What it calculates
Estimate round duct diameter from airflow and target velocity.
Round Duct Size Planner
Estimate round duct diameter from airflow and target air velocity.
Planning and estimating aid only. Verify project requirements, manufacturer instructions, adopted codes, permits, and field conditions before performing regulated work.
HVAC planning
How to Use This Calculator
Enter the measurements and job information you know, select Calculate, then use the result as a starting point for estimating, purchasing or project planning.
How the calculation works
Required duct area = CFM ÷ target velocity. The area is converted to an equivalent round diameter.
What the result means
Final duct design should also account for friction rate, fittings, static pressure and noise.
Use real project measurements
Start with the actual field dimensions or equipment values from the job. Run the calculator once with your measured quantity, then test a second scenario with your normal waste, routing or safety allowance. This makes the tool useful for both the takeoff and the customer estimate.
Contractor tip: Save the measurements that produced your estimate in the job notes so the quantity can be checked later if the scope changes.
Verify the final quantity
These calculations are useful for early estimating and comparison. Equipment selection and final duct/airflow design should use appropriate load calculations, manufacturer data and applicable mechanical/energy requirements.
Good practice: Compare the calculated quantity with plans, specifications, product packaging and current field conditions before purchasing material.