VPD Calculator
Dial in vapor pressure deficit — the single best indicator of whether your grow room climate is letting plants transpire, drink, and eat properly. Enter temperature and humidity, get your current VPD and the right target range for your plants' stage.
Calculate your current VPD
Leaf offset matters
Leaves almost never sit at air temperature. Under LED they run 2–3°F cooler; under HPS they run 1–2°F warmer. Using air-only VPD will mislead your climate decisions by 0.1–0.2 kPa.
Your reading is
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Enter temperature and humidity to see guidance.
Common VPD mistakes
- Reading at ceiling height — canopy conditions differ 3–8°F and 10–20% RH. Always measure at canopy.
- Ignoring leaf temp — leaves run 2–3°F cooler under LED. Air-only VPD misleads by 0.1–0.2 kPa.
- Changing temp and humidity together — you won't know which variable actually moved VPD. One at a time.
- Not waiting after adjustments — rooms take 20–30 minutes to reach equilibrium. Don't chase readings.
Current VPD
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kPa
Enter values to see status.
Four steps to a room that breathes right
Measure air temperature and humidity at canopy height
Don't use the temp reading from a controller mounted near the ceiling or the floor — conditions at your plants' canopy can be 3–8°F and 10–20% RH different. Put your thermometer/hygrometer at canopy height, in the airflow path, away from the light's direct heat. Let it stabilize for at least 10 minutes before reading.
Enter the readings into the calculator
Type your air temperature and relative humidity. The default leaf temperature offset (−2°F) assumes standard LED lighting with decent airflow — accurate for most home setups. If you're running HPS, HID, or a different lighting style, switch to Pro mode and adjust the offset.
Compare your VPD to your plant's stage target
The meter shows your current VPD and the green zone for vegetative growth by default. In Pro mode, select your actual stage — clones want much lower VPD than late flower. If your pointer is inside the green zone, you're dialed. If it's outside, use the guidance below the meter to adjust.
Adjust temperature or humidity — not both
Change one variable at a time so you can see what's actually moving the VPD. Raising temperature by 2°F shifts VPD about 0.1 kPa higher. Dropping humidity by 10% shifts VPD about 0.2–0.3 kPa higher. Wait 30 minutes after any adjustment before re-reading — your room needs time to reach equilibrium.
The leaf temperature trap
VPD charts that ignore leaf temperature will tell you to target air-temp-based numbers, which will systematically push your environment 0.1–0.2 kPa away from what your plants actually experience. Under LED, leaves run cooler than air — so your real VPD is lower than a simple air-VPD chart suggests. Always account for leaf offset, especially when dialing in flower.
Quick adjustment guide
VPD too low? Raise temperature, run dehumidifier, increase exhaust fan speed.
VPD too high? Lower temperature, run humidifier, reduce exhaust fan speed, add a wet towel or water tray.
Why VPD matters more than temperature or humidity alone
Vapor pressure deficit measures the drying power of the air around your plants. It's the difference between how much water vapor the air currently holds (relative humidity) and how much it could hold at a given temperature. That gap is what pulls water out of your plants' leaves — and with it, nutrients up from the roots.
How plants drink
Plants don't actively pump water. They open stomata on their leaves, water evaporates into the surrounding air, and the negative pressure this creates pulls more water up from the roots — carrying dissolved nutrients with it. This is transpiration, and it's the entire engine of nutrient uptake. Too little VPD and transpiration stalls: roots stop pulling, nutrients accumulate in the medium, plants droop from root rot and salt buildup. Too much VPD and plants panic: stomata close to conserve water, transpiration stops entirely, nutrients stop moving, and the plant stresses.
The formula
VPD is calculated from saturation vapor pressure at the leaf's surface minus the actual vapor pressure in the surrounding air. The industry-standard Tetens equation gives saturation vapor pressure in kPa:
SVP(T) = 0.6108 × exp((17.27 × T) / (T + 237.3))
VPD = SVP(leaf) − SVP(air) × (RH ÷ 100)
The leaf's temperature almost always differs from air temperature. Under LED lighting, leaf temp typically runs 2–3°F cooler than air. Under HPS or HID, leaves can run 1–2°F warmer. Using air temperature in place of leaf temperature introduces 0.1–0.2 kPa of error — enough to push you outside your target range without knowing it. This calculator applies a configurable leaf offset to give you the honest number.
VPD reference table (standard −2°F leaf offset)
Target VPD by temperature and humidity, assuming a standard LED leaf offset of −2°F. These values match what this calculator produces.
| Air Temp °F | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH |
|---|---|---|---|---|---|
| 68 | 1.00 | 0.83 | 0.67 | 0.50 | 0.33 |
| 72 | 1.12 | 0.94 | 0.75 | 0.56 | 0.37 |
| 75 | 1.22 | 1.02 | 0.82 | 0.61 | 0.41 |
| 78 | 1.33 | 1.11 | 0.89 | 0.67 | 0.44 |
| 82 | 1.48 | 1.24 | 0.99 | 0.74 | 0.50 |
| 85 | 1.60 | 1.34 | 1.07 | 0.80 | 0.54 |
| 88 | 1.74 | 1.45 | 1.16 | 0.87 | 0.58 |
Stage-by-stage targets
0.4 – 0.8
Clones & Seedlings
Weak roots can't supply much water. Keep transpiration low by running a humid environment. Propagation domes and small enclosures maintain these levels naturally.
0.8 – 1.2
Vegetative
Fastest growth window. Plants have established root systems and full leaf area to transpire. Moderate VPD drives strong nutrient uptake without stressing the plant.
