How do I calculate CFM from FPM?

The basic formula is CFM = Area (sq ft) × Velocity (FPM). For example, if you have a 12"x12" grille (1 sq ft) and measure an average air velocity of 500 FPM, the airflow is 1 × 500 = 500 CFM. Always remember to convert dimensions to square feet (divide square inches by 144).

What is the Ak Factor (Effective Area)?

The Ak Factor (or K-factor) represents the effective open area of a grille or register. Since the metal vanes block some airflow, the actual open area is less than the physical size. Manufacturers provide Ak factors for their products. If unknown, a common rule of thumb is 0.70 to 0.85 for standard residential registers.

How many velocity readings should I take?

For accurate results, take multiple readings across the face of the grille (a 'traverse'). For a standard residential register, 4-6 readings (corners and center) averaged together is usually sufficient. For large commercial ducts, use the 'Equal Area Method' with at least 16 points.

Why is my measured CFM lower than the unit rating?

Measured CFM is often lower due to: 1) Duct leakage (typically 10-30%), 2) Dirty filters or coils restricting flow, 3) High static pressure from poor duct design, or 4) Inaccurate measurement technique (not using an Ak factor).

What is a good velocity for supply registers?

For residential supply registers, a face velocity of 500-700 FPM is ideal. Below 500 FPM may have poor 'throw' (mixing), while above 700 FPM can cause objectionable noise and drafts.

💨 Airflow CFM Calculator

Calculate CFM from Velocity (FPM) and Area

Opening Shape

Width (inches)

Height (inches)

Ak Factor (Effective Area)

1.0 = Open Duct, 0.75 = Standard Register

Velocity Readings (FPM)

Enter one average reading or multiple points to average automatically.

📐 How to Calculate Airflow (CFM)

Airflow is calculated using the formula Q = V × A, where:

Q = Airflow in CFM (Cubic Feet per Minute)
V = Velocity in FPM (Feet per Minute)
A = Area in Square Feet

The Importance of Ak Factor

If you are measuring airflow at a grille or register, you cannot use the simple physical area (Width × Height). The metal vanes block some of the air. The Ak Factor (or Effective Area) accounts for this blockage.

Open Duct: Ak = 1.0
Commercial Grille: Ak ≈ 0.80 - 0.90
Residential Stamped Register: Ak ≈ 0.65 - 0.75

Always check the manufacturer's engineering data for the exact Ak factor if available.

💡 From Jeff Martinez, HVAC Technician (18 years):

"When using a vane anemometer on a supply register, don't just hold it in the center! The air moves fastest in the center and slower at the edges. I use a 'snake' pattern to traverse the entire face of the grille over 10-15 seconds to get a true average. If you only measure the 'hot spot,' you'll overestimate your CFM by 20% or more."

⚠️ Common Mistakes

1. Forgetting to convert to Square Feet: Velocity is in Feet/Min, so Area must be in Square Feet. If you use Square Inches, you must divide by 144. (This calculator does it for you!)

2. Ignoring the Ak Factor: Measuring a 12x12 register (1 sq ft) at 500 FPM does NOT mean 500 CFM. With an Ak of 0.75, it's actually 375 CFM.

3. Measuring too close to a bend: Turbulence destroys accuracy. Try to measure in a straight section of duct if possible, or take more readings to average out the turbulence.

❓ FAQ

What is a good CFM for a bedroom?

A typical bedroom needs about 1 CFM per square foot for cooling. A 12x12 room (144 sq ft) usually needs 100-150 CFM depending on windows and sun exposure.

Can I use an anemometer app on my phone?

No. Phone microphones cannot accurately measure air velocity. You need a dedicated anemometer (hot wire or vane type) for meaningful results.

Reviewed by Jeff Martinez

Licensed HVAC Technician, 18 years experience in residential and light commercial air balancing.

📐 How to Calculate Airflow (CFM)

The Importance of Ak Factor

⚠️ Common Mistakes

❓ FAQ

What is a good CFM for a bedroom?

Can I use an anemometer app on my phone?

Embed this Calculator