Drip irrigation calculator: water per plant and run time
A drip system is only as good as its schedule: run it too short and the root zone dries, too long and water and fertilizer drain away. This drip irrigation calculator works out how many litres each plant needs per day and how long to run the drip, and sizes the system — number of plants, drippers or inline emitters, metres of lateral pipe, total flow in litres per hour and how many zones your pump can supply at once.
Water demand comes from open-pan evaporation with a pan factor, as in the formula used by Tamil Nadu Agricultural University, or from a reference evapotranspiration (ET₀) value. Fields can be in acres, hectares, bigha, guntha or other local units.
How to use the drip irrigation calculator
- Enter the field area, the row spacing (one lateral per row) and the plant spacing in the row.
- Choose Drippers per plant for online drippers, or Inline drip line and enter the emitter spacing. Enter the emitter discharge in L/h.
- Enter pan evaporation and pan factor, or switch to ET₀. Set the crop factor (1 for fully grown plants) and the wetted area fraction.
- Keep drip efficiency at 90% or set 100% to match the TNAU formula exactly.
- Add your pump discharge to see how many zones (shifts) the field needs and the total pumping time.
Drip Irrigation Calculator features
Litres per plant per day
Pan evaporation × pan factor × crop factor × area per plant × wetted fraction ÷ efficiency.
Run time
Water per plant ÷ (drippers per plant × discharge), shown in hours and minutes.
System bill of quantities
Plants, drippers or emitters, lateral metres and drip line rolls.
Flow and zones
Total flow in L/h, m³/h and L/s, application rate in mm/h, and the zones your pump can run.
Inline or online drippers
Works for drip line with fixed emitter spacing and for drippers placed at each plant.
Drip irrigation calculation formula
Water requirement (L/plant/day) = Ep × Kp × Kc × A × Wp, where Ep is open-pan evaporation (mm/day), Kp the pan factor (about 0.7), Kc the crop factor, A the area per plant (row spacing × plant spacing, m²) and Wp the fraction of that area wetted. Run time (h) = water requirement ÷ (drippers per plant × dripper discharge in L/h).
- Rows 1.5 m apart and plants every 0.6 m: A = 0.9 m².
- With 6 mm pan evaporation, Kp 0.7, Kc 1 and 80% wetting: 6 × 0.7 × 1 × 0.9 × 0.8 = 3.02 litres per plant per day.
- Two 4 L/h drippers give 8 L/h, so run time = 3.02 ÷ 8 = 0.38 h, about 23 minutes (25 minutes at 90% efficiency).
- One acre holds 4,047 ÷ 0.9 ≈ 4,497 plants, 8,993 drippers and about 2,698 m of lateral; all running together they draw about 36 m³/h (10 L/s), so a 5 L/s pump needs two zones.
Choosing the inputs
Pan factor, crop factor and wetted fraction follow Tamil Nadu Agricultural University’s drip guidance; efficiency is from FAO’s irrigation scheduling manual. For crop-stage water use with FAO-56 coefficients, use the irrigation water calculator; for plants per acre at other layouts, the plant spacing calculator.
| Input | Typical values | Notes |
|---|---|---|
| Pan factor Kp | 0.6–0.8 | Varies with the season; TNAU uses 0.7 as the default. |
| Crop factor Kc | 0.3–1.2 | 1 for fully grown plants; young plants use much less. See FAO-56 values in the irrigation water calculator. |
| Wetted area fraction | 0.3–0.8 | 0.3 for widely spaced trees, 0.7–0.8 for closely spaced row crops. |
| Emitter discharge | 1–8 L/h | Choose a rate the soil can absorb without puddling or runoff. |
| Drip efficiency | 90% | FAO indicative field application efficiency for drip irrigation. |
Practical tips
- Split long run times into two sessions on sandy soil so water stays in the root zone.
- Check emitter flow at the start and end of laterals once a season; clogged or uneven emitters change the real run time.
- Fertigation needs the system to run long enough to flush the fertilizer from the lines — plan it into the schedule.
Drip Irrigation Calculator FAQ
How do you calculate drip irrigation water requirement?
Multiply pan evaporation (mm/day) × pan factor × crop factor × area per plant (m²) × wetted area fraction. The result is litres per plant per day.
How long should I run drip irrigation?
Divide the litres each plant needs by the litres per hour it receives. 3 litres with two 4 L/h drippers is about 23 minutes a day; adjust as the weather and crop change.
How many drippers per acre do I need?
Plants per acre × drippers per plant. At 1.5 × 0.6 m spacing there are about 4,500 plants per acre, so two drippers each means about 9,000 drippers.
How much lateral pipe is needed per acre?
Area ÷ row spacing. With laterals 1.5 m apart, one acre needs about 2,700 m of lateral.
How do I calculate drip line flow (LPH)?
Total emitters × discharge per emitter. 9,000 drippers × 4 L/h = 36,000 L/h (36 m³/h).
What if my pump is smaller than the system flow?
Divide the field into zones and irrigate them one after another. The calculator shows the number of zones and the total pumping time.