---
title: "Evapotranspiration Calculator: Stage-by-Stage Crop Water Demand Using Real Kc Values"
canonical: "https://theyieldgrid.com/evapotranspiration-calculator/"
model_id: "tyg-792"
model_version: "1.0.0"
last_reviewed: "2026-08-24T08:27:16"
reviewed_by: "Umer Hayiat"
---

# Evapotranspiration Calculator: Stage-by-Stage Crop Water Demand Using Real Kc Values

> Canonical calculator: [https://theyieldgrid.com/evapotranspiration-calculator/](https://theyieldgrid.com/evapotranspiration-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Evapotranspiration Calculator: Stage-by-Stage Crop Water Demand Using Real Kc Values A flat weekly irrigation schedule treats a tiny seedling the same as a fully canopied, fruiting plant. That mismatch is not a minor calibration error; it is a fundamental misreading of plant biology. Evapotranspiration-based irrigation works by translating a single local weather measurement, the reference ET (ET₀), into a crop-specific water demand figure that shifts every few weeks as the plant’s canopy size, root depth, and metabolic rate change. The crop coefficient (Kc) is the bridge between those two numbers.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Reference ET (ET₀) | `etcrop_eto` | number | Inches | 0.01 to 1.5 | No |
| Crop Type | `etcrop_crop` | select |  | — Select crop — = ``; Cool Season Turf (Fescue/Bluegrass) = `cool_turf`; Warm Season Turf (Bermuda/Zoysia) = `warm_turf`; Tomatoes = `tomatoes`; Citrus = `citrus`; Field Corn = `corn`; Bell Peppers = `peppers`; Soybeans = `soybeans`; Lettuce / Leafy Greens = `lettuce` | No |
| Crop Growth Stage | `etcrop_stage` | select |  | — Select stage — = ``; Initial (seedling / early establishment) = `initial`; Mid-Season (peak canopy / flowering) = `mid`; Late Season (ripening / senescence) = `late` | No |
| Irrigation System Efficiency (%) | `etcrop_eff` | number | % | 40 to 100 | No |
| Irrigated Area (sq ft) | `etcrop_area` | number | sq ft | 1 to 10000000 | No |

## Outputs

| Output ID | Default state |
|---|---|
| `etcrop_eto_err` |  |
| `etcrop_crop_err` |  |
| `etcrop_stage_err` |  |
| `etcrop_eff_err` |  |
| `etcrop_area_err` |  |
| `etcrop_results` | Weekly Irrigation Requirement inches/week Crop Demand (ET꜀) in/day Crop Coefficient (Kc) unitless multiplier Weekly Volume — gallons/week Water Demand vs. Reference ET (percentage of baseline) 0% 50% (Low Kc) 100% (ET₀) 150%+ (High Kc) ✓ Looks Good Kc Reference Table — Your Crop & All Stages Stage Kc Value ET꜀ (in/day) Weekly Req. (in) Demand vs ET₀ Recommended Equipment for ET-Based Irrigation Rachio / Hunter Smart Wi-Fi controllers with ET scheduling Davis Instruments Pro weather stations for |
| `etcrop_out_primary` |  |
| `etcrop_out_etc` |  |
| `etcrop_out_kc` |  |
| `etcrop_out_gallons` | — |
| `etcrop_warnings_box` | ✓ Looks Good |
| `etcrop_warnings_title` | ✓ Looks Good |
| `etcrop_warnings_list` |  |

