VPD Calculator for Indoor Farming
Calculate Vapor Pressure Deficit from air temperature, humidity, and leaf offset. Compare your VPD against crop-specific targets by growth stage, with instant recommendations to optimize transpiration and yield. Want it on the wall instead? The same math is available as a free printable VPD chart in PDF — °F and °C, US Letter and A4.
What Is VPD?
Vapor Pressure Deficit (VPD) measures the difference between how much moisture the air currently holds and how much it could hold at saturation. In indoor farming, VPD is the primary driver of transpiration — the process by which plants pull water and nutrients through their roots, transport them through the stem, and release water vapor through stomatal pores in their leaves. A VPD that is too low (below 0.4 kPa) means the air is nearly saturated, transpiration stalls, and disease risk increases. A VPD that is too high (above 1.6 kPa) forces plants to close stomata to conserve water, halting photosynthesis and nutrient uptake. The optimal range for most indoor crops is 0.8–1.2 kPa during active growth.
How to Use This Calculator
- 1 Set your room’s air temperature and relative humidity. The leaf offset accounts for evaporative cooling — most leaves run 1–4°F below air temperature.
- 2 Choose your crop and growth stage. Each combination has a different optimal VPD range that the calculator targets.
- 3 Review the VPD result and zone classification. The heatmap shows how VPD changes across temperature and humidity combinations.
- 4 Follow the targeted recommendations to raise or lower VPD based on your current conditions.
VPD Targets by Crop & Growth Stage (kPa)
| Crop | Stage | Low | High |
|---|---|---|---|
| Butterhead | Propagation | 0.4 | 0.8 |
| Butterhead | Grow-out | 0.8 | 1.2 |
| Butterhead | Pre-Harvest | 1.0 | 1.3 |
| Romaine | Propagation | 0.4 | 0.8 |
| Romaine | Grow-out | 0.8 | 1.2 |
| Romaine | Pre-Harvest | 1.0 | 1.3 |
| Basil | Propagation | 0.4 | 0.8 |
| Basil | Vegetative | 0.9 | 1.3 |
| Basil | Harvest | 1.0 | 1.4 |
| Arugula | Propagation | 0.4 | 0.8 |
| Arugula | Grow-out | 0.8 | 1.1 |
| Arugula | Harvest | 0.9 | 1.2 |
| Kale | Propagation | 0.4 | 0.8 |
| Kale | Grow-out | 0.8 | 1.2 |
| Kale | Harvest | 1.0 | 1.3 |
| Microgreens | Germination | 0.2 | 0.5 |
| Microgreens | Greening | 0.5 | 0.9 |
| Microgreens | Harvest | 0.6 | 1.0 |
| Strawberry | Establish | 0.5 | 0.9 |
| Strawberry | Flower | 0.9 | 1.3 |
| Strawberry | Fruit | 1.0 | 1.5 |
| Tomato | Transplant | 0.4 | 0.8 |
| Tomato | Vegetative | 0.9 | 1.3 |
| Tomato | Flower | 1.0 | 1.5 |
| Tomato | Fruit | 1.2 | 1.6 |
| Cucumber | Transplant | 0.4 | 0.8 |
| Cucumber | Vegetative | 0.8 | 1.2 |
| Cucumber | Fruit | 1.0 | 1.5 |
| Pepper | Transplant | 0.4 | 0.8 |
| Pepper | Vegetative | 0.9 | 1.3 |
| Pepper | Flower | 1.0 | 1.5 |
| Pepper | Fruit | 1.2 | 1.6 |
Printable VPD Chart — Free PDF Download
This calculator answers one reading at a time. For the wall next to the room, the same Tetens math is drawn as a printable VPD chart pack: twelve PDFs, ungated, with every cell printing its own value in kPa. Each pack is five pages — four wall charts, one per leaf-temperature offset (0, −1, −2 and −3 °C), plus a reference sheet.
- Cannabis & general
- Banded on the familiar 0.4 / 0.8 / 1.2 / 1.6 kPa spine: clones and seedlings, vegetative, flower, and the two zones you do not want to be in.
- Greenhouse produce
- Tomato, cucumber, pepper and strawberry. Built for fruiting crops, which want the higher deficit that carries calcium into the fruit.
