Compare duct sizes
Use the Duct Calculator when airflow and a design friction rate are known. Compare round and rectangular options, then check velocity, fittings and project requirements before selecting a size.
HVAC Tools
HVAC Calculators & Tools
Free, practical tools for HVAC professionals, engineers, designers, technicians, and students.
Choose a tool below to get started. No account or sign-up required.
Available
Air Distribution
HVAC Duct Calculator
Size round and rectangular ductwork using airflow, friction rate, and velocity.
Available
Air Distribution
Air Distribution Tools
Airflow, velocity, air changes, friction rate, pressure relationships, and practical air-distribution calculations.
Available
Air Properties
Air Properties
Moist-air and psychrometric tools for humidity, temperature, enthalpy, and HVAC analysis.
Available
Air Properties
Mixed Air Calculator
Combine outdoor air and return air using psychrometric mass and energy conservation.
Choose the tool for your engineering question.
Start with the quantities you know and the result you need. All tools are free to use without an account; results still need to be checked against the conditions and requirements of your project.
Check airflow and pressure inputs
Choose Air Distribution to relate airflow, velocity and area, calculate nominal air changes, or work through a design pressure budget. Establish the friction rate here before using it to size ducts; measured TESP is not a design pressure allowance.
Describe air or combine streams
Choose the Psychrometric Calculator for one moist-air state. Use the Mixed Air Calculator for outdoor and return air together, with flow inputs and a common project pressure. Air Properties links to both tools.