VPD Calculator: Leaf and Air Vapor Pressure Deficit for Grow Rooms

How it works

Saturation vapor pressure e_s(T) = 0.61078 × exp(17.27 × T ÷ (T + 237.3)) kPa, T in °C · actual vapor pressure e_a = e_s(T_air) × RH ÷ 100 · Leaf VPD = e_s(T_leaf) − e_a · Air VPD = e_s(T_air) − e_a · RH for a target VPD = (e_s(T_leaf) − VPD) ÷ e_s(T_air) × 100

Vapor pressure deficit (VPD) is how far the air is from being saturated with water vapor, measured as a pressure in kilopascals. It drives transpiration better than relative humidity alone, because the same 60% RH is “drier” at 30 °C than at 20 °C. The calculator finds the saturation vapor pressure with the Tetens equation (the one used in FAO-56), multiplies it by the relative humidity to get the vapor actually in the air, and subtracts. Growers care most about leaf VPD: the difference between the saturated air inside the leaf, at leaf temperature, and the room air. Transpiring leaves are usually 1–3 °C cooler than the air, which lowers leaf VPD, so enter a leaf offset or a measured canopy temperature from an infrared thermometer. The result is compared with common targets: about 0.4–0.8 kPa for seedlings and clones, 0.8–1.2 kPa in vegetative growth and 1.2–1.6 kPa in flowering. The calculator also works out the humidity range that would hit that target at your temperatures. The VPD chart table uses your leaf offset, so you can read it like a printed grow-room chart. The Relative Humidity and Absolute Humidity calculators convert between moisture measures. This one is built for plant climate control: leaf versus air VPD, stage targets and the humidity setpoint to use. The Tetens equation is accurate to about 0.1% between 0 and 50 °C. The targets are general guidelines, not crop-specific limits.

Worked example

The room is at 77 °F (25 °C) and 60% RH, with leaves 3.6 °F (2 °C) cooler at 73.4 °F (23 °C). Saturation vapor pressure of the air is 0.61078 × e^(17.27 × 25 ÷ 262.3) = 3.168 kPa, so the air holds 3.168 × 0.60 = 1.901 kPa of vapor. At the leaf, saturation vapor pressure is 2.809 kPa, so leaf VPD = 2.809 − 1.901 = 0.91 kPa. That is inside the 0.8–1.2 kPa vegetative range. Air VPD is 1.27 kPa and the dew point is 62.1 °F (16.7 °C). Any humidity from 51% to 63% keeps leaf VPD in the vegetative range at these temperatures.

Sources: FAO Irrigation and Drainage Paper 56, ch. 3: saturation vapour pressure (eq. 11) and vapour pressure deficit · Omni Calculator: VPD (Tetens equation, 22 °C / 55% / 20 °C leaf example) · Hydrobuilder: VPD calculator (leaf offset and growth-stage target ranges)

Frequently asked questions