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Calcimator

Damper Sizing Calculator

Damper area from CFM and velocity limits.

About this calculator

This calculator sizes an HVAC control damper from Airflow (CFM) and Target Velocity (FPM): required free area equals Airflow divided by Target Velocity, then the resulting width and height are rounded up to the nearest 2-inch standard increment for the given Aspect Ratio. Airflow is the dominant input on every dimension and area output -- it enters linearly and its declared range (1 to 1,000,000 CFM) dwarfs the other inputs, so raising Airflow raises Free Area, Damper Width, and Damper Height in lockstep. System Delta P has no effect on sizing at all; it exists purely to compute Damper Authority, the ratio of the damper's own pressure drop (from the AMCA formula deltaP = (V/4005)^2 at the actual, rounded-up face velocity) to the total circuit pressure drop.

A higher System Delta P pulls authority down, since the damper's fixed drop becomes a smaller share of the total -- that is why the helpText notes "higher = better authority" is really about the damper's own drop relative to the system, not about System Delta P directly raising anything. Leakage Class I and II are simple multiples of Free Area (4 and 10 CFM per square foot per AMCA 511) and always move together with damper size. Required Torque scales with Free Area and roughly doubles for opposed-blade dampers versus parallel-blade dampers at the same area, reflecting the extra mechanical resistance of independently pivoting blade linkages.

Inputs

Results

Damper width (in)

24

Damper height (in)

16

Free area (ft²)2.67
Actual velocity (FPM)1,875
Damper drop (in. w.g.)0.22
Damper authority0.31
Authority rating (0-2)1
Equivalent round dia. (in)22.1
Leakage Class I (CFM)10.7
Leakage Class II (CFM)26.7
Required torque (in-lb)19
How to Use This Calculator
  1. Enter Airflow (CFM), Target velocity (FPM), and System delta P (in. w.g.).
  2. Set Aspect ratio (W:H), Blade Type, and Opposed blade (modulating control).
  3. Adjust Parallel blade (2-position/mixing) as needed.
  4. Review Damper width (in) and Damper height (in).
  5. Use Free area (ft²) and Actual velocity (FPM) to inform your decision.

How the result changes with Airflow (CFM)

Airflow (CFM)Damper width (in)Damper height (in)
2,5001812
3,7502214
7,5003020
12,5003826

What each input means

Airflow (CFM)
Design airflow through the damper in cubic feet per minute.
Target velocity (FPM)
Target face velocity. OA intake: 1000-1500, mixing: 1500-2500, duct: 2000-3000 FPM.
System delta P (in. w.g.)
System pressure drop the damper must modulate against (coil, duct, filters). Higher = better authority.
Aspect ratio (W:H)
Width-to-height ratio of rectangular damper. 1.0 = square. Keep <=4:1 per AMCA guidelines.
Blade Type
Select damper blade configuration

What each result means

Damper width (in)
Damper width rounded to the nearest 2-inch standard increment.
Damper height (in)
Damper height rounded to the nearest 2-inch standard increment.
Free area (ft²)
Actual damper free area at the rounded dimensions.
Actual velocity (FPM)
Actual face velocity at the selected damper size.
Damper drop (in. w.g.)
Pressure drop across the damper when wide open.
Damper authority
Ratio of damper drop to total circuit drop. Target >= 0.5 for good modulating control.
Authority rating (0-2)
0 = Poor (<0.3), 1 = Fair (0.3-0.5), 2 = Good (>=0.5).
Equivalent round dia. (in)
Equivalent round duct/damper diameter for the same free area.
Leakage Class I (CFM)
Maximum leakage at AMCA Class I (4 CFM/ft²) - low leakage rating.
Leakage Class II (CFM)
Maximum leakage at AMCA Class II (10 CFM/ft²) - standard rating.
Required torque (in-lb)
Estimated actuator torque requirement based on damper area and blade type.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Airflow (CFM) = 5000, Target velocity (FPM) = 2000, System delta P (in. w.g.) = 0.5, Aspect ratio (W:H) = 1.5 = 5 input(s) provided
  2. Calculate Damper width
    Damper width = ceil(widthIn / 2) * 2
    24 = 24
  3. Calculate Damper height
    Damper height = ceil(heightIn / 2) * 2
    16 = 16
  4. Calculate Free area
    Free area = actualAreaSqIn / 144
    2.67 = 2.67
  5. Calculate Actual velocity
    Actual velocity = cfm / actualAreaSqFt
    1875 = 1875

Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

What determines the damper's physical size?

Airflow (CFM) divided by Target Velocity (FPM) sets the required free area, which the calculator then converts to a width and height at the chosen Aspect Ratio and rounds up to the nearest 2-inch standard increment. Airflow is the dominant factor: because its declared range runs from 1 CFM up to 1,000,000 CFM, it swings every dimension output far more than any other input.

Does the system pressure drop change the damper's size?

No. System Delta P never appears in the free-area or dimension formulas -- it only feeds Damper Authority, the ratio of the damper's own pressure drop to the total circuit drop (damper + system). Sizing the damper and rating its controllability are two separate calculations that share only the resulting damper size.

Why is a higher target velocity not automatically better?

A higher Target Velocity shrinks the damper (less free area for the same airflow), but it also raises the actual face velocity and therefore the damper's own pressure drop, per the AMCA formula deltaP = (velocity / 4005)^2. That extra drop can improve Damper Authority, but it also increases fan energy and noise -- AMCA guidelines cap OA intake dampers around 1,000-1,500 FPM and mixing dampers around 1,500-2,500 FPM for that reason.

What is the difference between the two leakage classes shown?

Leakage Class I (4 CFM per square foot of free area) and Leakage Class II (10 CFM per square foot) are both AMCA 511 low-leakage ratings computed directly from the damper's free area -- Class I is the tighter, lower-leakage rating. Neither reflects an actual measured leakage rate; they show the maximum allowed leakage for a damper of that certified class at the calculated size.

Why does blade type change the required actuator torque?

Opposed-blade dampers use roughly 7 in-lb of torque per square foot of free area in this calculator, versus about 4 in-lb per square foot for parallel-blade dampers, because opposed blades pivot independently against each other and require more force to overcome linkage friction and air pressure across the modulating range. Required Torque scales directly with Free Area for either blade type.

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