Evapotranspiration (ET) Calculator: Know Exactly How Much Water Your Crops Lose Every Day

Introduction

Most farmers irrigate by habit. Same time every day. Same duration every week. Rain or shine, hot or cold, the pump runs on the same schedule.

The result? Overwatering in cool months. Underwatering during peak summer heat. Crops stressed at the wrong time. Water wasted at the wrong time.

The real question you should be asking every single day is this: how much water did my crop actually lose today?

That number is called Evapotranspiration. It’s the total water lost from your crop through evaporation from the soil surface and transpiration through the plant leaves.

When you know it, you can irrigate exactly what’s needed. Not more. Not less.

I built the Evapotranspiration (ET) Calculator on moralinsights.com to give every farmer access to this calculation.

It uses the same scientific methods that irrigation engineers and agronomists use worldwide. You enter your weather data, select your crop and growth stage, and the tool tells you your crop’s daily water demand and how much you need to irrigate.

No guesswork. No fixed schedules. Just science-based irrigation every day.

Why ET-Based Irrigation Changes Everything

Irrigation without ET data is like driving with your eyes closed.

You might get there safely. But you’re relying on luck, not knowledge.

ET-based irrigation is the recommended standard worldwide. FAO Irrigation and Drainage Paper 56 is the global reference for this approach. It’s used by irrigation engineers, agronomists, and water managers in every country.

Here’s what changes when you start using ET data:

• You stop overwatering. On cool or cloudy days, ET is low. Your crop doesn’t need much water. Most farmers irrigate the same amount regardless. ET data tells you when to pull back.
• You stop underwatering during heat waves. On a hot, dry, windy day, ET can exceed 8 mm per day. That’s a lot of water loss. Fixed schedules miss this surge. ET data catches it.
• You reduce your water bill. Studies consistently show 20 to 40 percent water savings when farmers switch from fixed schedules to ET-based irrigation. That’s real money saved every season.
• Your yields improve. Crops under consistent water stress produce less. Crops that are overwatered develop root diseases and poor fruit quality. ET-based irrigation keeps the crop in its optimal zone.

According to the United Nations Environment Programme (UNEP), agriculture accounts for 70 percent of all freshwater withdrawals globally. ET-based irrigation is one of the most effective tools available to reduce that number without sacrificing food production.

What Are the Four Tabs and What Does Each One Do?

The calculator is organized into four tabs. Each tab builds on the previous one.

Tab 1: Reference ET (ET0)

This tab calculates ET0. That’s the daily water loss from a reference grass surface under your specific weather conditions.

Think of it as the baseline. ET0 tells you how thirsty the atmosphere is today.

You choose from three calculation methods based on the weather data you have available.

Tab 2: Crop ET (ETc)

ET0 is for a reference grass surface. Your actual crop is different.

This tab multiplies ET0 by the Crop Coefficient (Kc) for your specific crop and growth stage. The result is ETc. That’s your crop’s actual daily water demand.

ETc = ET0 x Kc. Simple formula. Powerful result.

Tab 3: Irrigation Deficit

Knowing ETc is just the start. This tab calculates how much of that demand you actually need to irrigate.

It subtracts effective rainfall and soil moisture contribution from ETc. Then it adjusts for your irrigation system’s efficiency. It also adds a leaching requirement for saline soils.

The final output is the exact volume of water to apply per irrigation event.

Tab 4: Kc Reference Table

This is your built-in reference library. It shows FAO-56 Kc values for 18 crops across all four growth stages.

It also includes Blaney-Criddle p values by latitude and an ET0 classification guide. Keep this tab open as you work through the other three.

The Three ET0 Calculation Methods Explained Simply

Tab 1 gives you three methods to choose from. Here’s how to pick the right one for your situation.

FAO Penman-Monteith

This is the gold standard. It’s the most accurate method and the one recommended by FAO worldwide.

You need: maximum and minimum temperature, relative humidity, wind speed, elevation, solar radiation, latitude, and day of year.

If you have a weather station or access to meteorological data, use this method. The accuracy is worth the extra inputs.

Hargreaves-Samani

You only need temperature data and your location coordinates.

This is the best choice when you don’t have humidity, wind, or radiation data. It’s widely used in developing regions and smallholder farming contexts.

The accuracy is slightly lower than Penman-Monteith but very practical for field use.

Blaney-Criddle

The simplest method. You need only temperature and sunshine hours.

Use this when you have very limited data. It gives a reasonable estimate and is useful for quick planning calculations.

The Kc Reference tab includes p values by latitude that you’ll need for this method.

What Do You Need to Enter?

