About Duct Chart

Duct Chart is built by a working refrigeration and HVAC technician who wanted a quick duct size calculator that works on a phone, gives straight answers and prints a one-page chart for the truck.

The site has a duct size calculator, a printable duct size chart, a round to rectangular converter, a flex duct calculator, a CFM per ton calculator and a duct velocity calculator, plus two short guides. Everything is in US units: CFM, inches, in. w.g. and fpm.

The calculations use published methods: Darcy-Weisbach friction with the Colebrook friction factor, the Huebscher equal-friction equation for rectangular duct, and the compression models published by ASHRAE, the Air Diffusion Council and LBNL for flex. The engine was checked against worked examples and tables from ASHRAE, ACCA Manual D, McGill AirFlow, Carrier and others. They are estimates only: always follow the design documents, manufacturer data and local code.

No sign-up, no tracking, free to use. Found a mistake or want a tool added? Email [email protected].

Sources and references

The methods, tables and worked examples behind the calculators draw on these published references:

• ASHRAE Handbook—Fundamentals and the ASHRAE Duct Systems Design Guide
• ACCA Manual D (Residential Duct Systems)
• SMACNA duct design guidance
• McGill AirFlow duct design references
• Carrier system design references
• LBNL flex duct research (Abushakra, Walker and Sherman)
• Air Diffusion Council (ADC) Flexible Duct Performance and Installation Standards

These are listed by name only. Each belongs to its publisher, and none of them endorses or certifies this site. Results are estimates only. Verify accuracy for your own application against the design documents, manufacturer data and local code.