This VPD calculator computes vapor pressure deficit from air temperature and relative humidity — VPD = SVP × (1 − RH), where SVP is the saturation vapor pressure at your temperature — the single number indoor growers steer their climate by.

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Why Growers Chase One Number Instead of Two

Temperature and humidity, quoted separately, lie to growers: 25 °C at 60% RH and 30 °C at 60% RH are the same "humidity" but utterly different drying power, because the air's capacity for water vapor grows exponentially with temperature (the Tetens saturation curve: SVP roughly doubles every 10 °C). VPD is the honest quantity — the difference between how much vapor the air could hold at saturation and how much it holds now, expressed in kilopascals — and it maps directly onto what the plant experiences at the leaf surface: the gradient that drives transpiration. The defaults (25 °C, 60% RH) give SVP 3.17 kPa and VPD = 1.27 kPa, at the warm edge of the vegetative band. The working grow-room targets: ~0.4–0.8 kPa for clones and seedlings, 0.8–1.2 for vegetative growth, 1.2–1.6 for flowering — the deliberate drift toward "drier" as the cycle advances, hardening the plant and protecting dense flowers from the mold that low VPD invites.

The refinement that separates air VPD from the number the plant actually lives is leaf temperature: transpiration happens at the leaf surface, whose saturation pressure is set by the LEAF's temperature, not the room's. Under modern LEDs leaves typically run about 2 °C cooler than air (no radiant heat load); the leaf VPD formula used here — SVP(leaf T) − RH × SVP(air T) — typically returns a value ~0.1–0.3 kPa LOWER than air VPD in LED rooms, which is why LED growers who steer by air VPD alone chronically over-dry their plants without knowing why. Under HPS the offset flips sign. Night matters too: lights-off temperature drops raise RH at constant moisture, VPD collapses toward the disease zone, and the standard counter is night heating or dehumidification sized to hold VPD, not a fixed RH setpoint.

Worked Examples (Verified)

  • Defaults: 25 °C, 60% → SVP 3.17 kPa, VPD = 1.27 kPa (late-veg/early-flower edge).
  • Same RH, hotter: 30 °C, 60% → SVP 4.24 kPa, VPD = 1.70 kPa — stress zone without the humidity percentage changing at all.
  • Clone dome: 24 °C, 85% → SVP 2.98, VPD = 0.45 kPa ✓ gentle, as unrooted cuttings need.
  • Leaf correction: 25 °C air, 60% RH, leaves −2 °C → leaf VPD = 2.81 − 1.90 = 0.91 kPa vs 1.27 air — a full zone apart.

VPD Discipline

  • Steer the pair, not the singles: any temperature change demands an RH change to hold VPD — controllers that chase RH alone fail every lights-on transition.
  • Measure at canopy height, inside the canopy's microclimate — a sensor on the wall reads the room you built, not the climate the leaves live in.
  • Leaf offset sign flips with lighting: LEDs ≈ −2 °C, HPS often +1 to +3 °C; an infrared thermometer reading beats any default.
  • Night VPD is the disease window: lights-off RH spikes are where botrytis is born — dehumidify or heat to hold the number.
  • VPD targets assume healthy roots and adequate water; a drought-stressed plant in "perfect" VPD is still a drought-stressed plant.

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Frequently Asked Questions

How do you calculate VPD?

Compute the saturation vapor pressure at your temperature with the Tetens formula, SVP = 0.6108 × e^(17.27T/(T+237.3)) kPa, then multiply by (1 − RH as a fraction): VPD = SVP × (1 − RH). At 25 °C and 60% RH: SVP = 3.17 kPa and VPD = 3.17 × 0.40 = 1.27 kPa. With a leaf-temperature offset, use the leaf VPD form: SVP(leaf temp) − RH × SVP(air temp). This calculator evaluates both.

What is a good VPD for growing plants indoors?

By growth stage (leaf VPD, in kPa): clones and seedlings 0.4–0.8 (gentle pull while roots are absent or tiny), vegetative growth 0.8–1.2 (brisk transpiration, strong nutrient flow), flowering 1.2–1.6 (drier air protects dense flowers and drives resin-era transpiration), and anything sustained above ~1.6 courts stress — stomata close, growth stalls, leaf edges burn — while below ~0.4 invites mold, mildew and calcium-starved new growth. The defaults on this page (1.27 kPa) sit exactly on the veg-to-flower transition, which is where many growers aim mid-cycle.

Why does the same humidity feel different at different temperatures?

Because relative humidity is relative — to a capacity that explodes with temperature. Air at 30 °C can hold roughly 60% more water vapor than air at 25 °C (SVP 4.24 vs 3.17 kPa), so 60% RH at 30 °C leaves a deficit of 1.70 kPa against 1.27 kPa at 25 °C: the hotter room pulls water from leaves a third harder at the identical humidity reading. That is the entire reason VPD exists — it converts the misleading percentage into the physical pressure difference the plant's stomata actually respond to.

What is leaf VPD and why is it different?

Leaf VPD uses the saturation pressure at the LEAF's surface temperature for the "supply" side, because evaporation happens there: leaf VPD = SVP(T_leaf) − RH × SVP(T_air). Leaves under LED lighting typically run ~2 °C cooler than the room (LEDs deliver little radiant heat), which lowers the leaf's saturation pressure and can pull leaf VPD 0.1–0.3 kPa below the air figure — a whole growth zone of difference, and the reason LED rooms often look "too humid" on paper while plants transpire perfectly. Under HPS, leaves run warm and the correction reverses. An infrared leaf reading makes it exact.

What happens if VPD is too high or too low?

Too high (dry pull): stomata close to conserve water, photosynthesis and growth stall, leaves curl ("taco"), edges crisp, and plants drink frantically while uptaking nutrients unevenly — tip burn follows. Too low (weak pull): transpiration slows so calcium (which moves only with the water stream) starves new growth, foliage stays wet longer, and powdery mildew and botrytis find their favorite climate; roots in constantly wet media compound it. The symptom pictures overlap with nutrient problems, which is why experienced growers check VPD before touching the feed recipe.

How do I raise or lower VPD in a grow room?

VPD rises when air gets hotter or drier, falls when cooler or more humid — four levers: temperature (heater/AC), moisture (humidifier/dehumidifier), and indirectly light intensity and airflow (which move leaf temperature). To raise VPD in flower: add heat or dehumidify. To lower it for clones: humidify and ease the temperature down. The pro move is a controller keyed to VPD itself (several climate controllers compute it live), because manual RH setpoints silently drift out of zone every time the temperature setpoint changes between day and night cycles.

Does VPD matter outdoors or in greenhouses?

Yes — greenhouse climate computers have steered by VPD for decades (it's a standard variable in commercial tomato and cucumber production, where it predicts both growth rate and disease pressure), and field agronomists use it to forecast irrigation demand and heat stress: midday field VPD regularly exceeds 3 kPa in arid heat, which is when crops close stomata regardless of soil moisture. Home growers adopted the greenhouse number; this calculator uses the same Tetens-based physics at any scale.

Is this VPD calculator free?

Yes — free, instant, no sign-up. Enter temperature and relative humidity (plus an optional leaf offset) for air and leaf VPD with the growth-zone reading.