VAV Box Sizing Calculator
VAV box size, minimum flow and reheat.
Enter your values
ⓘ Live results
Results
- Inlet velocity target
- 2000 FPM
- Calculated inlet
- 9.1 in
- Select box size
- 10 in
- Minimum airflow
- 270 CFM
- Reheat coil load
- 7,290 BTU/h
Quick answer
VAV boxes are selected by inlet size for the design airflow, with a minimum flow setting usually 20-30% of maximum.
inlet size from CFM at 2000 FPM inlet velocity
How to use the VAV Box Sizing Calculator
- 1Enter the zone's design cooling airflow.
- 2Set the minimum flow — usually 20-30%, or the ventilation requirement, whichever is higher.
- 3Size the reheat coil at the minimum flow, not the maximum.
Worked example
Using the values the calculator loads with:
Inputs
- Design airflow900 CFM
- Minimum flow30 %
- Reheat temperature rise25 °F
Results
- Inlet velocity target2000 FPM
- Calculated inlet9.1 in
- Select box size10 in
- Minimum airflow270 CFM
- Reheat coil load7,290 BTU/h
Frequently asked questions
Why size reheat at minimum?
Reheat only runs when the box is at minimum flow, so that is the design condition.
Fan-powered boxes?
Series boxes give constant air motion; parallel boxes only run the fan in heating.
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.