Duct Sizing Calculator (Equal-Friction Method) - Round & Rectangular from CFM
Undersize a duct and the system roars like a jet engine while the far rooms never get warm; oversize it and you waste money and space. Duct sizing is where good HVAC design is won or lost, and the most widely used approach — the one behind ACCA Manual D and most residential systems — is the equal-friction method . This guide gives you a professional-grade duct sizing calculator (validated against ACCA/ASHRAE formulas) that converts CFM into round and rectangular duct sizes , checks the velocity, and explains the method behind every number. Figure 1. In the equal-friction method, every duct section is sized to the same friction rate (e.g. 0.08 in.wg/100 ft). As CFM drops after each branch, the duct steps down — and the system stays balanced. Table of Contents The Duct Sizing Calculator What Is the Equal-Friction Method? The Sizing Formulas Choosing a Friction Rate Velocity Limits (Noise Control) Round to Rectangular Conversion Step...