- Leafy greens & herbs
- Butterhead, romaine, arugula, kale, basil and microgreens. Closes out at 1.40 kPa rather than 1.60 — above roughly 1.3 kPa lettuce buys tipburn, not growth.
Every edition ships in °F and °C, for US Letter and A4 portrait. No email, no form — free to print, post, photocopy and share.
What Leaf Temperature Offset Should You Use?
The offset field above is the one input growers guess at, and it is the input that moves the answer most. A plant does not transpire against the air’s temperature — water evaporates from inside the leaf, so the pressure driving it out is the saturation pressure at leaf temperature. At 25 °C and 60% RH, each 1 °C of offset moves VPD by about 0.18 kPa, which is a larger error than reading humidity wrong by five points.
| Situation | Offset (°C) | Offset (°F) | Why |
|---|---|---|---|
| Lights off / dark period | 0 to −0.5 | 0 to −0.9 | No radiant load. The leaf sits at, or a little under, air temperature. |
| Low-intensity LED, gentle airflow | −0.5 to −1.5 | −0.9 to −2.7 | Modest transpiration; the leaf runs slightly cool. |
| Full-intensity LED, good airflow | −1.5 to −2.5 | −2.7 to −4.5 | The common case in a working indoor room. Start here. |
| High airflow, dry air, open canopy | −2.5 to −3.5 | −4.5 to −6.3 | Vigorous transpiration pulls leaf temperature well below air. |
| HPS / high radiant load | −1.0 to +2.0 | −1.8 to +3.6 | Radiant heat can push a leaf above air temperature. Measure, do not assume. |
| Water stress, closed stomata | 0 to +3.0 | 0 to +5.4 | A leaf that has stopped transpiring stops cooling itself. |
Measure your own with an infrared thermometer set to emissivity 0.95: aim at a fully expanded, well-lit leaf from about 30 cm, slightly off perpendicular, average five leaves mid-photoperiod, and subtract air temperature at canopy height. Avoid the growing point and leaf edges — both run warm. The printable chart pack ships a separate sheet for each offset.
Frequently Asked Questions
Is there a printable VPD chart I can download?
Yes — a free printable VPD chart pack in PDF, with no email required. Twelve files: three crop editions (cannabis and general, greenhouse produce, leafy greens and herbs), each in °F and °C, each for US Letter and A4. Every file is five pages: four wall charts at leaf-temperature offsets of 0, −1, −2 and −3 °C, plus a reference sheet with the crop targets and the equation.
What is a good VPD for lettuce?
Most lettuce varieties (butterhead, romaine, leaf) do best at 0.8–1.2 kPa during grow-out. During propagation, keep VPD lower at 0.4–0.8 kPa to reduce stress on young seedlings. Drop slightly before harvest to keep leaves turgid.
How does leaf temperature offset affect VPD?
Leaves are typically 1–4°F cooler than air due to evaporative cooling from transpiration. This offset lowers the saturation vapor pressure at the leaf surface, which changes the effective VPD. A larger offset means a lower leaf VPD. Use an IR thermometer to measure your actual leaf offset for best accuracy.
What happens when VPD is too low?
When VPD drops below 0.4 kPa, the air is nearly saturated. Transpiration slows dramatically, which reduces nutrient transport through the plant. Condensation can form on leaf surfaces, creating ideal conditions for fungal diseases like botrytis and powdery mildew. Dehumidify, increase airflow, or raise temperature to increase VPD.
What happens when VPD is too high?
Above 1.6 kPa, plants close stomata to prevent excessive water loss. This halts both transpiration and photosynthesis, limiting growth and potentially causing wilting, tip burn, or leaf edge necrosis. Add humidity, lower temperature, or reduce light intensity to bring VPD down.
How is VPD calculated?
VPD is the difference between the saturation vapor pressure (SVP) at a given temperature and the actual vapor pressure (AVP) of the air. This calculator uses the Tetens equation: SVP = 0.6108 × exp((17.27 × T) / (T + 237.3)), where T is temperature in Celsius. Leaf VPD uses the leaf surface temperature instead of air temperature for the SVP calculation.
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