About this tool
The VPD Calculator computes vapor pressure deficit — the drying power of the air around your plants — from air temperature and relative humidity, with an optional leaf-temperature offset. VPD is one of the most useful climate metrics for grow rooms and greenhouses because it drives transpiration, nutrient uptake and disease pressure. Everything is calculated in your browser.
First the saturation vapor pressure is found with SVP = 0.61078 × exp(17.27 × T / (T + 237.3)), giving kPa for a temperature T in °C. The actual vapor pressure the air is holding is AVP = SVP(air) × RH / 100. VPD is then SVP(leaf) − AVP, where the leaf's SVP uses the leaf temperature (air temperature plus the offset you set; leaves are often a couple of degrees cooler than the air, so try −2 or −3 °C). With the offset at 0, VPD is calculated from air temperature alone.
The tool then compares your VPD against the target band for the growth stage you pick — roughly 0.4–0.8 kPa for propagation and clones, 0.8–1.2 kPa for vegetative growth and 1.2–1.6 kPa for flowering — and tells you whether you are below, inside or above the ideal zone. These bands are common horticultural guidelines; the best target varies by crop, strain and lighting, so treat them as a starting point.