Duct Size Calculator
Who it's for: HVAC installers, sheet metal crews, and field supervisors.
Calculator Inputs
Quick Summary
- Built for: HVAC installers, sheet metal crews, and field supervisors.
- Key outputs: Estimated duct area, Suggested round diameter, and more.
- Use this page for early planning estimates before final code and manufacturer verification.
Inputs
- Target airflow (CFM)
- Velocity target (fpm)
- Duct type (supply or return)
Outputs
- Estimated duct area
- Suggested round diameter
- Nearest standard duct size
How this calculator estimates results
- Converts airflow and target velocity into required duct cross-sectional area.
- Calculates equivalent round diameter from area using a geometric round-duct formula.
- Matches calculated diameter to a nearby standard round size for practical takeoff and install planning.
Cross-check assumptions with CFM Airflow Calculator for a more complete planning workflow.
Common ranges and tips
- Higher velocity can reduce duct size but may increase noise and pressure drop.
- Use return and supply velocity targets intentionally rather than one default for all runs.
- Finalize duct layout and static assumptions with ductulator/manufacturer data before installation.
Related Calculators
FAQ
Velocity directly affects required area; higher velocity means smaller duct but can increase acoustic and pressure penalties.
Nearest size is a starting point. You may choose larger sizes for lower noise or lower friction in longer runs.
This output is for equivalent round sizing; rectangular duct selection still needs aspect-ratio and friction checks.
No. Treat this as a planning estimate and validate final duct design with accepted design procedures and equipment data.
Disclaimer: Use results for planning only. Final design and installation decisions must follow local code and manufacturer documentation.
Back to HVAC hub