Dehumidifier Sizing Calculator (Grow Room & Drying Room)

Q: What is PPD and why does it matter?

PPD stands for pints per day and is the standard capacity rating for dehumidifiers. Size your unit so its nameplate PPD can remove your room’s daily moisture load after accounting for run hours and real-world efficiency.

Q: How do I estimate grow room moisture?

Plants transpire about 97% of what they drink. Convert daily gallons to pints (×8), add any ventilation load based on ACH and RH gap, and subtract A/C condensate removal.

Q: How do I size for drying rooms?

Fresh material is roughly 80% water and 60–70% of that moisture leaves in the first few days. Size for that peak window, then scale to your run hours/day and efficiency.

Q: What PPD should I target?

Use the calculator’s Required Nameplate. As a quick guide: Small ≤70 PPD, Medium ≤120 PPD, Large ≤190 PPD, Commercial >190 PPD.

Q: Do I need extra capacity for lights-off spikes?

Yes—cooler air holds less moisture, so RH rises when lights turn off. Add a buffer (e.g., 15–30%) or use the calculator’s Lights-Off Spike Capacity.

Size dehumidifiers with confidence. Estimate daily PPD from plant transpiration or a harvest batch, then scale for real-world efficiency and run windows. Prefer help? Talk to a Grow Expert.

Shop Dehumidifiers Open Calculator Read the Full Guide

Dehumidifier Sizing (Grow Room & Drying Room)

Tip: Plants transpire ~97% of what they drink. Start with daily watering to estimate PPD (pints/day). Add headroom for lights-off spikes.

Preset (optional)

Also seeds default Grow inputs.

Length (ft)

Width (ft)

Height (ft)

Plants (qty)

Gallons per plant per watering

Days between wateringSoil every other day? Enter 2. Hydro: use weekly/7 rule.

Transpiration fraction (0–1) i Share of irrigation that returns to air via plants. 0.97 is a common baseline.

A/C condensate removal (PPD) i If your A/C removes humidity (condensate), it offsets dehu load. Enter average pints/day.

Dehumidifier run window (hrs/day) i If you don't run 24/7, nameplate capacity must cover daily moisture within this window.

Lights-off hours/day

Spike buffer during lights-off (%) i Extra headroom to handle RH spikes when lights go off (cool air holds less moisture).

Device efficiency (% effective)Real output can be below nameplate.

Optional ventilation load
Estimate extra moisture to remove from fresh-air exchange using ACH and RH gap.

Air Temperature (°F)

Ventilation (ACH) i ACH = air changes per hour. Exhaust or intake cycles increase moisture exchange.

Extra loss (%)Leaky rooms, frequent purges, etc.

Ambient RH (%) i Incoming or current RH before control.

Target RH (%)

1 gal = 8 pints PPD = Pints / Day

Wet weight (lbs) i Total fresh biomass entering the room.

% water in fresh material Typical ~80%

% of water leaving in first days i Share of total water that evaporates during the initial drying window (peak load). 60–70% is common.

Peak window (days) i Length of the first high-moisture phase you want to size for.

Dehumidifier run window (hrs/day)

Device efficiency (% effective)

A/C condensate removal (PPD)

Target RH (%) Dry low & slow ≈ 60°F / 60% RH

1 gal water ≈ 8.34 lb 1 gal = 8 pints

How to use this calculator (step-by-step)

Pick a view: Grow Room (plant transpiration) or Drying Room (harvest batch).
Grow → enter plants, gal/plant, days between watering; add optional ACH & RH gap in Advanced.
Drying → enter wet weight, % water, % leaving in first days, and peak window (days).
Set run hours/day, device efficiency, and any A/C condensate to scale nameplate PPD.
Click Calculate → use Required Nameplate and Suggested Class to choose a unit.

Rule of thumb: Water in ≈ water out. Soil every other day → divide by 2. Hydro → base on weekly change and divide by 7.

Shop Dehumidifiers Read the Full Guide

Frequently Asked Questions

What is PPD and why does it matter?

PPD stands for pints per day and is the standard capacity rating for dehumidifiers. Size your unit so its nameplate PPD can remove your room’s daily moisture load after accounting for run hours and real-world efficiency.

How do I estimate grow room moisture?

Start with irrigation: plants transpire ~97% of what they drink. Convert daily gallons to pints (×8). Add optional ventilation load (ACH × room volume × RH gap) and subtract any A/C condensate removal.

How do I size for drying rooms?

Fresh material is ~80% water. Roughly 60–70% of that leaves in the first few days—size your dehumidifier for that peak window, then scale to your run hours/day and efficiency.

What PPD should I target?

Use the calculator’s Required Nameplate. As a quick guide: Small ≤70 PPD, Medium ≤120 PPD, Large ≤190 PPD, Commercial >190 PPD. Multiple units improve redundancy and distribution.

Do I need extra capacity for lights-off spikes?

Yes—cooler air holds less moisture, so RH rises at lights-off. Add a buffer (e.g., 15–30%) or use the calculator’s Lights-Off Spike Capacity to ensure stable RH.

Shop Dehumidifiers Read the Full Guide

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Dehumidifier Sizing Calculator (Grow Room & Drying Room)

Dehumidifier Sizing (Grow Room & Drying Room)

Frequently Asked Questions

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