What the number means
Air at a given temperature can hold only so much water vapour; the pressure that vapour exerts when the air is saturated is the saturation vapour pressure, and it rises steeply with temperature. The vapour pressure the air actually holds is that figure scaled by the relative humidity. FAO Irrigation and Drainage Paper 56, the reference for crop water calculations, defines relative humidity as the ratio of the actual to the saturation vapour pressure at the same temperature and calls the difference between the two the vapour pressure deficit or saturation deficit. That difference, in kilopascals, is what a leaf responds to: a large deficit pulls water out of it fast, a small one barely at all. The plant VPD calculator computes it live with a leaf temperature offset; the dew point calculator uses the same vapour pressure arithmetic to find the temperature at which that vapour would condense.
The formula
At 24 °C the saturation vapour pressure is 2.98 kPa. At 60 percent humidity the air holds 1.79 kPa, so the deficit is 1.19 kPa. Fahrenheit readings are converted to Celsius first; the second chart below does that for you. When the leaf is warmer or cooler than the air, the first term is evaluated at the leaf temperature and the second at the air temperature, which the calculator handles with its leaf offset.
VPD chart in Celsius
Air VPD in kPa, leaf temperature taken equal to air temperature. Rows are air temperature, columns relative humidity.
| °C | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 15 °C | 1.02 | 0.94 | 0.85 | 0.77 | 0.68 | 0.60 | 0.51 | 0.43 | 0.34 | 0.26 | 0.17 |
| 16 °C | 1.09 | 1.00 | 0.91 | 0.82 | 0.73 | 0.64 | 0.55 | 0.45 | 0.36 | 0.27 | 0.18 |
| 17 °C | 1.16 | 1.07 | 0.97 | 0.87 | 0.78 | 0.68 | 0.58 | 0.48 | 0.39 | 0.29 | 0.19 |
| 18 °C | 1.24 | 1.14 | 1.03 | 0.93 | 0.83 | 0.72 | 0.62 | 0.52 | 0.41 | 0.31 | 0.21 |
| 19 °C | 1.32 | 1.21 | 1.10 | 0.99 | 0.88 | 0.77 | 0.66 | 0.55 | 0.44 | 0.33 | 0.22 |
| 20 °C | 1.40 | 1.29 | 1.17 | 1.05 | 0.94 | 0.82 | 0.70 | 0.58 | 0.47 | 0.35 | 0.23 |
| 21 °C | 1.49 | 1.37 | 1.24 | 1.12 | 0.99 | 0.87 | 0.75 | 0.62 | 0.50 | 0.37 | 0.25 |
| 22 °C | 1.59 | 1.45 | 1.32 | 1.19 | 1.06 | 0.93 | 0.79 | 0.66 | 0.53 | 0.40 | 0.26 |
| 23 °C | 1.69 | 1.55 | 1.40 | 1.26 | 1.12 | 0.98 | 0.84 | 0.70 | 0.56 | 0.42 | 0.28 |
| 24 °C | 1.79 | 1.64 | 1.49 | 1.34 | 1.19 | 1.04 | 0.90 | 0.75 | 0.60 | 0.45 | 0.30 |
| 25 °C | 1.90 | 1.74 | 1.58 | 1.43 | 1.27 | 1.11 | 0.95 | 0.79 | 0.63 | 0.48 | 0.32 |
| 26 °C | 2.02 | 1.85 | 1.68 | 1.51 | 1.34 | 1.18 | 1.01 | 0.84 | 0.67 | 0.50 | 0.34 |
