How it works
SVP(T) = 0.6108 · e^(17.27·T / (T + 237.3)); air VPD = SVP(air) · (1 − RH/100); leaf VPD = SVP(leaf) − SVP(air) · RH/100
VPD is the difference between how much water vapour the air could hold when saturated and how much it actually holds. Saturation vapour pressure rises steeply with temperature and is given by the Tetens/Magnus equation adopted in FAO Irrigation & Drainage Paper 56: at 25 °C it is about 3.17 kPa. Air VPD multiplies that saturation value by one minus the fractional humidity, so warm dry air has a high deficit and cool damp air a low one. Leaf VPD is the number that matters to the plant: because a transpiring leaf is cooler than the surrounding air — roughly 2 °C under good airflow — its internal air is saturated at a lower temperature, so the calculator evaluates saturation vapour pressure at the leaf temperature and subtracts the air’s actual vapour pressure. A leaf running 2 °C below 25 °C air at 50% humidity sits near 1.2 kPa, comfortably in the vegetative range, while the same air read as air-only VPD looks closer to 1.6 kPa. Propagation and seedlings prefer a gentle 0.4–0.8 kPa so tender, poorly rooted tissue does not dry out; vegetative growth likes roughly 0.8–1.2 kPa; and flowering or fruiting plants tolerate a drier 1.2–1.6 kPa that discourages mould on dense canopies. Because leaf temperature and species behaviour vary, treat these as target zones rather than exact set points.
Sources
- FAO Irrigation & Drainage Paper 56 — saturation vapour pressure Allen, Pereira, Raes & Smith (1998), Chapter 3, Eq. 11: e°(T) = 0.6108·exp[17.27·T/(T+237.3)] kPa; vapour pressure deficit = saturation vapour pressure minus actual vapour pressure.
- Michigan State University Extension — Vapor pressure deficit VPD combines temperature and humidity into the water-vapour gradient that drives transpiration; a transpiring leaf is cooler than the surrounding air, and greenhouse crops grow best within moderate VPD ranges managed by growth stage.
FAQ
What is a good VPD range for plants?
As a starting point, keep leaf VPD near 0.4 to 0.8 kPa for propagation and seedlings, 0.8 to 1.2 kPa during vegetative growth, and 1.2 to 1.6 kPa for flowering or fruiting plants. Tender cuttings want the humid low end so they do not dry out before they root, while mature, leafy canopies tolerate a drier deficit that keeps mould down. These are target zones; adjust to how your specific plants respond.
What is the difference between air VPD and leaf VPD?
Air VPD uses air temperature for both the saturation and actual vapour pressure, so it describes the drying power of the room. Leaf VPD instead evaluates saturation at the cooler leaf-surface temperature, because that is where water actually evaporates from. Since a transpiring leaf sits a degree or two below air temperature, leaf VPD is usually lower than air VPD, and it is the figure that best predicts transpiration and stress.
Why is the leaf cooler than the air?
Evaporative cooling. As water evaporates from the stomata it carries away heat, so a well-watered, transpiring leaf runs below the surrounding air temperature — often about 2 °C under good airflow, more when humidity is low and transpiration is fast. If plants are water-stressed, stomata close, transpiration drops and the leaf warms toward air temperature. The default −2 °C offset is a reasonable estimate; measure it with an infrared thermometer for precision.
How do I lower a VPD that is too high?
A high VPD means the air is too dry for the stage. Raise the relative humidity by adding a humidifier, grouping plants, or reducing extraction, and lower the air temperature if it is running hot. Both moves shrink the gap between saturation and actual vapour pressure. Recheck with the calculator after each change — humidity has a larger effect at warm temperatures because saturation vapour pressure climbs steeply with heat.
How do I raise a VPD that is too low?
A low VPD means the air is too humid, which stalls transpiration and invites damping-off and fungal disease. Increase ventilation or dehumidification to drop the humidity, and let the air temperature rise a little so saturation vapour pressure increases. Improving airflow across the canopy also helps by refreshing the boundary layer around each leaf. Aim back into the target band for your growth stage.
Do I need to measure leaf temperature?
Not to get started. The calculator assumes the leaf is about 2 °C below air, which is typical for healthy plants with decent airflow, and you can change that offset. For the most accurate leaf VPD, point an inexpensive infrared thermometer at a shaded upper leaf and enter the measured difference. On still, humid days the leaf may be only slightly cooler; under bright light with dry air it can be several degrees cooler.
Estimates only. VPD targets vary by species, cultivar and airflow, and leaf temperature depends on light, water status and air movement. Use these ranges as starting points for home growing, greenhouse and houseplant care, verify with your own instruments, and observe how your plants respond — general growing guidance, not professional agronomic advice.