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Review the methodology to ensure it aligns with your project's requirements.

  1. Select US or Metric units

  2. Select the Diffuser or Grill Type, Return Grill, 1-Way Diffuser, 2-Way Diffuser, etc.

  3. Enter the velocity criteria, FPM [m/s]

  4. Enter the flow rate, CFM [l/s]

  5. The Results Table will populate with Diffusers that meet the specified criteria. 
    A link to a sample specification sheet is located above the table.


Click here to jump to the table of Noise Criterion (NC) based on the ASHRAE Handbook.

duct and diffuser



Size flexible duct, duct board, and metal ducts

Diffuser Size Calculator


Ps-Static Pressure [in. wg]

NC-Noise Criterion

Throw-Throw estimates based on terminal velocities of 100 FPM and 50 FPM respectively [ft]

Adicot is not affiliated nor do we recommend any diffuser/grille manufacturers.  Any references or links to specific manufacturers are for illustrative purposes only. 


The Diffuser Size Calculator is a technical tool designed to assist engineers and HVAC professionals in calculating the appropriate size of diffusers for use in commercial and residential air distribution systems. The tool calculates the optimal diffuser size, taking into account factors such as air velocity, throw distance, static pressure, and air pattern dispersion. A properly sized diffuser is essential for ensuring that conditioned air is distributed evenly and efficiently throughout a building, which can help to improve indoor air quality and reduce energy costs. This tool can save time and effort for professionals who need to select and install the appropriate diffusers for their HVAC systems.

Diffusers can be sized based on Noise Criterion (click on the table to the right for ASHRAE Fundamentals-based recommendations based on room type), available static pressure, and/or throw. You can vary the FPM input to hone in on the correct diffuser size for your project. Below is a table of recommended noise criteria values based on Room Type.  The values are based on ASHRAE's Handbook, Chapter 49, Noise and Vibration Control, Table 1,

Table of Diffuser/Grille Noise Criterion

Design Guidelines for HVAC-Related Background Sound in Rooms

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