| 27 °C | 2.14 | 1.96 | 1.78 | 1.60 | 1.43 | 1.25 | 1.07 | 0.89 | 0.71 | 0.53 | 0.36 |
| 28 °C | 2.27 | 2.08 | 1.89 | 1.70 | 1.51 | 1.32 | 1.13 | 0.94 | 0.76 | 0.57 | 0.38 |
| 29 °C | 2.40 | 2.20 | 2.00 | 1.80 | 1.60 | 1.40 | 1.20 | 1.00 | 0.80 | 0.60 | 0.40 |
| 30 °C | 2.55 | 2.33 | 2.12 | 1.91 | 1.70 | 1.49 | 1.27 | 1.06 | 0.85 | 0.64 | 0.42 |
| 32 °C | 2.85 | 2.62 | 2.38 | 2.14 | 1.90 | 1.66 | 1.43 | 1.19 | 0.95 | 0.71 | 0.48 |
| 34 °C | 3.19 | 2.93 | 2.66 | 2.39 | 2.13 | 1.86 | 1.60 | 1.33 | 1.06 | 0.80 | 0.53 |
VPD chart in Fahrenheit
| °F | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 60 °F | 1.06 | 0.97 | 0.88 | 0.80 | 0.71 | 0.62 | 0.53 | 0.44 | 0.35 | 0.27 | 0.18 |
| 62 °F | 1.14 | 1.04 | 0.95 | 0.85 | 0.76 | 0.66 | 0.57 | 0.47 | 0.38 | 0.28 | 0.19 |
| 64 °F | 1.22 | 1.12 | 1.02 | 0.92 | 0.81 | 0.71 | 0.61 | 0.51 | 0.41 | 0.31 | 0.20 |
| 66 °F | 1.31 | 1.20 | 1.09 | 0.98 | 0.87 | 0.76 | 0.65 | 0.55 | 0.44 | 0.33 | 0.22 |
| 68 °F | 1.40 | 1.29 | 1.17 | 1.05 | 0.94 | 0.82 | 0.70 | 0.58 | 0.47 | 0.35 | 0.23 |
| 70 °F | 1.50 | 1.38 | 1.25 | 1.13 | 1.00 | 0.88 | 0.75 | 0.63 | 0.50 | 0.38 | 0.25 |
| 72 °F | 1.61 | 1.47 | 1.34 | 1.21 | 1.07 | 0.94 | 0.80 | 0.67 | 0.54 | 0.40 | 0.27 |
| 74 °F | 1.72 | 1.58 | 1.43 | 1.29 | 1.15 | 1.00 | 0.86 | 0.72 | 0.57 | 0.43 | 0.29 |
| 76 °F | 1.84 | 1.69 | 1.53 | 1.38 | 1.23 | 1.07 | 0.92 | 0.77 | 0.61 | 0.46 | 0.31 |
| 78 °F | 1.96 | 1.80 | 1.64 | 1.47 | 1.31 | 1.15 | 0.98 | 0.82 | 0.65 | 0.49 | 0.33 |
| 80 °F | 2.10 | 1.92 | 1.75 | 1.57 | 1.40 | 1.22 | 1.05 | 0.87 | 0.70 | 0.52 | 0.35 |
| 82 °F | 2.24 | 2.05 | 1.87 | 1.68 | 1.49 | 1.31 | 1.12 | 0.93 | 0.75 | 0.56 | 0.37 |
| 84 °F | 2.39 | 2.19 | 1.99 | 1.79 | 1.59 | 1.39 | 1.19 | 1.00 | 0.80 | 0.60 | 0.40 |
| 86 °F | 2.55 | 2.33 | 2.12 | 1.91 | 1.70 | 1.49 | 1.27 | 1.06 | 0.85 | 0.64 | 0.42 |
| 88 °F | 2.71 | 2.49 | 2.26 | 2.03 | 1.81 | 1.58 | 1.36 | 1.13 | 0.90 | 0.68 | 0.45 |
| 90 °F | 2.89 | 2.65 | 2.41 | 2.17 | 1.93 | 1.69 | 1.44 | 1.20 | 0.96 | 0.72 | 0.48 |
| 92 °F | 3.07 | 2.82 | 2.56 | 2.31 | 2.05 | 1.79 | 1.54 | 1.28 | 1.02 | 0.77 | 0.51 |
| 95 °F | 3.37 | 3.09 | 2.81 | 2.53 | 2.25 | 1.97 | 1.69 | 1.41 | 1.12 | 0.84 | 0.56 |
Read either chart along a row to see what a humidifier or dehumidifier does at a fixed temperature, and down a column to see what a heater or the lights do at a fixed humidity. The two directions are not symmetrical: at 70 percent humidity, warming from 20 to 28 °C raises the deficit from 0.70 to 1.13 kPa, while at 24 °C dropping humidity from 70 to 60 percent raises it from 0.90 to 1.19 kPa.
