Dehumidifier Calculator

VPD and Transpiration

Vapor Pressure Deficit is the difference between how much moisture the air can hold and how much it currently holds. Higher VPD means drier air, which pulls more water from leaves — up to a point. Above ~1.5 kPa, stomata close defensively, reducing transpiration.

Leaf Area Index (LAI)

LAI represents total leaf surface area relative to ground area. Seedlings have an LAI of ~0.5, while mid-flower plants can reach 5.0 — meaning 10x more leaf surface driving transpiration.

Light-Driven Transpiration

Light energy opens stomata and powers the transpiration process. More light = more transpiration. During dark periods, transpiration drops to roughly 10% of daytime rates.

What size dehumidifier do I need?

It depends on room size, number of plants, growth stage, and VPD. A 4x4 tent with 4 plants in mid-flower typically needs a 30-50 pint unit. Larger rooms or more plants may need 70+ pint commercial units.

Why is humidity harder to control in flower?

Plants transpire more during flowering due to larger leaf area (higher LAI) and heavier water uptake driven by VPD. A plant in mid-flower can transpire 3-5x more than a seedling.

Does VPD affect transpiration?

Yes — VPD is the primary driver. Higher VPD means more transpiration, but above ~1.5 kPa stomata begin closing as a defense mechanism, which actually reduces transpiration. The sweet spot depends on growth stage.

Should I use a sealed room?

Sealed rooms retain all plant-produced moisture, requiring more dehumidification. But they also allow CO₂ supplementation and eliminate pest pathways. If sealed, adequate dehumidification is critical.