Crop Coefficient Calculator

This calculator implements the FAO-56 single crop coefficient method, published by the U.N. Food and Agriculture Organization as Irrigation and Drainage Paper No. 56, "Crop Evapotranspiration" — the standard approach agronomists and irrigation engineers use to translate a generic reference evapotranspiration value (ETo — how much a well-watered grass reference surface would lose to the atmosphere) into the actual water demand of a specific crop at a specific point in its life (ETc = ETo × Kc). Rather than using one fixed coefficient all season, it splits the growing season into four stages — initial, development, mid-season, and late season, the same staging FAO-56's own Kc-ini/Kc-mid/Kc-end framework uses — each with its own characteristic Kc. During the initial stage, when the crop is small and most of the field is bare soil, Kc stays low and constant.

During development and late season, the calculator linearly interpolates Kc day by day between the surrounding stage values, modeling the gradual increase in canopy cover as the crop grows in, and the gradual decline as it senesces toward harvest. Mid-season holds Kc at its peak, constant value while the canopy is fully established. Enter the day number since planting and the calculator tells you exactly where you are in that curve, the current daily crop water use in both inches and millimeters, and a season-weighted average Kc used to estimate total seasonal water use. A common mixup: Kc values from FAO-56 tables are calibrated for well-watered, disease-free, optimally fertilized crops under standard conditions — if your field's canopy is thinner, stressed, or the microclimate differs meaningfully from the tables' assumptions, actual Kc may run lower, and this calculator has no way to detect that without a hands-on canopy or stress adjustment.