulness, especially during short, intense storms. Matching the rainfall period to your irrigation period keeps the comparison valid and avoids overwatering.
Start with a target depth for the same window you measured rainfall, such as a day, three days, or a week. Targets can come from local evapotranspiration reports, crop coefficients, or past experience with your soil. Shallow-rooted seedlings often need smaller, more frequent depths, while established shrubs may prefer deeper, less frequent watering. The calculator treats the target as a total depth to replace for that window.
Use higher effective factors for light, steady rain on mulched beds and level ground. Use lower factors for compacted soil, clay that seals, steep slopes, or heavy downpours where most water runs off. After dry spells, the first rain may soak in well, but later rain can shed if the soil becomes saturated. Adjust the factor when you change mulch, cultivation, or drainage.
Irrigation systems rarely deliver the full applied depth to the plant. By dividing the net need by system efficiency, the calculator estimates gross depth at emitters so the root zone receives the intended amount. Typical efficiency is higher for drip and lower for wind-affected sprinklers. If distribution is uneven, use a conservative efficiency until you perform a catch-can test.
Depth helps planning, while liters and minutes help controllers and pumps. One millimeter over one square meter equals one liter, so area converts gross depth into volume for each zone. If you enter an application rate, runtime is calculated to show minutes to apply and minutes saved. Review soil moisture after watering and split long runtimes into cycles to improve infiltration accurately.
A rainfall offset is the portion of your planned irrigation that rain already supplied. The calculator estimates effective rainfall, subtracts it from the target depth, then converts the remaining need into applied depth and liters.
Start with 50–70% for heavy clay or slopes, 70–90% for mulched beds, and lower it for intense storms. Refine it by observing runoff, puddling, and soil moisture the next day.
Efficiency represents losses from drift, evaporation, non-uniform coverage, and leaks. Lower efficiency means more gross depth must be applied to deliver the same net depth to the root zone.
Yes. Enter an efficiency that matches your method. Drip is typically higher, sprinklers lower in wind, and hand watering varies. The depth-to-liters conversion still works for any delivery method.
Enter an application rate in mm per hour. The calculator converts the required gross depth into minutes, showing both the new runtime and the minutes saved compared with no rainfall.
If the rain was very light, highly intercepted by foliage, or the soil surface dried quickly under heat and wind, effective rainfall may be near zero. Always verify with a finger test or probe before skipping irrigation.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.