1.2 – 1.6
Early Flower
Transition to bloom needs stronger transpiration to move phosphorus and potassium. Raising VPD slightly also signals the plant to shift resources from leaf to flower production.
1.4 – 1.8
Late Flower & Ripening
Drier environment reduces mold and botrytis risk as buds pack on density. Plants slow transpiration naturally as they approach harvest, so slightly higher VPD tolerances work.
Accuracy and practical limits
VPD calculation accuracy depends mostly on the accuracy of your inputs. A hygrometer with ±3% RH tolerance (common for budget units) can shift calculated VPD by 0.05–0.1 kPa — enough to matter at the edges of your target range. Invest in a quality combo thermometer/hygrometer, calibrate annually with a salt solution, and measure at canopy height.
VPD targets are also strain-dependent. Sativa-dominant strains tolerate higher VPD (drier conditions) than indica-dominant strains. If you're running genetics from a specific breeder, check whether they publish VPD recommendations — the ranges above are good defaults but not universal.
Go deeper: Read our full guide — What is VPD? The ideal vapor pressure deficit chart for cannabis growers
VPD questions growers actually ask
What is VPD and why does it matter?
Vapor pressure deficit measures how much water the air could hold compared to how much it currently holds. That gap is what pulls water out of your plants' leaves through transpiration — and transpiration is what drives nutrient uptake through the roots. VPD matters because it's the single best indicator of whether your climate is letting your plants drink and eat properly. Temperature and humidity on their own don't tell you the full story; VPD combines them into one number that reflects what your plants actually experience.
What's a good VPD for cannabis?
Target ranges by stage: clones and seedlings 0.4–0.8 kPa, vegetative 0.8–1.2 kPa, early flower 1.2–1.6 kPa, late flower 1.4–1.8 kPa. Exact targets depend on strain — sativas tolerate higher VPD than indicas. Stay inside your stage's range and plants grow and drink steadily; drift outside and you'll see stalled growth (low VPD) or wilting and leaf curl (high VPD).
How do I raise my VPD?
Higher VPD means drier air. Raise temperature, lower humidity, or both. Practical options: run a dehumidifier, increase exhaust fan speed to pull out humid air, raise the heat setpoint 2–3°F, or reduce your number of active plants (each plant transpires water back into the air). Change one variable at a time and wait 30 minutes before re-reading — rooms take time to equilibrate.
How do I lower my VPD?
Lower VPD means more humid air. Lower temperature, raise humidity, or both. Practical options: run a humidifier (ultrasonic or evaporative), reduce exhaust fan speed to retain moisture, put wet towels or water trays in the room, or drop the temp setpoint 2–3°F. In sealed rooms you can also add a CO2 system, which allows running higher humidity without negative effects.
What's the difference between air VPD and leaf VPD?
Air VPD assumes the leaf temperature equals the surrounding air temperature. Leaf VPD accounts for the fact that leaves almost always run at a different temperature than air — cooler under LED (by 2–3°F), warmer under HPS or HID (by 1–2°F). Leaf VPD is what the plant actually experiences, so it's the number that matters for climate decisions. The difference between air VPD and leaf VPD is typically 0.1–0.2 kPa, which is enough to push you outside your target range if you're not accounting for it. This calculator uses leaf VPD by default.
What happens if my VPD is too low?
Low VPD means the air is too humid for your plants' current stage. Transpiration slows or stops, which means nutrient uptake slows or stops. Symptoms include stalled growth, leaf droop, pale new growth, and increased risk of powdery mildew, bud rot, and fungal pathogens. In extreme cases (flower with VPD under 0.8 kPa), you're setting up a botrytis outbreak. Raise temperature, lower humidity, or improve airflow.
What happens if my VPD is too high?
High VPD means the air is too dry. Plants close their stomata to prevent dehydration, which halts transpiration and nutrient uptake entirely. Symptoms include leaf curl (especially tips curling up or down), wilting despite wet medium, tip burn, and stalled growth. Prolonged high VPD stresses the plant and can trigger hermaphroditism in flower. Lower temperature, raise humidity, or reduce ventilation.
What tools do I need to manage VPD?
At minimum: a quality combo thermometer/hygrometer positioned at canopy height. Budget units with ±3% RH tolerance are usable but introduce error; research-grade units (±1%) are worth it for serious growers. For active VPD control: a humidifier, a dehumidifier, and an exhaust fan with variable speed. Smart environmental controllers like AC Infinity Controller 69 Pro, TrolMaster Hydro-X, or Inkbird IHC-200 automate the adjustments and log VPD over time.
Gear to dial in and maintain your VPD
Calculating VPD is free. Controlling it takes the right equipment. Start with accurate sensors so you know where you are, add active control gear for the direction you need to move, and automate with a smart controller once you're running multiple rooms or cycles.
Climate Controllers
AC Infinity, TrolMaster, and smart controllers that monitor temp/RH and automatically run your fans, humidifiers, and dehumidifiers to hit target VPD.
Temp & Humidity Meters
Combo thermometers and hygrometers — the input to every VPD calculation. Accuracy here determines accuracy everywhere downstream.
Humidifiers & Dehumidifiers
The active side of VPD control. Humidifiers lower VPD for veg and propagation; dehumidifiers raise it for flower and prevent bud rot.
Inline Fans & Ventilation
Variable-speed exhaust fans let you adjust how fast humid air gets pulled out. Paired with a controller, they're the fastest lever for VPD correction.
Circulation Fans
VPD assumes uniform canopy conditions. Oscillating and clip fans move air through leaves, preventing the humid microclimates where mold and mildew start.
CO₂ Systems
In sealed rooms, CO₂ enrichment lets you run higher VPD targets without stressing plants — unlocking faster growth at tighter climate tolerances.