Hot Wire Anemometer CFM Calculator
CFM from averaged hot-wire velocity readings at low speed.
Enter your values
Enter the average velocity reading in FPM.
Enter the gross opening area in sq ft.
Enter the free area ratio in %.
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.
- Free area
- 1.5 sq ft
- Airflow
- 270 CFM
- Effective velocity (gross)
- 135 FPM
Quick answer
CFM equals the average hot-wire velocity reading multiplied by the measured free area of the opening.
CFM = V_avg x Free Area
At a glance
| What it does | CFM from averaged hot-wire velocity readings at low speed. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Average velocity reading, Gross opening area, Free area ratio |
| Main output | Free area |
| Formula | CFM = V_avg x Free Area |
| Cost | Free — no sign-up, no download |
How to use the Hot Wire Anemometer CFM Calculator
- 1Take multiple hot-wire readings across the grille or opening and average them.
- 2Enter the gross opening area and the grille's published free-area percentage.
- 3Multiply free area by average velocity to get CFM.
Inputs explained
- Average velocity reading(FPM)
- Enter the average velocity reading in FPM.
- Gross opening area(sq ft)
- Enter the gross opening area in sq ft.
- Free area ratio(%)
- Enter the free area ratio in %.
Worked example
Using the values the calculator loads with:
Inputs
- Average velocity reading180 FPM
- Gross opening area2.0 sq ft
- Free area ratio75 %
Results
- Free area1.5 sq ft
- Airflow270 CFM
- Effective velocity (gross)135 FPM
Frequently asked questions
Why free area, not gross area?
Hot-wire probes usually read velocity through the open portion of a grille, not the full frame area.
Hot wire vs vane anemometer?
Hot wire is more accurate at low velocities (below ~150 FPM) where vane anemometers lose accuracy.
What do I need to enter into the Hot Wire Anemometer CFM Calculator?
Just 3 values: average velocity reading, gross opening area, free area ratio. 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 Hot Wire Anemometer CFM Calculator work out the answer?
CFM equals the average hot-wire velocity reading multiplied by the measured free area of the opening. It applies the formula CFM = V_avg x Free Area and shows the working so you can check each step by hand.
Is the Hot Wire Anemometer 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.