Tab 1 Inputs: Reference ET

• Maximum and Minimum Temperature: The day’s high and low temperatures. Supports Celsius, Fahrenheit, and Kelvin.
• Relative Humidity (Max and Min): For the Penman-Monteith method only. Most weather stations record this. Enter the daily maximum and minimum humidity percentages.
• Wind Speed: Supports m/s, km/h, mph, and knots. If you don’t have wind data, the default is 2 m/s which is a reasonable average for many agricultural regions.
• Elevation: Your farm’s altitude above sea level. Supports metres and feet. Higher elevations have lower atmospheric pressure which affects ET0.
• Solar Radiation: The amount of solar energy reaching your location. Supports MJ/m2/day, W/m2, cal/cm2/day, and kWh/m2/day. Your weather station or national meteorological service should have this.
• Latitude and Day of Year: Used to calculate extraterrestrial radiation. Latitude supports degrees and radians. Day of year is 1 for January 1st and 365 for December 31st.

Tab 2 Inputs: Crop ET

• Reference ET0: Copy the result from Tab 1, or enter a value from your weather station data.
• Field Area: Supports hectares, acres, square metres, square feet, and Guntha.
• Crop Type and Growth Stage: Select your crop and current growth stage. The tool auto-fills the FAO-56 Kc value. You can override it with your own local data.
• Effective Rainfall: Any rainfall that actually reaches the root zone. Rainfall that runs off or evaporates before reaching roots doesn’t count.

Tab 3 Inputs: Irrigation Deficit

• Crop ET (ETc): The value from Tab 2.
• Effective Rainfall and Soil Moisture: Both reduce the amount you need to irrigate.
• Irrigation System Efficiency: Drip at 90 to 95 percent. Sprinkler at 80 percent. Flood at 60 percent. This accounts for losses before water reaches the root zone.
• Leaching Requirement: For saline soils, extra water is needed to flush salts below the root zone. Choose 0 for normal soils and up to 20 percent for highly saline conditions.
• Irrigation Frequency: Daily, every 2 days, every 3 days, weekly, or every 2 weeks. The tool calculates the volume per irrigation event based on this frequency.

What Do Your Results Tell You?

ET0 Results (Tab 1)

You get ET0 in six unit formats simultaneously: mm/day, inch/day, mm/week, mm/month, m3/ha/day, and L/m2/day.

An ET0 below 2 mm/day means very low atmospheric demand. Irrigation can be minimal. An ET0 above 8 mm/day means extreme conditions. Your crops are losing water rapidly and need urgent attention.

The tool classifies your ET0 as Very Low, Low, Moderate, High, or Very High and gives you a practical tip based on that classification.

ETc Results (Tab 2)

ETc is your crop’s actual daily water demand in mm/day, inch/day, and m3/ha/day.

The net irrigation need subtracts your effective rainfall from ETc. If it rains 2 mm and your crop needs 5 mm, you only need to irrigate 3 mm.

If you enter your field area, the tool calculates the daily and weekly water volume in litres.

Irrigation Deficit Results (Tab 3)

This tab gives you the complete water balance for your field.

You see exactly how much water your crop demands, how much rainfall and soil moisture are contributing, and the net deficit after accounting for your irrigation system’s efficiency.

The volume per irrigation event is shown in litres, cubic metres, US gallons, or acre-feet. Choose the unit your pump or flow meter uses.

Weekly and monthly volumes help you plan your water source requirements ahead of time.

What Makes This Calculator Stand Out

Three Scientific Methods in One Tool

Most online ET calculators offer only one method. This tool gives you three.

Use Penman-Monteith when you have full weather data. Switch to Hargreaves-Samani when you only have temperature. Use Blaney-Criddle for the simplest estimate.

The same tool works for a farmer with a basic thermometer and one with a full weather station.

FAO-56 Kc Values Built In

The Kc values in Tab 2 come directly from FAO Irrigation and Drainage Paper 56. That’s the internationally accepted standard.

You don’t need to look up Kc values separately. Select your crop and growth stage and the tool fills them in automatically.

Full Water Balance in Tab 3

Tab 3 is what most ET tools miss completely.

Knowing ETc tells you how thirsty your crop is. But what you actually need to irrigate depends on rainfall, soil moisture, system efficiency, and leaching requirements too.

Tab 3 brings all of those together in one calculation.

Leaching Requirement for Saline Soils

This is a feature I specifically included for farmers in regions with saline soils or saline irrigation water.

When salts build up in the root zone, they damage crops even when water is available. Extra irrigation is needed to flush salts downward. Tab 3 adds this leaching requirement automatically when you select a saline soil level.