## Formula and method

This visualization demonstrates how reference weather data is scaled by dimensionless Kc multipliers to reach precise demand. Show the calculation steps Step 1: Look up the Kc value The crop coefficient is pulled from a lookup table indexed by crop type and growth stage. The values in this tool follow FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998), the internationally recognized standard for Penman-Monteith-based ET calculations. Kc is dimensionless; it has no units. Step 2: Calculate crop ET demand (ET₮) ET₮ (in/day) = ET₀ (in/day) x Kc This is the water the crop is removing from the soil and atmosphere per day, adjusted for canopy size and physiology. It is not the irrigation volume yet. Step 3: Scale to a weekly demand Weekly ET₮ (in/week) = ET₮ (in/day) x 7 This assumes ET₀ is approximately constant across the week. For more accuracy, average ET₀ over the actual forecast period. Step 4: Adjust for system efficiency Irrigation Requirement (in/week) = Weekly ET₮ / (Efficiency / 100) Dividing by the efficiency fraction inflates the required application depth to compensate for delivery losses. At 75% efficiency, you must apply 1.33 inches for every 1.00 inch of crop demand. Step 5: Convert to gallons (if area is provided) Gallons/week = Irrigation Requirement (in/week) x Area (sq ft) x 0.6233 The conversion factor 0.6233 gallons per inch-foot-squared is derived from the volumetric relationship: 1 acre-inch = 27,154 gallons, and 1 acre = 43,560 sq ft, giving 27,154 / 43,560 = 0.6233 gallons per square foot per inch. Rounding rules used in this tool: ET₮ is displayed to 3 decimal places to preserve precision. The weekly irrigation requirement is displayed to 2 decimal places. Gallons are rounded to the nearest whole number. Assumptions and Limits ET₀ must come from a Penman-Monteith calculation or a calibrated weather station reporting standardized reference ET. Using pan evaporation data without a pan coefficient adjustment will overestimate demand. The Kc values in this tool are the FAO-56 “standard” values for well-watered crops under non-stressed conditions. If your crop is already water-stressed, Kc will be lower; the tool will over-predict demand in that case. System efficiency is assumed uniform across the entire irrigated area. In zones with significant slope, pressure variation, or clogged emitters, actual distribution uniformity will differ from the entered value. Consider a soil infiltration rate check if runoff is observed before the run cycle ends. This tool does not subtract rainfall or soil water credits. If significant rain has occurred, the calculated irrigation requirement will overstate your actual need for that week. The Kc stage boundaries (Initial, Mid-Season, Late Season) are user-defined. In practice, stage transitions are gradual. The tool assumes you have correctly identified the dominant stage at the time of calculation. Gallons output assumes that the entire entered area is served by the same crop type, stage, and system efficiency. Mixed-use zones should be calculated separately. This calculator is a planning and scheduling tool. It does not replace a licensed irrigation designer for system sizing, hydraulics, or large-scale agricultural design work.

## Verified worked examples

### Example 1: Cool Season Turf in Peak Summer

Reference ET₀: 0.28 in/day Crop: Cool Season Turf (Fescue/Bluegrass) Stage: Mid-Season System Efficiency: 75% Area: 8,000 sq ft Kc = 0.85. ET₮ = 0.28 x 0.85 = 0.238 in/day. Weekly ET₮ = 0.238 x 7 = 1.666 in. Irrigation required = 1.666 / 0.75 = 2.221 in/week. Gallons = 2.221 x 8,000 x 0.6233 = 11,075 gal/week. Result: 2.22 inches per week, approximately 11,075 gallons This is a typical late-July demand for tall fescue in a mild inland climate. A homeowner running a rotor system at 75% efficiency needs to apply 2.22 inches of water across that 8,000 sq ft to replace what the turf is losing to the atmosphere each week.

### Example 2: Tomato Seedlings in May (The Under-Watering Trap)

Reference ET₀: 0.22 in/day Crop: Tomatoes Stage: Initial (seedling) System Efficiency: 75% Area: 2,000 sq ft Kc = 0.50. ET₮ = 0.22 x 0.50 = 0.11 in/day. Weekly ET₮ = 0.11 x 7 = 0.77 in. Irrigation required = 0.77 / 0.75 = 1.027 in/week. Gallons = 1.027 x 2,000 x 0.6233 = 1,281 gal/week. Result: 1.03 inches per week, approximately 1,281 gallons Early-season tomatoes have a small root mass and minimal canopy. Applying turf-level schedules at this stage saturates the soil around undeveloped roots, which is a common cause of early-season root rot and stunted establishment.

