How it works
SVP(T) = 0.6108 · e^(17.27·T / (T + 237.3)); cell = SVP(T_air + offset) − SVP(T_air) · RH/100; target RH = 100 · (SVP(T_leaf) − VPD) / SVP(T_air)
Read the grid as a map of drying power. Going down a column warms the air, and because saturation vapor pressure climbs exponentially — 2.06 kPa at 18 °C, 3.17 kPa at 25 °C and 4.24 kPa at 30 °C by FAO-56 Equation 11 — the numbers grow toward the bottom of the table. Going left dries the air, which also widens the deficit. The two moves are not worth the same everywhere: one 5-point humidity step is worth about 0.21 kPa at 30 °C but only about 0.10 kPa at 18 °C, so a warm room needs a tighter humidistat than a cool one to stay inside the same band. On the leaf basis each cell takes saturation at the leaf (air temperature plus your offset) and subtracts the vapor actually in the air, the definition Ohio State uses in e-GRO Edible Alert 8(16); the air basis subtracts that same vapor from saturation at the air temperature. The gap between the two views is e°(T_air) − e°(T_leaf), and it does not depend on humidity: at 25 °C with a leaf 2 °C cooler it is 0.36 kPa in every column of that row. Bold cells sit inside the selected band, a dotted underline marks cells just outside it and a solid underline marks cells well outside it. The highlighted row and the hinted column show your reading, snapped to the nearest grid line. The target humidity readout runs the equation backwards, solving for the RH that puts the band midpoint at your exact temperature: at 25 °C with the default offset that is 62% RH for early vegetative growth, with 57–67% still in band. Four of the five bands are displayed planning assumptions that circulate without a published source; the fifth, 0.45–1.15 kPa, is the greenhouse range published by the University of Georgia.
Sources
- FAO Irrigation & Drainage Paper 56 — Chapter 3, Eq. 11 Allen, Pereira, Raes & Smith (1998): saturation vapour pressure e°(T) = 0.6108·exp[17.27·T/(T+237.3)] kPa, and the vapour pressure deficit as saturation minus actual vapour pressure. Every cell in the grid is computed with this equation.
- e-GRO Edible Alert 8(16) — VPDleaf vs. VPDair Kubota, C. (2023), The Ohio State University, Vol. 8 No. 16: defines VPDair = SVPair − VPair and VPDleaf = SVPleaf − VPair. Its worked example (20 °C air, 60% RH, 18 °C leaf → 0.94 and 0.66 kPa; 22 °C leaf → 1.24 kPa) is reproduced exactly by this chart.
- e-GRO Alert 7(24) — Vapor Pressure Deficit? Why Should I Track That? Thomas, P. (2018), University of Georgia, Vol. 7 No. 24: gives 0.45–1.15 kPa as the ideal greenhouse VPD range, varying by crop species and growing protocol, with 1.15 kPa or above counted as high. Source of the Greenhouse band.
- Greenhouse Product News — Water Vapor-Pressure Deficit Runkle, E. (2021), Michigan State University, November 2021: below 0.3 kPa suits mainly propagation; 0.3–0.7 kPa is the minimum desirable for most plant growth; 0.7–1.5 kPa is a target for most plant growth; above 1.5 kPa is dry air.
FAQ
How do I read a VPD chart?
Find the row for your air temperature, slide across to the column for your relative humidity, and read the number where they meet: that is the deficit in kPa. Bold numbers fall inside the band you picked, so a run of bold cells along a row shows the humidity range that works at that temperature. A dotted underline means a cell is within 0.3 kPa below or 0.4 kPa above the band; a solid underline means it is further out. Values rise toward the bottom-left, where the air is warm and dry, and fall toward the top-right, where it is cool and damp.
Should I read leaf VPD or air VPD?
They answer different questions. Air VPD describes the room: how far its air is from saturation at its own temperature. Leaf VPD describes the gradient a leaf works against, using saturation at leaf temperature, which e-GRO Edible Alert 8(16) calls a vapor pressure difference rather than a true deficit. Published greenhouse targets, such as the University of Georgia and Michigan State ranges, are stated as air VPD, while the familiar stage bands are usually quoted on a leaf basis. Set the toggle to match whichever target you are comparing against.
What humidity should I hold at 24 °C (75 °F)?
With the leaf 2 °C below the air and the early vegetative band selected, hold about 60% RH; anything from roughly 55% to 65% keeps the leaf between 0.7 and 1.0 kPa. That ten-point window is the headroom you give your humidistat, so set it near the middle and normal swings stay in band. Switch to the published greenhouse range and the window opens to roughly 50–74%. Enter your own temperature and band above and the target humidity readout does this lookup for you; for the reading itself with the full working, open the VPD calculator.
Why do two VPD charts disagree?
Mostly because each assumes a different leaf offset, and few say which. At 25 °C and 50% RH, a chart that puts the leaf 1 °C below the air prints 1.40 kPa, while this page’s default 2 °C offset prints 1.23 kPa: the same room, about 12% apart. Charts that ignore leaf temperature print air VPD, 1.58 kPa for that cell. Smaller gaps come from the saturation formula; this page uses FAO-56, and other approximations can move values by a few hundredths. Match the offset and the basis before comparing two charts.
Which VPD ranges are actually published?
The University of Georgia’s e-GRO Alert 7(24) gives 0.45 to 1.15 kPa as the ideal greenhouse range and counts 1.15 kPa or more as high. Erik Runkle at Michigan State treats anything below 0.3 kPa as suited mainly to propagation, 0.3 to 0.7 kPa as low, and 0.7 to 1.5 kPa as the target for most plant growth. The four stage bands offered here are an industry convention with no primary source, and the 1.2–1.6 kPa flowering band sits at or beyond both extension ceilings. Treat those bands as planning assumptions, not crop limits.
What does a negative number in the chart mean?
A negative leaf VPD means the leaf is colder than the dew point of the surrounding air, so water vapor condenses onto it instead of evaporating from it. It appears only on the leaf basis, with a large negative offset and very humid air: at 25 °C and 80% RH, a leaf 10 °C below the air reads −0.83 kPa. The chart prints the value rather than rounding it to zero because wet foliage invites disease. Lower the humidity, or move air across the canopy so the leaves warm back toward room temperature.
Can I print this VPD chart?
Yes. Use your browser’s print command: the site header, footer, FAQ, embed and feedback blocks drop away, leaving your inputs and the grid in black on white. Bold in-band cells, the dotted and solid underlines and the bold highlighted row all survive printing, so the chart still reads without colour. The twelve-column grid is designed to fit a portrait page; if print preview clips the last column, switch to landscape. Set the unit, basis and band before printing, because the paper copy shows exactly what is on screen.
Does CO₂ or airflow change the target?
Airflow changes the reading more than the target. Moving air thins the still layer on each leaf and pulls its temperature toward the air, so the offset tends to shrink as circulation improves; update the offset field, ideally from an infrared thermometer. CO₂ enrichment changes how far stomata open, but setting CO₂ levels is outside CanopyMath’s scope. A stable deficit pays off most when the canopy also gets enough light, which the PPFD calculator helps you plan.
Estimates only. Grow-environment targets vary by crop, cultivar, airflow and setup, and the four stage bands on this chart are planning assumptions rather than universal crop limits. Confirm readings with your own instruments; this is general growing guidance for home and greenhouse use, not professional agronomic advice.