Six Area Units and Multiple Volume Formats

Field area inputs support hectares, acres, square metres, square feet, and Guntha.

Volume outputs support litres, cubic metres, US gallons, and acre-feet. The tool speaks the unit language of farmers worldwide.

Who Gets the Most from This Tool?

• Irrigated Crop Farmers: Any farmer who irrigates wheat, maize, rice, vegetables, or fruit crops can use this tool to plan daily irrigation amounts based on actual crop water demand.
• Drip and Sprinkler Irrigation Users: Precision irrigation systems only deliver their full benefit when they’re operated with correct runtimes. ET-based scheduling is how you get those runtimes right.
• Farmers in Water-Scarce Regions: When water is limited, every litre matters. ET-based irrigation prevents waste and ensures water goes where it’s needed most.
• Farmers with Saline Soils or Water: The leaching requirement feature in Tab 3 is directly useful for farmers dealing with soil salinity problems.
• Irrigation Engineers and Agronomists: The three-method ET0 calculation with full unit support makes this a useful desktop tool for professional irrigation design and farm advisory work.
• Agricultural Students and Researchers: The Penman-Monteith implementation follows the exact FAO-56 methodology. It’s a practical learning tool for anyone studying irrigation science.

Step-by-Step: How to Use the ET Calculator

Here’s a complete example. You’re a tomato farmer. Today’s weather shows a maximum temperature of 35°C and minimum of 22°C. Relative humidity is 80 percent max and 40 percent min. Wind speed is 2 m/s. Solar radiation is 20 MJ/m2/day. Your farm is at 500 metres elevation, latitude 18.5 degrees, and today is day 180 of the year. Your field is 2 hectares. There was 2 mm of effective rainfall today. You use drip irrigation at 90 percent efficiency.

Step 1: Calculate ET0 (Tab 1)

1. Open the ET Calculator on moralinsights.com.
2. Stay on Tab 1 (Reference ET).
3. Select FAO Penman-Monteith as your method.
4. Enter Tmax as 35, Tmin as 22, unit as Celsius.
5. Enter RH Max as 80 and RH Min as 40.
6. Enter Wind Speed as 2 m/s.
7. Enter Elevation as 500 metres.
8. Enter Solar Radiation as 20 MJ/m2/day.
9. Enter Latitude as 18.5 degrees, Day of Year as 180.
10. Click Calculate ET0.

Result: ET0 is approximately 5.8 mm/day. The tool classifies this as Moderate to High. It advises you to plan irrigation to match crop ETc.

Step 2: Calculate Crop ETc (Tab 2)

1. Click Tab 2 (Crop ET).
2. Enter ET0 as 5.8 mm/day.
3. Enter Field Area as 2 hectares.
4. Select Tomato as crop type and Mid Season as growth stage.
5. The tool auto-fills Kc as 1.15.
6. Enter Effective Rainfall as 2 mm/day.
7. Click Calculate ETc.

Result: ETc = 5.8 x 1.15 = 6.67 mm/day. Net irrigation need after rainfall = 6.67 minus 2 = 4.67 mm/day. Daily water volume for your 2-hectare field = 93,400 litres.

Step 3: Calculate Exact Irrigation Volume (Tab 3)

1. Click Tab 3 (Irrigation Need).
2. Enter ETc as 6.67 mm/day.
3. Enter Field Area as 2 hectares.
4. Enter Effective Rainfall as 2 mm/day.
5. Set System Efficiency to 90% (drip).
6. Set Leaching Requirement to 0% (normal soil).
7. Set Irrigation Frequency to Daily.
8. Click Calculate Irrigation Need.

Result: Gross deficit = 4.67 mm/day. After efficiency adjustment = 5.19 mm/day gross depth. Volume per irrigation event = 103,800 litres per day for your 2-hectare field.

That’s your number. Run your drip system until it delivers 103,800 litres today.

All three methods in this calculator are based on the FAO Irrigation and Drainage Paper 56 (FAO-56). This is the globally recognized standard for crop water requirement calculations. For Hargreaves-Samani method validation, refer to Hargreaves and Samani (1985) as documented through the National Library of Medicine. For regional weather data to feed into your calculations, the NASA POWER climate data platform provides free daily weather data for any location on Earth.

