Free Irrigation Calculator
Enter your field's location, crop, and soil, and get a live 7-day watering plan: how much water your crop will use, how much rain will cover, when to irrigate, and exactly how much to apply. Built on the FAO-56 standard and real forecast data. No signup required.
The math behind the numbers
Crop water use: ETc = ETo × Kc
Open-Meteo supplies each day's reference evapotranspiration (ETo) computed with the FAO-56 Penman-Monteith equation. Multiplying by your crop's stage-specific coefficient (Kc) converts that into what your crop actually transpires. With a planting date we interpolate Kc through the initial, development, mid-season, and late-season stages; without one we assume peak mid-season use.
Effective rainfall
Not all rain reaches the root zone. Following the FAO rule of thumb, days with under 2 mm are ignored (lost to evaporation) and larger events are credited at 80% to account for runoff and deep percolation.
Soil water: TAW and the refill point
Total available water (TAW) is the water your soil can hold between field capacity and wilting point across your crop's root depth. Crops start to stress well before TAW is exhausted, so the refill trigger (RAW) is TAW scaled by a crop-specific management-allowed depletion: irrigate when the running deficit crosses it.
Application amount
The gross amount to apply is the refill depth divided by your system's efficiency: drip delivers about 90% of pumped water to the root zone, sprinklers about 75%, flood as little as 50%. That way the root zone actually refills instead of coming up short.
Frequently asked questions
- How does the irrigation calculator work?
- It pulls a live 7-day forecast for your coordinates from Open-Meteo, including FAO-56 Penman-Monteith reference evapotranspiration (ETo). Each day's ETo is multiplied by your crop's growth-stage coefficient (Kc) to get actual crop water use, expected rainfall is credited using the FAO effective-precipitation rule, and the refill amount is sized from your soil's water-holding capacity and your irrigation system's efficiency.
- What is ETo (reference evapotranspiration)?
- ETo is the combined evaporation and plant transpiration from a standard well-watered grass surface, computed from temperature, humidity, wind, and solar radiation. It is the weather side of the water-demand equation: multiply ETo by a crop coefficient (Kc) and you get how much water your specific crop is actually using each day.
- How accurate is the “irrigate in N days” countdown?
- The countdown assumes your soil profile starts full (at field capacity), because a one-shot calculator has no history of your field. If you irrigated or got rain recently, that is close to true; if not, you may need water sooner. The full RainRule product tracks your actual running water balance day by day, so its countdown reflects your real deficit.
- Where does the weather data come from?
- All weather comes from Open-Meteo, a free public API built on global weather models, including pre-computed FAO-56 Penman-Monteith reference evapotranspiration. No account or API key is needed.
- What is the difference between this and the full product?
- This calculator is a one-shot estimate from a full soil profile. RainRule itself tracks a running day-by-day water balance for every field you add, refreshes it hourly with fresh weather, logs your irrigation events, and emails you a digest before your crop hits water stress.
Stop recalculating. Start tracking.
RainRule runs this math for every field you have, every hour, against your field's real running water balance, and emails you before your crop hits stress. Set it up once in about two minutes.