### Example 3: Peak-Season Tomatoes Switched to Drip

Reference ET₀: 0.35 in/day Crop: Tomatoes Stage: Mid-Season (fruiting) System Efficiency: 90% (drip) Area: 2,000 sq ft Kc = 1.15. ET₮ = 0.35 x 1.15 = 0.4025 in/day. Weekly ET₮ = 0.4025 x 7 = 2.818 in. Irrigation required = 2.818 / 0.90 = 3.131 in/week. Gallons = 3.131 x 2,000 x 0.6233 = 3,904 gal/week. Result: 3.13 inches per week, approximately 3,904 gallons The same 2,000 sq ft plot that needed 1,281 gallons in May now demands more than 3,900 gallons in August. This is the Kc biology in action: a mature, fruiting tomato plant has a fully developed canopy transpiring at a rate 15% above the reference grass. Missing this upward shift is the single most common cause of late-season blossom drop and yield loss from water stress.

## Assumptions

This visualization demonstrates how reference weather data is scaled by dimensionless Kc multipliers to reach precise demand. Show the calculation steps Step 1: Look up the Kc value The crop coefficient is pulled from a lookup table indexed by crop type and growth stage. The values in this tool follow FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998), the internationally recognized standard for Penman-Monteith-based ET calculations. Kc is dimensionless; it has no units. Step 2: Calculate crop ET demand (ET₮) ET₮ (in/day) = ET₀ (in/day) x Kc This is the water the crop is removing from the soil and atmosphere per day, adjusted for canopy size and physiology. It is not the irrigation volume yet. Step 3: Scale to a weekly demand Weekly ET₮ (in/week) = ET₮ (in/day) x 7 This assumes ET₀ is approximately constant across the week. For more accuracy, average ET₀ over the actual forecast period. Step 4: Adjust for system efficiency Irrigation Requirement (in/week) = Weekly ET₮ / (Efficiency / 100) Dividing by the efficiency fraction inflates the required application depth to compensate for delivery losses. At 75% efficiency, you must apply 1.33 inches for every 1.00 inch of crop demand. Step 5: Convert to gallons (if area is provided) Gallons/week = Irrigation Requirement (in/week) x Area (sq ft) x 0.6233 The conversion factor 0.6233 gallons per inch-foot-squared is derived from the volumetric relationship: 1 acre-inch = 27,154 gallons, and 1 acre = 43,560 sq ft, giving 27,154 / 43,560 = 0.6233 gallons per square foot per inch. Rounding rules used in this tool: ET₮ is displayed to 3 decimal places to preserve precision. The weekly irrigation requirement is displayed to 2 decimal places. Gallons are rounded to the nearest whole number. Assumptions and Limits ET₀ must come from a Penman-Monteith calculation or a calibrated weather station reporting standardized reference ET. Using pan evaporation data without a pan coefficient adjustment will overestimate demand. The Kc values in this tool are the FAO-56 “standard” values for well-watered crops under non-stressed conditions. If your crop is already water-stressed, Kc will be lower; the tool will over-predict demand in that case. System efficiency is assumed uniform across the entire irrigated area. In zones with significant slope, pressure variation, or clogged emitters, actual distribution uniformity will differ from the entered value. Consider a soil infiltration rate check if runoff is observed before the run cycle ends. This tool does not subtract rainfall or soil water credits. If significant rain has occurred, the calculated irrigation requirement will overstate your actual need for that week. The Kc stage boundaries (Initial, Mid-Season, Late Season) are user-defined. In practice, stage transitions are gradual. The tool assumes you have correctly identified the dominant stage at the time of calculation. Gallons output assumes that the entire entered area is served by the same crop type, stage, and system efficiency. Mixed-use zones should be calculated separately. This calculator is a planning and scheduling tool. It does not replace a licensed irrigation designer for system sizing, hydraulics, or large-scale agricultural design work. ET₀ must come from a Penman-Monteith calculation or a calibrated weather station reporting standardized reference ET. Using pan evaporation data without a pan coefficient adjustment will overestimate demand. The Kc values in this tool are the FAO-56 “standard” values for well-watered crops under non-stressed conditions. If your crop is already water-stressed, Kc will be lower; the tool will over-predict demand in that case. System efficiency is assumed uniform across the entire irrigated area. In zones with significant slope, pressure variation, or clogged emitters, actual distribution uniformity will differ from the entered value. Consider a soil infiltration rate check if runoff is observed before the run cycle ends. This tool does