Related Tools on MoralInsights.com

Use the ET Calculator as the starting point for your complete irrigation planning workflow:

• Irrigation and Fertigation Calculator — Use your ETc value from Tab 2 as the daily water requirement input for your drip or sprinkler irrigation and fertigation schedule.
• Drip Irrigation Layout Calculator — Design your drip system layout and use the daily volume from Tab 3 to set your irrigation timer correctly.
• Universal Sprinkler Spacing, Rate and Runtime Calculator — For sprinkler-irrigated fields, use the ETc value from this calculator to determine your daily runtime.
• Crop Water Requirement Calculator — Calculate your crop’s full seasonal water requirement alongside the daily ET values from this tool.
• Irrigation Scheduling Calendar — Build a full-season irrigation calendar using ET-based daily water requirements.
• Water Storage Capacity Calculator — Use your monthly water volume from Tab 3 to check whether your farm pond or tank can support your irrigation needs.
• Soil Moisture Depletion Calculator — Combine ET data with soil moisture tracking to know exactly when to irrigate next.
• Rainwater Harvesting Calculator — Use your monthly ET and rainfall data to plan how much rainwater storage you need.

Frequently Asked Questions

What is the difference between ET0 and ETc?

ET0 is Reference Evapotranspiration. It measures water loss from a standard short grass surface under your weather conditions. It tells you how dry the atmosphere is today.

ETc is Crop Evapotranspiration. It’s ET0 multiplied by the crop coefficient Kc. It tells you how much water your specific crop actually loses under those conditions.

ET0 is the input. ETc is the answer you act on.

I don’t have a weather station. Which method should I use?

Use the Hargreaves-Samani method. You only need a maximum and minimum thermometer plus your location’s latitude and the day of the year.

You can find your latitude from any mapping application on your phone. The day of the year is simply how many days have passed since January 1st.

It’s not as accurate as Penman-Monteith but it’s far better than guessing.

What is a Crop Coefficient (Kc) and why does it change by growth stage?

Kc is a number that reflects how much water your specific crop uses compared to the reference grass surface.

It changes by growth stage because crops use different amounts of water at different times. A seedling with little leaf area uses less water than a full-canopy crop at peak flowering. The mid-season Kc is almost always the highest because canopy cover and transpiration are at their maximum.

All Kc values in this tool come directly from FAO-56, the global standard reference.

What is effective rainfall and how is it different from total rainfall?

Total rainfall is what falls from the sky. Effective rainfall is what actually reaches the crop root zone and stays there.

Rain that falls harder than the soil can absorb runs off as surface runoff. Rain on hard, compacted soil may not infiltrate at all. Light rainfall in hot, dry conditions may evaporate before reaching the roots.

As a general rule, effective rainfall is about 70 to 80 percent of total rainfall for moderate rain events on cultivated soil. For light showers on dry soil, effective rainfall can be much lower.

What does the leaching requirement mean for my irrigation planning?

When you irrigate with saline water or when salts build up in your soil over time, they accumulate in the root zone and damage crops even when water appears available.

To prevent this, you need to apply extra water to push salts below the root zone. This extra water is the leaching requirement.

Tab 3 calculates this automatically. A 10 percent leaching requirement means you apply 10 percent more water than your ETc-based net irrigation need. That extra water flushes salts downward and protects your crop.

Conclusion

Water is your most valuable input. Wasting it costs money. Underapplying it costs yield.

The Evapotranspiration Calculator on moralinsights.com gives you the science to get it right every single day. Tab 1 tells you how thirsty the atmosphere is. Tab 2 tells you exactly what your crop needs. Tab 3 tells you the precise volume to apply after accounting for rainfall, soil moisture, your system’s efficiency, and your soil’s salt level. And Tab 4 gives you the reference data to back it all up.

Use it daily during your growing season and you’ll irrigate smarter, save water, and grow better crops. That’s the goal I had in mind when I built it, and I hope it serves your farm well.

Disclaimer

The Evapotranspiration Calculator on moralinsights.com implements the FAO Penman-Monteith, Hargreaves-Samani, and Blaney-Criddle methods as described in FAO Irrigation and Drainage Paper 56. Results are estimates for planning purposes only. Actual field evapotranspiration varies with local microclimatic conditions, crop variety, canopy characteristics, soil type, and field topography.

The calculator assumes standard reference conditions including wind speed measured at 2 metres height and a well-watered crop surface. Kc values are sourced from FAO-56 and may differ from locally calibrated values for your specific crop variety and region. Always validate ET estimates with local weather station data and agronomic observations. The author and moralinsights.com accept no liability for irrigation decisions, water use, or crop losses made on the basis of this calculator’s output.

About the Author

Lalita Sontakke is the founder of moralinsights.com, a global agriculture-focused platform offering 47+ free tools and calculators for farmers, irrigation engineers, agronomists, and agricultural professionals worldwide. Her mission is to make science-based farm management accessible to every farmer, free, practical, and available from any device, anywhere in the world.