Transpiration Calculator

What it does

A transpiration calculator that uses the FAO Penman-Monteith equation, adjusted with crop coefficients and environmental stress factors. It works in metric or US units.

When to use it

Use it to estimate daily and hourly plant water loss for irrigation planning and dehumidifier sizing.

How it calculates

• FAO-56 Penman-Monteith reference evapotranspiration, with psychrometric constant γ = 0.000665 × 101.3 kPa
• Saturation vapor pressure = 0.6108·exp(17.27T/(T+237.3)); aerodynamic resistance = 208 ÷ wind speed
• Crop coefficient = leaf area index ÷ the crop's optimal LAI, capped at 1.4
• Surface resistance adjusted for growth stage, VPD and CO₂; light intensity converted as PPFD × 0.219 = W/m²

What you enter

Temperature(°C or °F)

Relative Humidity(%)

Wind Speed(m/s or mph)

Solar Radiation(W/m² or BTU/hr/ft²)

Light Intensity(μmol/m²/s)

Crop Type(Tomato, Lettuce, Cannabis, Cucumber, Herbs)

Growth Stage

Leaf Area Index

Photoperiod(hours)

CO₂ concentration, air circulation, day of year and latitude

What you get

Daily Transpiration(mm/day or in/day)

Hourly Rate(mm/h or in/h)

Water Use(L/m²/day or gal/ft²/day)

WUE(g/L)

VPD(kPa)

Net radiation, latent heat, sensible heat and soil heat flux(W/m²)

Aerodynamic and Surface Resistance(s/m)

Plant status and recommendations

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• Diurnal Forecast Estimate
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