Calculator
Example data table
| Scenario | Plant | Setup | Soil | Temp | Sun | Interval | Approx. volume |
|---|---|---|---|---|---|---|---|
| 1 | Vegetables | In-ground, 3 m² | Loamy | 30°C | 8 h | ~1.6 days | ~15–25 L/event |
| 2 | Indoor Houseplant | Pot 25×25 cm | Loamy | 22°C | 3 h | ~5–7 days | ~0.8–1.6 L/event |
| 3 | Succulent / Cactus | Pot 20×18 cm | Sandy | 26°C | 6 h | ~10–14 days | ~0.3–0.8 L/event |
| 4 | Shrubs | In-ground, 10 m² | Clay | 28°C | 6 h | ~4–7 days | ~60–120 L/event |
Formula used
- Reference demand: ET0 = 2.5 × f(temp) × f(sun) × f(humidity) × f(wind) × f(mulch) × f(setup)
- Plant demand: ETc = ET0 × Kp × f(stage)
- Soil storage: TAW = AWC × root_depth, and RAW = TAW × TDF
- Watering interval: interval_days = RAW ÷ ETc (then clamped to plant limits)
- Water per event: water_mm = (ETc × interval_days) − effective_rain
- Volume: liters = water_mm × area_m² (in-ground). For pots, liters use pot surface area and a safety cap.
How to use this calculator
- Select your units, plant type, growth stage, setup, and soil.
- Enter sun hours, average temperature, humidity, wind, and recent rain.
- Provide either garden area (in-ground) or pot dimensions (container).
- Click Calculate Watering Plan to see results above.
- Download CSV or PDF to save your plan and dates.
- Adjust interval if soil stays wet or dries too fast.
Water demand drivers
Water demand comes from daily evapotranspiration (ET). This calculator starts with an ET0 baseline of 2.5 mm/day, then multiplies by temperature, sun, humidity, wind, mulch, and container setting. Hot weeks (30–40 °C) can lift the temperature factor near 1.3, and wind can add ~15%. Dry air increases demand, while humid air reduces it. Mulch typically cuts surface loss around 10%.
Soil storage and depletion
Soil texture controls storage between events. The model uses available water capacity (AWC) of about 70 mm/m for sand, 140 mm/m for loam, and 180 mm/m for clay. To limit stress, only a fraction is treated as “readily available” using a total depletion fraction (TDF): 0.35 (sand), 0.50 (loam), and 0.60 (clay). Lower TDF protects plants during heat but shortens the interval.
Root depth and interval stability
Root depth links storage to resilience. Total available water is TAW = AWC × root depth, and readily available water is RAW = TAW × TDF. The interval is RAW ÷ ETc, where ETc is ET0 adjusted by plant coefficient (Kp) and growth stage. Stage factors used are about 1.10 (seedling), 1.00 (active), 0.95 (mature), and 0.80 (dormant), so interval targets should change across the season.
Rain credit and infiltration realism
Recent rain can replace part of irrigation. The calculator credits “effective rain” using about 70% (sand), 80% (loam), and 60% (clay) to reflect runoff and shallow wetting. The credit is capped so event depth never goes negative, reducing overwatering after storms.
Turning depth into volume and runtime
Depth becomes volume by multiplying by area: 1 mm over 1 m² equals 1 liter. For containers, surface area estimates refill and a safety cap keeps each event near 80% of pot volume; containers also apply a small demand increase to reflect faster drying. If drip flow is provided (L/h), runtime is liters ÷ flow × 60 minutes. Re-run after weather shifts, and adjust 10–20% using root-zone moisture checks.
FAQs
1) Why does the interval change so much with soil type?
Different soils store different usable water. Sand drains quickly, so RAW is smaller and intervals shorten. Loam stores more evenly. Clay can store more, but infiltration and runoff reduce how much rain actually helps.
2) How do I calibrate results to my garden?
After a deep watering, check moisture at root depth on the suggested due date. If it is still moist, extend the interval by 10–20%. If it is dry before the date, shorten the interval or increase volume modestly.
3) What if there was heavy rain yesterday?
Enter the last 48-hour rainfall so the calculator credits effective rain. If soil is saturated or puddling, delay watering until the top few centimeters dry and roots have oxygen, even if the schedule suggests sooner.
4) Can I use this for drip irrigation systems?
Yes. Add your drip flow rate (L/h) to get an estimated runtime per event. If you have multiple emitters, use their total combined flow. Always verify uniform wetting around the root zone.
5) My potting mix is not sand/loam/clay. What should I choose?
Most potting mixes behave like “loamy” for storage but can dry faster in heat. Start with loam, then adjust interval by observation. Containers also warm faster, so summer intervals are often shorter than in-ground.
6) How often should I update the inputs?
Update whenever weather shifts noticeably—such as a 5 °C temperature swing, a change in sun exposure, or a windy spell. Also update when plants move stages (seedling to active growth) or when you repot or mulch.