VAV Box Sizing Calculator
VAV box size, minimum flow and reheat.
Enter your values
Enter the design airflow in CFM.
Enter the minimum flow in %.
Enter the reheat temperature rise in °F.
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.
- 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
At a glance
| What it does | VAV box size, minimum flow and reheat. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Design airflow, Minimum flow, Reheat temperature rise |
| Main output | Inlet velocity target |
| Formula | inlet size from CFM at 2000 FPM inlet velocity |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Design airflow(CFM)
- Enter the design airflow in CFM.
- Minimum flow(%)
- Enter the minimum flow in %.
- Reheat temperature rise(°F)
- Enter the reheat temperature rise in °F.
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.
What do I need to enter into the VAV Box Sizing Calculator?
Just 3 values: design airflow, minimum flow, reheat temperature rise. 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 VAV Box Sizing Calculator work out the answer?
VAV boxes are selected by inlet size for the design airflow, with a minimum flow setting usually 20-30% of maximum. It applies the formula inlet size from CFM at 2000 FPM inlet velocity and shows the working so you can check each step by hand.
Is the VAV Box Sizing 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.