VPD CalculatorVapor Pressure Deficit Calculator
Calculate Vapor Pressure Deficit - the true drying power of air. VPD combines temperature and humidity into a single number that predicts how fast moisture leaves wood, making it the key control variable for kiln and storage climate.
Why this calculation matters
Vapour pressure deficit is the real drying force. Two loads at the same relative humidity can dry at very different rates.
VPD Calculator
Vapor Pressure Deficit Calculator
Air temperature in the kiln or store.
Relative humidity of the air (1-100%).
| Property | Value | Meaning |
|---|
About VPD Calculator
Vapor Pressure Deficit (VPD) is the difference between how much moisture the air could hold when saturated and how much it actually holds. It is the real driving force that pulls moisture out of wood. Two rooms at the same humidity but different temperatures have very different drying power - VPD captures this in one number, which is why professional kiln and climate control increasingly targets VPD rather than RH alone. Kiln operators who normally read a VPD chart can work the vapour pressure deficit formula directly here and see what the number means for drying speed.
VPD quick chart (kPa)
| Temp | 30% RH | 50% RH | 70% RH |
|---|---|---|---|
| 20 °C | 1.64 | 1.17 | 0.70 |
| 30 °C | 2.97 | 2.12 | 1.27 |
| 40 °C | 5.16 | 3.69 | 2.21 |
| 50 °C | 8.64 | 6.17 | 3.70 |
| 60 °C | 13.95 | 9.97 | 5.98 |
How to calculate VPD — worked example
At 40 °C, saturation vapour pressure es = 0.6108 × e^(17.27×40 ÷ (40+237.3)) = 7.38 kPa. At 60% relative humidity, VPD = 7.38 × (1 − 0.60) = 2.95 kPa. The calculator above does this for any pair of readings.
Where Is This Used?
Formulas Used
Saturation Vapor Pressure (Magnus/Tetens):SVP = 0.6108 x exp( (17.27 x T) / (T + 237.3) ) T in deg C, result kPaActual Vapor Pressure: AVP = SVP x (RH / 100)VPD = SVP - AVP = SVP x (1 - RH/100)Wet-bulb to RH (psychrometric, simplified Bogel approximation)How the calculation works
Vapour pressure deficit is the gap between how much moisture the air could hold and how much it does hold — the actual driving force pulling water out of the timber. It rises steeply with temperature, which is why the same relative humidity dries far harder in a warm kiln than in a cool store.
Show the formula
Saturation vapour pressure SVP = 0.6108 x exp(17.27 T / (T + 237.3)) kPa (Tetens). Actual vapour pressure AVP = SVP x RH/100. VPD = SVP - AVP. From a wet bulb, RH is derived from e = e_s(T_wet) - A P (T_dry - T_wet) with A = 0.00066 (1 + 0.00115 T_wet) and P = 101.325 kPa. Dew point inverts the Tetens equation.The Tetens saturation equation, the psychrometer relation and the dew-point inversion are all published. The drying-power bands are not: no standard divides VPD into named severity levels, so they are this site’s convention for reading the number at a glance. Use the VPD figure and your own schedule, not the label.
References and what each one provides
- Hailwood–Horrobin sorption modelPublished in USDA FPL Wood Handbook, Ch. 4The two-hydrate equation relating temperature and relative humidity to equilibrium moisture content.
- USDA Wood Handbook (FPL GTR-190)USDA Forest Products LaboratoryPublished density, moisture relationships, shrinkage coefficients and mechanical properties for a wide range of species. The reference dataset behind most wood-property figures on this site.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Reports vapour pressure deficit for the conditions entered. It does not account for air velocity, stack geometry, or the moisture content of the timber itself, all of which affect actual drying rate.
Do not use this for Kiln control or process validation.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
- Tetens equation
- FAO-56 saturation vapour pressure
Formula checked against the sources above by an automated regression test (tests/test-calculators.js) that derives each expected value independently of this page. Last reviewed .
How to use this tool
- Enter your wood and climate details below, then press Calculate.
- Pick your rh input mode and application from the dropdown.
- Fill in dry-bulb temperature, relative humidity, wet-bulb temperature.
- Press Calculate.
Worked example
Common mistakes to avoid
- Chasing a 'perfect' VPD without watching the wood. VPD guides drying speed, but boards tell you if it is too fast.
- Using air temperature instead of wet-bulb data where the formula needs both.
- Ignoring airflow. The right VPD with dead air still dries unevenly.
Frequently Asked Questions
Wood species data
Density, hardness and movement for 60 timbers