not subtract rainfall or soil water credits. If significant rain has occurred, the calculated irrigation requirement will overstate your actual need for that week. The Kc stage boundaries (Initial, Mid-Season, Late Season) are user-defined. In practice, stage transitions are gradual. The tool assumes you have correctly identified the dominant stage at the time of calculation. Gallons output assumes that the entire entered area is served by the same crop type, stage, and system efficiency. Mixed-use zones should be calculated separately. This calculator is a planning and scheduling tool. It does not replace a licensed irrigation designer for system sizing, hydraulics, or large-scale agricultural design work. Critical Warnings The single-schedule trap: Applying one fixed weekly irrigation depth from planting through harvest ignores the fact that Kc can shift by a factor of 2 or more across a crop’s life. A tomato plant’s Kc moves from 0.50 at seedling stage to 1.15 at peak fruiting. Irrigating at mid-season rates during the initial stage will over-water young root systems; under-irrigating at peak canopy causes fruit stress and yield loss. Update your schedule every three to four weeks for annual crops. Efficiency inflation: Growers and landscapers frequently enter system efficiency values that reflect design intent rather than measured reality. An aging rotor system rated at 80% may deliver distribution uniformity closer to 60% due to head misalignment, pressure variation, and nozzle wear. If you are entering a number you have not verified with a catch-can audit, treat your result as a minimum volume estimate, not a precise target. Use the catch-can test calculator to get a real field number before relying on this output for a programmed schedule. High ET₀ with overhead irrigation: When ET₀ exceeds 0.40 in/day, the atmospheric vapor pressure deficit is high and wind speeds are often elevated. Under these conditions, overhead sprinkler systems lose a larger fraction of applied water to direct evaporation before it reaches the soil. The calculated irrigation requirement does not model this additional atmospheric loss. Irrigating in the early morning reduces this effect. Late-season under-watering: Many growers reduce irrigation aggressively in late season, assuming senescence means low demand. For crops like tomatoes (late Kc = 0.80) and bell peppers (late Kc = 0.90), demand remains substantial through ripening. Premature reduction causes blossom end rot in tomatoes and tip burn in peppers. Minimum Standards ET₀ inputs below 0.05 in/day should trigger a source verification check; values that low are unusual except in coastal, foggy, or winter conditions. System efficiency inputs below 60% indicate a system that needs redesign or rehabilitation before ET-based scheduling is meaningful; the irrigation requirement will be so inflated by losses that the calculation loses practical precision. For commercial vegetable production, updating the growth stage designation at least three times per crop cycle (at transplant, at canopy closure, and at the start of fruit ripening) is the minimum standard for ET-based irrigation management. Competitor Trap: Most ET calculators online provide a single Kc number for a crop species and call it done. That approach is only accurate at peak canopy. It systematically over-waters every crop in its early stage and under-waters any crop in a high-Kc mid-season phase. The stage-specific Kc lookup in this tool is not a refinement; it is the core of what makes ET-based scheduling different from calendar-based guessing. A calculator that skips stage selection is solving the wrong problem. ET₀ inputs below 0.05 in/day should trigger a source verification check; values that low are unusual except in coastal, foggy, or winter conditions. System efficiency inputs below 60% indicate a system that needs redesign or rehabilitation before ET-based scheduling is meaningful; the irrigation requirement will be so inflated by losses that the calculation loses practical precision. For commercial vegetable production, updating the growth stage designation at least three times per crop cycle (at transplant, at canopy closure, and at the start of fruit ripening) is the minimum standard for ET-based irrigation management. Competitor Trap: Most ET calculators online provide a single Kc number for a crop species and call it done. That approach is only accurate at peak canopy. It systematically over-waters every crop in its early stage and under-waters any crop in a high-Kc mid-season phase. The stage-specific Kc lookup in this tool is not a refinement; it is the core of what makes ET-based scheduling different from calendar-based guessing. A calculator that skips stage selection is solving the wrong problem.

