Duct size calculator for round duct.
Free duct size calculator: round duct diameter and air velocity from airflow (CFM) and friction rate, the equal-friction method used in Manual D. Plus Manual D with J and S for $100.
How it works
The calculator uses the equal-friction method: for a given airflow and friction rate it solves for the round duct diameter, rounds up to the next standard size and shows the air velocity. Tons are converted at 400 CFM per ton.
Choosing the friction rate
In Manual D the friction rate is not picked from a habit. It comes from the available static pressure of the blower after coils, filters, registers and grilles, divided by the total effective length of the longest run. Many residential systems land between 0.06 and 0.10 inches per 100 feet.
Flex duct and fittings
Flexible duct has more friction than smooth metal, especially when it is compressed or sags. Every elbow and takeoff adds equivalent length. Manual D accounts for both; a quick calculator does not.
When you need Manual D
For new construction most codes reference ACCA Manual D for duct design (IRC M1601.1). We prepare Manual D together with Manual J and Manual S for $100 per house.
Estimate only. Last reviewed November 2026.
Manual J, S & D for $100.
Permit-ready load calculation, equipment selection and duct design from your plans, delivered as one PDF.
- J + S + D for $100
- Permit-ready PDF
- 2–3 business days
- No subscription
Free quote confirmation, no account needed. You pay per house, no subscription.
Duct Size Calculator: answered.
How many CFM per ton?
About 400 CFM per ton is the common starting point; humid climates often use less to remove more moisture, dry climates more. Manual S and the equipment data set the final number.
What size duct for 400 CFM?
At a friction rate of 0.08 inches per 100 feet, roughly a 10-inch round metal duct. Flex duct or a lower friction rate needs a larger size.
Can I size every room with this?
You can size a single run. Room airflow comes from the room-by-room Manual J loads, and the friction rate from the whole system, which is what Manual D does.
How much does a Manual J cost?
$100 per house for Manual J, Manual S and Manual D together. No subscription and no minimum order.
How fast do I get the report?
Usually 2–3 business days after we receive complete plans and specs.
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