Water Potential Calculator
Water always moves from less negative potential to more negative — never uphill.
The formula
VPD = SVP(leaf temperature) - SVP(air) * relative humidity
The pull on the plant
Vapour pressure deficit is the gap between how much water vapour the air could hold and how much it does. That gap is what drives transpiration: too small and the plant cannot move water or the nutrients dissolved in it; too large and it closes its stomata to avoid drying out, which stops photosynthesis at the same time.
Leaf temperature, not air temperature
A transpiring leaf is typically a degree or two cooler than the air around it, because evaporation costs energy. Computing VPD from air temperature alone therefore overstates the deficit the plant actually experiences, sometimes by a third. Growers who measure leaf temperature with an infrared thermometer are correcting for exactly this.
Why relative humidity alone is not enough
Sixty per cent humidity at 20 °C and at 30 °C are entirely different growing conditions, because warm air holds far more water. VPD collapses temperature and humidity into the single number the plant responds to, which is why greenhouse control has largely moved to it.
Rules of thumb, and the field
Every figure here describes an average season on an average site. Soil type, aspect, drainage, variety and weather each move the answer by more than the arithmetic does, and a grower who has kept records for a few years on one piece of ground knows it better than any formula. Use these to plan an order or size an implement, then trust what the ground has actually done before.