Humidifier CFM Calculator
Duct humidifier capacity from airflow and target RH gain.
Enter your values
Enter the system airflow in CFM.
Enter the grains of moisture to add per lb air in grains/lb.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Estimated output needed
- 370.3 gallons/day
- Grains added per minute
- 1800
- Suggested unit class
- Steam humidifier
Quick answer
Duct humidifiers are sized in gallons per day based on system CFM, outdoor design temperature and the target indoor relative humidity.
GPD approx = CFM x grains removed x 60 / 7000 / 24
At a glance
| What it does | Duct humidifier capacity from airflow and target RH gain. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | System airflow, Grains of moisture to add per lb air |
| Main output | Estimated output needed |
| Formula | GPD approx = CFM x grains removed x 60 / 7000 / 24 |
| Cost | Free — no sign-up, no download |
How to use the Humidifier CFM Calculator
- 1Confirm the furnace or air handler's rated CFM.
- 2Estimate the grains of moisture per pound of air you need to add for your climate.
- 3Match the gallons-per-day figure to a humidifier's rated output.
Inputs explained
- System airflow(CFM)
- Enter the system airflow in CFM.
- Grains of moisture to add per lb air(grains/lb)
- Enter the grains of moisture to add per lb air in grains/lb.
Worked example
Using the values the calculator loads with:
Inputs
- System airflow1200 CFM
- Grains of moisture to add per lb air20 grains/lb
Results
- Estimated output needed370.3 gallons/day
- Grains added per minute1800
- Suggested unit classSteam humidifier
Frequently asked questions
What grains figure should I use?
Colder climates with dry winter air often need 15-25 grains/lb added; mild climates need less.
Steam vs bypass humidifiers?
Steam units add moisture independent of airflow and suit tighter, well-insulated homes.
What do I need to enter into the Humidifier CFM Calculator?
Just 2 values: system airflow, grains of moisture to add per lb air. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Humidifier CFM Calculator work out the answer?
Duct humidifiers are sized in gallons per day based on system CFM, outdoor design temperature and the target indoor relative humidity. It applies the formula GPD approx = CFM x grains removed x 60 / 7000 / 24 and shows the working so you can check each step by hand.
Is the Humidifier CFM Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.