An hvac duct calculator matches duct diameter to airflow (CFM) at a target friction rate. Searchers use different names — hvac ductwork calculator, ductwork calculator, duct calculator cfm, ductwork calculator cfm, air duct calculator, air duct size calculator, and air duct design calculator — but every version answers: “What size duct carries this much air without excessive noise or static pressure?”
Quick CFM-to-Duct Reference
Round metal duct at ~0.10 in.wg/100 ft friction rate:
| CFM | Round duct size |
|---|---|
| 100 | 6 in |
| 225 | 8 in |
| 350 | 9 in |
| 450 | 10 in |
| 700 | 12 in |
| 1,100 | 14 in |
| 1,700 | 16 in |
These numbers come from the same math behind any ductwork size calculator. Flex duct needs one size larger due to higher friction from ribbed walls.
Round duct size by CFM at 0.10 in.wg/100 ft
The Two Rules Every Duct Calculator Uses
Friction rate: How much static pressure the duct consumes per 100 ft. Residential design typically targets 0.08–0.10 in.wg/100 ft on branch runs.
Velocity: Air speed in feet per minute. Exceed ~900 fpm on branch runs and registers whistle. Returns run slower — size them larger than supplies for the same CFM.
An hvac duct calculator that ignores velocity will oversize trunks and undersize branches. Always check both.
What a duct calculator needs
Supply vs Return Sizing
- Supply ducts carry conditioned air to rooms — sized from room CFM requirement
- Return ducts carry air back to the air handler — typically sized for 100–125% of supply CFM split across returns
- Return drop near the air handler is often the largest single duct in the system
See Return Air Duct Sizing for return-specific rules.
Rectangular Duct Equivalent
When ceiling space limits round duct, convert using equal cross-sectional area:
Area (sq in) = π × (diameter ÷ 2)²A 10 in round duct (78.5 sq in) ≈ 8 × 10 rectangular (80 sq in). Our Duct Size Chart covers round-to-rect conversion in detail.
Round ↔ Rectangular duct equivalents
Whole-System vs Single Run
A duct calculator cfm tool sizes one segment. Manual D (ACCA) sizes the entire system accounting for fitting losses, longest run, and total external static pressure. Use segment calculators during design; validate the whole system against blower capacity.
Use the Free Calculators
HVAC Duct Size Calculator — hvac duct calculator for round and rectangular duct.
Duct Friction Rate Calculator — check friction on existing runs.
For the full Manual D workflow, see HVAC Duct Sizing Guide.
FAQ
How do I calculate duct size from CFM?
Enter CFM and target friction rate (0.10 in.wg/100 ft default) into a duct calculator. Verify velocity stays under 900 fpm on branch runs. Upsize one increment if velocity is too high.
What is the difference between hvac duct calculator and ductwork calculator?
Nothing meaningful — both size duct from airflow. “Ductwork calculator” often implies whole-home scope; “hvac duct calculator” usually means a single run or segment.
Can I use a duct calculator for flex duct?
Yes, but derate capacity 15–20% or select one size larger than the rigid-metal chart suggests.
What CFM per ton of AC?
Roughly 400 CFM per ton of cooling (e.g. 3 tons → 1,200 CFM total system airflow). Verify against equipment specs — some high-efficiency units want 350–450 CFM/ton.