Humidity for a target VPD
Turn the formula around and you get the humidity that produces a chosen deficit at a given temperature: RH = 100 × (1 − VPD ÷ e°(T)). This is the setting for a humidity controller once you have decided on a target.
| Air temperature | RH for 0.6 kPa | RH for 0.8 kPa | RH for 1.0 kPa | RH for 1.2 kPa | RH for 1.4 kPa |
|---|---|---|---|---|---|
| 18 °C (64 °F) | 71% | 61% | 52% | 42% | 32% |
| 20 °C (68 °F) | 74% | 66% | 57% | 49% | 40% |
| 22 °C (72 °F) | 77% | 70% | 62% | 55% | 47% |
| 24 °C (75 °F) | 80% | 73% | 66% | 60% | 53% |
| 26 °C (79 °F) | 82% | 76% | 70% | 64% | 58% |
| 28 °C (82 °F) | 84% | 79% | 74% | 68% | 63% |
| 30 °C (86 °F) | 86% | 81% | 76% | 72% | 67% |
The same target needs a higher humidity the warmer the room: holding 1.0 kPa takes 57 percent at 20 °C and 74 percent at 28 °C. That is the practical reason to control temperature first: every degree of extra heat from the lights has to be paid for in humidity, and a humid, warm room is where condensation and mould begin when the lights go off and the temperature falls.
Which VPD to aim for
The charts give numbers, not targets, because the right deficit depends on the crop and its stage, and published recommendations differ by source. The direction is consistent: cuttings and seedlings with little root have no way to replace water quickly, so they are kept at a low deficit in humid air; established plants in full growth transpire freely and are run at a higher deficit so water and nutrients keep moving; and very humid air around dense flowers or fruit is where fungal problems start. The plant VPD calculator uses stage-based rules of thumb and shows how much humidity has to change to land in each band; treat its ranges, and anyone else's, as starting points and watch the plants.
Lights, heat and the leaf
Grow lights heat the canopy, and the leaf is what the deficit acts on. A leaf a few degrees warmer than the air has a higher saturation pressure, so its deficit is larger than the air chart shows; a leaf transpiring hard can run cooler, and its deficit smaller. An infrared thermometer pointed at the canopy gives the number to use. Lighting is also where the running cost of a grow room sits, and the grow light cost calculator turns the fixture's wattage and hours into a monthly figure, which is worth knowing before adding heat that the humidifier then has to answer.
Frequently asked questions
What is VPD?
Vapour pressure deficit is the difference between the water vapour pressure the air would hold if saturated at its temperature and the vapour pressure it actually holds, in kilopascals. FAO Paper 56 calls it the vapour pressure deficit or saturation deficit. It describes the drying power of the air as a plant leaf feels it, which relative humidity alone does not, because warm air holds far more water than cool air at the same percentage.
How is VPD calculated?
First the saturation vapour pressure at the air temperature, with the FAO-56 equation e°(T) = 0.6108 × exp(17.27 T ÷ (T + 237.3)) in kPa, T in degrees Celsius. Then the actual vapour pressure, e°(T) × RH ÷ 100. The deficit is the first minus the second. At 24 °C and 60 percent humidity: 2.98 kPa × (1 − 0.6) = 1.19 kPa.
What is a good VPD for plants?
It depends on the species and the stage of growth, and published targets differ by source and crop; the chart on this page gives the numbers, not the targets. As a direction, cuttings and seedlings with few roots are kept at a low deficit so they do not dry out, and plants in full growth at a higher one so they transpire and move nutrients. The plant VPD calculator applies stage-based rules of thumb and shows the humidity change needed to reach them.
Why does the same humidity give a different VPD at different temperatures?
Because saturation vapour pressure rises steeply with temperature: about 1.7 kPa at 15 °C, 2.3 at 20 °C, 3.2 at 25 °C and 4.2 at 30 °C. Sixty percent humidity leaves 40 percent of that as deficit, so the same 60 percent is 0.68 kPa at 15 °C and 1.70 kPa at 30 °C. Reading humidity without temperature says nothing about how hard the air is pulling water from a leaf.
Should I use air temperature or leaf temperature?
Leaf temperature, when you can measure it. The deficit the plant experiences is the saturation pressure at the temperature of the leaf surface minus the actual vapour pressure of the air. Leaves under strong lights can run warmer than the air and leaves transpiring freely a little cooler, which shifts the saturation side of the subtraction. The charts here assume leaf and air are equal; the VPD calculator takes a leaf offset.