How much water does your crop need this week?
Calculated with the weather forecast for your location and with FAO-56, the method the FairGrow platform also uses.
Unless you choose, we estimate your location from your internet connection. That gives a city, not an address, and we do not store anything extra for it. “My precise location” asks your browser for permission and uses your location only for this calculation.
Pick the closest match. You can adjust everything afterward.
From an answer to a rule that waters by itself
The toolbox calculates for one week. The platform recalculates every day, with your sensor and the forecast included, and only waters when the soil has run dry and no rain is coming.
Your automation in YAML
Paste this into Automations on the platform. The engine links the example names to your own sensor and valve and shows you what it links before anything starts running. The format is the Home Assistant format, so you can also bring it over from there.
Water needs differ by growth stage
A young plant drinks little, a flowering plant the most, and during ripening the need drops again. Click a stage.
Your soil decides how often, not how much
How much water your crop uses depends on the weather and the crop. Your soil decides how much of that it can buffer, and therefore how often you need to water and how much each time.
Sand, light soil
Holds little water, about 50 to 110 mm per meter of root zone, and nutrients leach out easily.
So: small amounts, often.
Loam, mixed soil
Holds water well and still lets excess water drain. Forgives most watering mistakes.
So: medium amounts, regularly.
Clay, heavy soil
Can store a lot of water, but absorbs it slowly. Too much at once causes puddles and a lack of oxygen at the roots.
So: larger amounts, less often, and apply slowly.
Not sure what soil you have? Two tests
The ribbon test
Moisten a handful of soil, knead it and push it up between your thumb and index finger into a ribbon.
- Falls apart, feels gritty: sand.
- Short, brittle ribbon under 2.5 cm: loam.
- Long, flexible and sticky ribbon over 5 cm: clay.
The jar test
Fill a glass jar one third with soil, top it up with water and a drop of dish soap, shake and let it stand for 48 hours. The sand settles at the bottom, silt above it, clay on top. The layer thicknesses give the ratio.
How the toolbox calculates
Every step can be checked. Where we estimate, we say so.
- The weather for your location. Reference evapotranspiration ETo (FAO Penman-Monteith), rain and temperature for the next seven days, from the DWD ICON model via our own weather server. For a monthly choice: the 2025 averages at your location from ERA5. [1] [10] [11]
- The crop. Crop evapotranspiration ETc = ETo × Kc, with Kc per growth stage from FAO-56 Table 12. Outdoors we adjust Kc for wind and humidity (Eq. 62). [1]
- Under glass. A greenhouse lets through about 70% of the sunlight, so we calculate with 0.7 × the outdoor ETo. With substrate, 20 to 30% drain is added. [6] [7]
- The rain. Not all rain counts. A shower smaller than 0.2 × ETo evaporates from the soil the same day (FAO-56, chapter 8). For an average month we use the FAO/AGLW formula from CROPWAT. [1] [12]
- The soil. Total available water TAW and readily available water RAW from FAO-56 Eq. 82/83 and Table 19. With drip irrigation only the wetted part counts (Table 20). The roots grow along with the stage. [1] [9]
- Salinity and heat. Yield loss from salt according to Maas and Hoffman, degree days according to McMaster and Wilhelm. [3] [4]
All sources
- Allen, R.G., Pereira, L.S., Raes, D. & Smith, M. (1998). Crop evapotranspiration: guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56. FAO, Rome. fao.org/4/x0490e. Eq. 6, 62, 82/83 and Table 12, 19, 20, 22.
- Monteith, J.L. (1965). Evaporation and environment. Symposia of the Society for Experimental Biology, 19, 205-234.
- Maas, E.V. & Hoffman, G.J. (1977). Crop salt tolerance: current assessment. Journal of the Irrigation and Drainage Division (ASCE), 103(IR2), 115-134.
- McMaster, G.S. & Wilhelm, W.W. (1997). Growing degree-days: one equation, two interpretations. Agricultural and Forest Meteorology, 87(4), 291-300. doi:10.1016/S0168-1923(97)00027-0
- Meier, U. (ed.) (2001). Growth stages of mono- and dicotyledonous plants. BBCH Monograph, 2nd edition. doi:10.5073/20180906-074619
- Stanghellini, C. (1987). Transpiration of greenhouse crops: an aid to climate management. PhD thesis, Wageningen Agricultural University. doi:10.18174/202121
- Katsoulas, N. & Stanghellini, C. (2019). Modelling crop transpiration in greenhouses: different models for different applications. Agronomy, 9(7), 392. doi:10.3390/agronomy9070392
- Ayers, R.S. & Westcot, D.W. (1985). Water quality for agriculture. FAO Irrigation and Drainage Paper 29 Rev.1, Rome.
- Saxton, K.E. & Rawls, W.J. (2006). Soil water characteristic estimates by texture and organic matter for hydrologic solutions. Soil Science Society of America Journal, 70(5), 1569-1578. doi:10.2136/sssaj2005.0117
- Deutscher Wetterdienst, ICON model, via Open-Meteo (open-meteo.com), on FairGrow’s own server.
- Hersbach, H. et al. (2020). The ERA5 global reanalysis. Quarterly Journal of the Royal Meteorological Society, 146(730), 1999-2049. doi:10.1002/qj.3803
- Smith, M. (1992). CROPWAT: a computer program for irrigation planning and management. FAO Irrigation and Drainage Paper 46, Rome. Effective rainfall according to the FAO/AGLW formula.
Honest about the limits
Roses, ornamentals, herbs and seedlings are not in FAO-56; their rooting depth and salt tolerance are well-founded estimates from related crops. The weather forecast applies to a grid cell of a few kilometers, not to your plot, and the toolbox does not know how wet your soil is right now. So use the result as a starting point. A soil moisture sensor measures what is actually happening.
Frequently asked questions
What is the Growers Toolbox?
A calculator that tells you how much water your crop needs this week, how often to water and how much each time. It calculates with the weather forecast for your location and with FAO-56, the same method as the FairGrow platform. You do not need an account.
How does the toolbox know where I am?
As long as you do not choose anything yourself, we estimate your location from your internet connection. That gives a city, not an address, and we do not store anything extra for it. If it is wrong, search for your town or let your browser share your precise location. We use it only for the calculation.
Why does the toolbox sometimes say I do not need to water?
Outdoors, the expected rain is included. If more rain falls this week than your crop evaporates, watering is only needed if your soil is already dry now. The toolbox cannot see that; a soil moisture sensor can.
Why does the toolbox calculate differently under glass?
In a greenhouse less sunlight gets in and it does not rain. So we calculate with about 70% of the outdoor evapotranspiration. If you grow on substrate, you give several small amounts a day with 20 to 30% drain, and the toolbox takes that into account too.
How accurate is the result?
It is a well-founded starting point, not a measurement. The forecast applies to an area of a few kilometers, and rooting depth and soil differ from plot to plot. The page shows for each step which source we use and where we estimate.
Can FairGrow do this automatically?
Yes. In Automations the same calculation runs every day with your sensor and the forecast included. A rule only waters when the soil has run dry and no rain is coming. The YAML above is the start of that.