## Limitations and safety

ET₀ must come from a Penman-Monteith calculation or a calibrated weather station reporting standardized reference ET. Using pan evaporation data without a pan coefficient adjustment will overestimate demand. The Kc values in this tool are the FAO-56 “standard” values for well-watered crops under non-stressed conditions. If your crop is already water-stressed, Kc will be lower; the tool will over-predict demand in that case. System efficiency is assumed uniform across the entire irrigated area. In zones with significant slope, pressure variation, or clogged emitters, actual distribution uniformity will differ from the entered value. Consider a soil infiltration rate check if runoff is observed before the run cycle ends. This tool does not subtract rainfall or soil water credits. If significant rain has occurred, the calculated irrigation requirement will overstate your actual need for that week. The Kc stage boundaries (Initial, Mid-Season, Late Season) are user-defined. In practice, stage transitions are gradual. The tool assumes you have correctly identified the dominant stage at the time of calculation. Gallons output assumes that the entire entered area is served by the same crop type, stage, and system efficiency. Mixed-use zones should be calculated separately. This calculator is a planning and scheduling tool. It does not replace a licensed irrigation designer for system sizing, hydraulics, or large-scale agricultural design work. Critical Warnings The single-schedule trap: Applying one fixed weekly irrigation depth from planting through harvest ignores the fact that Kc can shift by a factor of 2 or more across a crop’s life. A tomato plant’s Kc moves from 0.50 at seedling stage to 1.15 at peak fruiting. Irrigating at mid-season rates during the initial stage will over-water young root systems; under-irrigating at peak canopy causes fruit stress and yield loss. Update your schedule every three to four weeks for annual crops. Efficiency inflation: Growers and landscapers frequently enter system efficiency values that reflect design intent rather than measured reality. An aging rotor system rated at 80% may deliver distribution uniformity closer to 60% due to head misalignment, pressure variation, and nozzle wear. If you are entering a number you have not verified with a catch-can audit, treat your result as a minimum volume estimate, not a precise target. Use the catch-can test calculator to get a real field number before relying on this output for a programmed schedule. High ET₀ with overhead irrigation: When ET₀ exceeds 0.40 in/day, the atmospheric vapor pressure deficit is high and wind speeds are often elevated. Under these conditions, overhead sprinkler systems lose a larger fraction of applied water to direct evaporation before it reaches the soil. The calculated irrigation requirement does not model this additional atmospheric loss. Irrigating in the early morning reduces this effect. Late-season under-watering: Many growers reduce irrigation aggressively in late season, assuming senescence means low demand. For crops like tomatoes (late Kc = 0.80) and bell peppers (late Kc = 0.90), demand remains substantial through ripening. Premature reduction causes blossom end rot in tomatoes and tip burn in peppers. Minimum Standards ET₀ inputs below 0.05 in/day should trigger a source verification check; values that low are unusual except in coastal, foggy, or winter conditions. System efficiency inputs below 60% indicate a system that needs redesign or rehabilitation before ET-based scheduling is meaningful; the irrigation requirement will be so inflated by losses that the calculation loses practical precision. For commercial vegetable production, updating the growth stage designation at least three times per crop cycle (at transplant, at canopy closure, and at the start of fruit ripening) is the minimum standard for ET-based irrigation management. Competitor Trap: Most ET calculators online provide a single Kc number for a crop species and call it done. That approach is only accurate at peak canopy. It systematically over-waters every crop in its early stage and under-waters any crop in a high-Kc mid-season phase. The stage-specific Kc lookup in this tool is not a refinement; it is the core of what makes ET-based scheduling different from calendar-based guessing. A calculator that skips stage selection is solving the wrong problem.

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## Provenance

- Model ID: `tyg-792`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-08-24T08:27:16
- Runtime SHA-256: `d1164cfc0a4f15a93d5eccb262486b13a08600da0a2cb4172c9dd40c40a9f112`

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