Supply/Return Ratio Calculator
Check supply-to-return airflow balance.
Enter your values
Enter the total supply cfm in CFM.
Enter the total return cfm in CFM.
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.
- Supply/return ratio
- 0.89
- Airflow imbalance
- 150 CFM
- Status
- Return-starved — building will pressurize
Quick answer
A well-balanced duct system keeps return airflow within about 10% of total supply airflow to avoid pressurizing or depressurizing the building.
Ratio = return CFM / supply CFM
At a glance
| What it does | Check supply-to-return airflow balance. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Total supply CFM, Total return CFM |
| Main output | Supply/return ratio |
| Formula | Ratio = return CFM / supply CFM |
| Cost | Free — no sign-up, no download |
How to use the Supply/Return Ratio Calculator
- 1Total the supply CFM across every register.
- 2Total the return CFM across every return grille.
- 3Compare the ratio to the 0.9-1.1 balanced range; add return path capacity if it falls outside.
Inputs explained
- Total supply CFM(CFM)
- Enter the total supply cfm in CFM.
- Total return CFM(CFM)
- Enter the total return cfm in CFM.
Worked example
Using the values the calculator loads with:
Inputs
- Total supply CFM1400 CFM
- Total return CFM1250 CFM
Results
- Supply/return ratio0.89
- Airflow imbalance150 CFM
- StatusReturn-starved — building will pressurize
Frequently asked questions
Why does imbalance matter?
A pressurized house pushes conditioned air out through gaps, while a depressurized one pulls unconditioned or combustion air in.
What's a quick fix for return-starved systems?
Add return grilles or transfer ducts in closed-door rooms before adding more supply capacity.
What do I need to enter into the Supply/Return Ratio Calculator?
Just 2 values: total supply cfm, total return cfm. 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 Supply/Return Ratio Calculator work out the answer?
A well-balanced duct system keeps return airflow within about 10% of total supply airflow to avoid pressurizing or depressurizing the building. It applies the formula Ratio = return CFM / supply CFM and shows the working so you can check each step by hand.
Is the Supply/Return Ratio 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.