Grow Room Humidifier Sizing Calculator

Grow Room Humidifier Sizing Calculator

Use this free grow room humidifier calculator to size the right unit for your target relative humidity. Start with room dimensions, temperature, air changes per hour (ACH), and ambient vs. target RH. Switch to Advanced to account for plant transpiration, any running dehumidifier, and extra loss from leaks or purge cycles.

Shop Humidifiers Grow Room Humidifier Guide

Humidifier Sizing Calculator

Size a humidifier to maintain your target RH given room size, temperature, and ventilation (ACH). Advanced mode accounts for plant moisture add, dehumidifier removal, and extra loss.

Tip: Humidifier load is driven by ACH and the gap between ambient RH and your target RH. Plants add moisture; dehumidifiers remove it.
Advanced adds/subtracts internal moisture sources
Plants add; dehumidifiers remove. Also account for leaks/purges (extra loss %).
Share of irrigation released to air by plants.
Leaky rooms, frequent purges, intake cracks, etc.
Real output vs nameplate after placement/controls.
1 L ≈ 2.11 pints ACH = Air Changes / Hour
How to use this calculator (step-by-step)
  1. Enter room size or pick a preset.
  2. Set temperature, ACH, ambient RH, and target RH.
  3. Advanced: add plants (transpiration), any running dehumidifier, and extra loss %.
  4. Click Calculate to see liters/hour, pints/day, and a suggested class.
  5. Select a humidifier with capacity ≥ Net need within your run window and efficiency.
Core math: We compute humidity ratio (kg/kg) from RH & temp, multiply by dry-air flow (ρ×Volume×ACH) to get kg/h water. Converted to L/h and PPD; then adjusted for plant add and dehu removal.
Humidifier sizing FAQs
What is ACH and why does it matter?

ACH stands for air changes per hour. Higher ACH brings in more fresh air at ambient RH, which increases the moisture your humidifier must add to reach your target RH.

How do plants affect humidifier sizing?

Plants transpire moisture into the air, offsetting some of the humidifier load. In Advanced mode, enter plants, irrigation per plant per day, and transpiration fraction to subtract this add-back.

What’s the difference between L/h and PPD?

L/h is liters of water per hour (instantaneous rate). PPD is pints per day (daily total). 1 liter ≈ 2.113 pints.

How do I account for a dehumidifier that’s also running?

Enter its actual daily removal (PPD) at your setpoint. The calculator converts to L/h and adds that to the required humidifier load.

What efficiency should I use?

If placement or controls limit output, use 80–90% as a realistic effective range. Lower efficiency increases the nameplate capacity you’ll need.

Can I run my humidifier less than 24 hours?

Yes. Set your daily run window (e.g., 12 hours). The required nameplate L/h will scale up so you can still meet the 24‑hour average need.

Does temperature change my humidifier need?

Yes. Warmer air holds more moisture. The calculator uses temperature to compute saturation vapor pressure and humidity ratio.

Why is my required L/h higher than the product’s spec?

Manufacturers quote ideal conditions. Real‑world efficiency, ducting, and cycling reduce output. Use the efficiency field to account for this.