Duct Size Calculator
Calculate round and equivalent rectangular duct sizes based on CFM airflow, target friction loss, velocity noise constraints, and aspect ratio limits.
Use the embedded interactive HVAC Duct Size Calculator below — powered by ServiceTitan.
Duct Sizing Formulas
Duct sizing balances static pressure loss against air velocity to ensure efficient air distribution without generating objectionable air turbulence or whistling noises.
Formula:
D_round = 0.1091 × (CFM^0.38 / ΔP^0.19) | D_e = 1.30 × (a × b)^0.625 / (a + b)^0.25 | Velocity = CFM / Area Variables & Parameters
D (round) Equivalent round diameter = 0.109136 × (CFM^0.38 / Friction^0.19) × 12
D_e (rect) Huebscher formula: 1.30 × (a × b)^0.625 / (a + b)^0.25
Velocity (FPM) Airflow (CFM) / Cross-Sectional Area (sq ft)
Aspect Ratio Max(a, b) / Min(a, b) [Target < 2.5:1]
Frequently Asked Questions
How to calculate duct size?
To calculate HVAC duct size per ACCA Manual D and ASHRAE standards: (1) Determine required room airflow in CFM (Cubic Feet per Minute) from heating/cooling load calculations. (2) Choose design static friction rate (typically 0.08 to 0.10 in. w.g. per 100 ft for supply ducts). (3) Compute round duct diameter: D = 0.109136 × (CFM^0.38 / Friction^0.19) × 12. (4) Convert to rectangular duct dimensions using Huebscher’s equivalent formula: De = 1.30 × (a × b)^0.625 / (a + b)^0.25, maintaining an aspect ratio (a/b) of 2:1 or less for quiet, efficient airflow.
How does this free online HVAC duct size calculator work?
Our hvac duct size calculator online and hvac duct size calculator free utilizes ASHRAE and ACCA Manual D equal friction formulas to instantly compute round duct diameter, equivalent rectangular duct dimensions, and air velocity in FPM based on CFM airflow and static pressure loss.
What is the Equal Friction Method for HVAC duct design?
The equal friction method sizes duct systems such that the friction loss per unit length remains constant throughout (typically 0.10 in. w.g. per 100 ft of duct for supply systems, and 0.05 to 0.08 in. w.g. for return systems).
How do you convert round duct diameter to equivalent rectangular size?
Huebscher's equivalent diameter formula is used: D_e = 1.30 × (a × b)^0.625 / (a + b)^0.25, where a and b are the rectangular dimensions in inches, providing identical airflow and friction loss.