Cannabis VPD calculator for your grow tent.
You have a reading in hand — say 77°F and 60% at the canopy — and one question: is that right for where the plant is, and if not, what do I change? Pick your stage, enter the numbers your canopy sensor shows, and get VPD in kPa plus the single move that puts you back in range.
Air VPD
1.27 kPa Too high
Vegetative target: 0.80–1.20 kPa
0.07 kPa above the band. Raise RH to about 65% (from 60%), or drop temperature about 3.5°F.
1.27 air VPD 1.27 leaf VPD dew point 62.0°F
How to use it
- Pick the grow stage the plant is in — the target band updates with it.
- Enter the temperature and humidity from a sensor at canopy height, lights on.
- Read the big number and the line under the gauge — it names the single fastest change, usually a target humidity.
Target bands this calculator uses
These are the bands the calculator judges against. They hold whether JavaScript is on or off.
| Stage | VPD (kPa) | Relative humidity | Temperature |
|---|---|---|---|
| Seedling / clone | 0.40–0.80 kPa | 70–85% RH | 68–77°F / 20–25°C |
| Vegetative | 0.80–1.20 kPa | 55–70% RH | 72–82°F / 22–28°C |
| Early flower (wk 1–5) | 1.00–1.30 kPa | 50–60% RH | 72–79°F / 22–26°C |
| Late flower (wk 6+) | 1.20–1.50 kPa | 40–50% RH | 68–77°F / 20–25°C |
Below 0.4 kPa the canopy stays wet and mold pressure climbs; past about 1.6 kPa the plant starts to stress, and near 2.0 kPa it closes its stomata. For the full temperature-versus-humidity grid, see the full VPD chart for cannabis by stage.
How the calculation works
Every reading here runs through the Tetens equation for saturated vapor pressure — the same formula behind our printed VPD chart — but solved for your exact temperature and humidity instead of snapping to the nearest grid cell, and shown to two decimals because the band edges (0.80, 1.20) sit right on the tenths.
Air VPD uses your air temperature straight from the sensor: SVP(air) × (1 − RH/100). Leaf VPD subtracts your chosen offset from the air temperature to estimate the cooler leaf surface, then compares it against the air's moisture load: SVP(leaf) − (RH/100) × SVP(air). Set the offset to 0 and the two numbers are identical.
The fix line then targets a point just inside your stage's band and solves the humidity change exactly, because VPD moves in a straight line with relative humidity; the temperature alternative is found by scanning, so it stays honest even with a leaf offset in play.
Air VPD = SVP(Tair) · (1 − RH/100)
Leaf VPD = SVP(Tleaf) − (RH/100) · SVP(Tair)
24°C / 60% = 1.19 kPa
25°C / 60% = 1.27 kPa
26°C / 65% = 1.18 kPa
Air VPD vs leaf VPD
Air VPD uses the air temperature your sensor reports. Leaf VPD assumes the leaf runs a little cooler than the surrounding air under strong LED light — because transpiration pulls heat off the surface — and is the more physically correct number to steer by. The gap is small in veg and grows in late flower, when you are pushing the plant hardest.
Leave the offset at 0 to match the air-based chart on this site, or set −2°C to match the leaf-temperature charts many growers and controllers use. The only way to know your true offset is to measure the leaf directly — a clip-on leaf probe or an infrared thermometer does it, and a reliable canopy-height thermo-hygrometer is the first instrument to get right.
Calculator questions
What's the difference between a VPD chart and a VPD calculator?
A chart is a lookup: you find the cell where your temperature and humidity meet and read the nearest value. This calculator solves the same Tetens equation for your exact numbers, to two decimals, then compares the result against your stage's band and tells you which way to move humidity or temperature to get in range. Use the calculator to make a change now; use the full VPD chart for cannabis when you want to see the whole temperature-versus-humidity grid at a glance.
My grow controller shows a different VPD. Which one is right?
Two calculators usually disagree over one thing: leaf temperature. This tool shows air VPD by default (offset 0), while many controllers bake in a fixed 1–2°C leaf offset, which lowers the number. Set the same offset here and the two line up to within rounding. Any gap left over comes from sensor placement and calibration — trust the instrument actually sitting in your canopy over any single formula, and match this tool's offset to your controller's before comparing.
Should I run the numbers for lights-on or lights-off?
Lights-on. VPD matters while the stomata are open and the plant is transpiring, so enter your canopy reading during the lit period. Lights-off temperature drops and humidity rises, which lowers VPD — that overnight dip is expected and fine unless it falls below about 0.4 kPa for long stretches, where a wet canopy invites mildew. Steer by the lit-period average, and only worry about the dark period if condensation is forming.
How accurate is this, and does it work without JavaScript?
The math is exact to the Tetens formula; the accuracy limit is your input. A canopy sensor a few degrees off, or placed on the tent wall instead of at the plants, shifts VPD by 0.1–0.3 kPa — more than the tool's own rounding. Leaf VPD is an estimate unless you have a leaf or infrared thermometer for the true offset. And yes, the page works without JavaScript: the stage-target table, the formula, and the link to the full grid all load with scripts off — the live calculator is layered on top, not a gate in front.
For the deeper conceptual questions — what VPD physically measures, what happens when it runs too low or too high — see the deeper VPD FAQ on the chart page.
A calculator is only as good as the reading you feed it.
The number this tool returns is exact for the temperature and humidity you type in — so the weak link is the sensor, not the math. A dependable thermo-hygrometer placed at canopy height, not clipped to the tent wall, is what makes any VPD figure trustworthy. Leaf-surface temperature, for true leaf VPD rather than the air-based estimate, is a separate measurement and a step beyond a basic hygrometer. Start with a reliable canopy-height thermo-hygrometer and get its placement right before chasing tenths of a kPa.