Supply/Return Ratio Calculator
Check supply-to-return airflow balance.
Enter your values
ⓘ Live results
Results
- 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
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.
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.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.