Room Humidifier Size & Runtime Calculator
Enter the room size, indoor humidity, outdoor air, infiltration, and tank size to estimate the humidifier rating needed and how often the tank may need refilling.
Recommended humidifier rating (gal/day)1.79
- Room air volume
- 2,400
- Initial water addition (gal)
- 0.05
- Minimum maintenance output (gal/day)
- 1.34
- Minimum output for deadline (gal/day)
- 1.49
- Recovery time at recommended rating
- 2.7 hours
- Tank runtime during recovery
- 13.4 hours
- Tank runtime after reaching target
- 17.9 hours
Humidity balance details
| Quantity | Value |
|---|---|
| Room volume (m3) | 67.96 |
| Current indoor humidity ratio (kg/kg dry air) | 0 |
| Target indoor humidity ratio (kg/kg dry air) | 0.01 |
| Outdoor humidity ratio (kg/kg dry air) | 0 |
| Indoor dry-air mass (kg) | 81.02 |
| Outdoor dry-air density (kg/m3) | 1.32 |
| Initial water addition (kg) | 0.19 |
| Maintenance load (kg/hr) | 0.21 |
| Recommended output (kg/hr) | 0.28 |
Assumes standard sea-level pressure and steady room, outdoor, and infiltration conditions.
Recommended rating is 20% above the minimum deadline-based output.
How to use this calculator
- Measure the room floor area and use an average ceiling height for the air volume.
- Enter the current and target indoor relative humidity at the same indoor temperature.
- Use typical outdoor conditions for the time period the humidifier must handle.
- Choose an air-change rate and recovery deadline, then compare the required rating with the tank runtimes.
How the humidifier size is calculated
This calculator treats the room as an air volume that must be brought from its current humidity ratio to the target humidity ratio, while outdoor air leaking into the room keeps removing moisture. That is why the result can be larger than a simple square-footage chart: a drafty room on a cold day may need much more water than the same room with little air leakage.
The calculation uses saturation vapor pressure from ASHRAE psychrometric equations. For each air state, relative humidity is converted to vapor pressure and then to humidity ratio:
W = 0.621945 * pv / (P - pv)
The room volume is A * h. Dry-air mass in the room is estimated from standard pressure, indoor temperature, and vapor pressure. The one-time recovery water is:
I = max(0, dry-air mass * (Wtarget - Wcurrent))
Infiltration is handled separately. Outdoor dry-air density is calculated at the outdoor temperature and humidity. The continuing moisture load is:
L = max(0, ACH * room volume * outdoor dry-air density * (Wtarget - Woutdoor))
The minimum hourly output is the maintenance load plus the recovery water spread over the requested deadline: Cmin = L + I / H. The recommended gallons-per-day rating is 1.20 * Cmin, converted from kilograms per hour to gallons per day. The 20% margin is shown directly so you can separate the physics from the selection allowance.
What moves the answer most
Air changes per hour often dominate the result because infiltrating outdoor air is a continuous load. Outdoor temperature also matters: cold air can show high relative humidity but still contain little water. The recovery deadline mainly affects how fast the room catches up from a dry start; after the target is reached, the maintenance load controls refill frequency.
What this calculator leaves out
The estimate assumes standard sea-level pressure and steady mixed air. It does not model moisture from people, cooking, showers, plants, or laundry, and it does not calculate window or wall surface temperatures. If condensation appears, lower the humidity setting even when the moisture balance says the target is reachable.
Worked example
For a 300 sq ft room with an 8 ft ceiling, the air volume is 2,400 cubic feet. At 70 F, raising the room from 25% RH to 40% RH requires about 0.05 gallons of initial water in the air. With 20 F outdoor air at 70% RH and 0.5 ACH, the continuing maintenance load is about 1.34 gallons per day.
With an 8 hour recovery deadline, the minimum deadline-based output is about 1.49 gallons per day. Adding the 20% sizing allowance gives a recommended rating of about 1.79 gallons per day. A 1 gallon portable tank would last about 13.4 hours during full-output recovery and about 17.9 hours after the room reaches the target, before allowing for manufacturer rating differences or wick performance.
Common questions
What size humidifier do I need for my room?
Use the recommended gallons-per-day result as the rating to compare with manufacturer capacity. It includes the water needed to raise the room from the current humidity to the target and the continuing moisture lost through air leakage.
Why is humidifier capacity listed in gallons per day?
Portable and console humidifiers are commonly rated by how much water they can evaporate in 24 hours. The actual room demand is calculated hourly here, then converted to gallons per day so it matches product labels.
Why does outdoor temperature affect humidifier sizing?
Relative humidity is temperature dependent. Cold outdoor air can be 70% RH and still contain very little water, so when that air warms indoors it can pull moisture from the room unless the humidifier replaces it.
What air-change rate should I enter?
Use a measured air-change rate if you have one. A closed, tight room may be below 0.5 ACH, while open doors, exhaust fans, fireplaces, and leaky construction can push the value higher.
How often will I need to refill the tank?
During recovery, tank runtime is based on the recommended full-output rate. After the room reaches the target, runtime is based only on the maintenance load, so it is usually longer unless the room is very leaky.
Is 40% indoor humidity safe during winter?
Often it is comfortable, but safety depends on window quality, wall insulation, outdoor temperature, and hidden cold surfaces. EPA moisture guidance warns against sustained high indoor humidity because condensation and damp materials can support mold growth.