🌬️ What Size Duct Do You Need for Your Airflow?
Convert CFM into the right round duct diameter and rectangular equivalent, with the correct air velocity for supply trunks, branches, returns and exhaust. Enter CFM directly, or derive it from your AC tonnage or house size — and it adjusts for flexible vs rigid duct.
🔢1. How Do You Want to Enter Airflow?
Enter CFM directly if you know it, or let the calculator derive it from AC tonnage (400 CFM per ton) or from house floor area.
💨2. Airflow
Range: 50 – 4,000 CFM. A single room register is typically 100–200 CFM.
🧭3. Which Part of the Duct System?
Main trunks run faster (700–900 FPM) because they carry the whole system's air. Branches run slower (400–600 FPM) so air doesn't whistle at the registers.
🔩4. Duct Material
Flexible duct has a ribbed inner liner that creates far more friction than smooth sheet metal, so it must be sized one size larger for the same airflow. Keep flex runs short, pulled taut and free of sharp bends.
Duct Size Chart by CFM (Round & Rectangular)
Quick-reference duct sizes for a supply trunk in rigid sheet metal. Branch runs are sized slower (and so slightly larger per CFM); flexible duct needs one size up.
| Airflow | Round duct | Rectangular | AC size |
|---|---|---|---|
| 100 CFM | 6 inch | 6×8 | single register |
| 200 CFM | 8 inch | 8×8 | bedroom branch |
| 400 CFM | 10 inch | 12×8 | 1 Ton |
| 600 CFM | 12 inch | 16×8 | 1.5 Ton |
| 800 CFM | 14 inch | 22×8 | 2 Ton |
| 1,000 CFM | 15 inch | 24×8 | 2.5 Ton |
| 1,200 CFM | 16 inch | 30×8 or 24×10 | 3 Ton |
| 1,600 CFM | 18 inch | 30×10 | 4 Ton |
| 2,000 CFM | 22 inch | 30×14 | 5 Ton |
Based on the velocity method at roughly 800 FPM for trunks. Standard airflow is 400 CFM per ton of cooling. Rectangular equivalents use the ASHRAE equivalent-diameter formula, so they carry slightly more cross-sectional area than the round size they replace.
How Duct Sizing Works
Duct size comes from two numbers: how much air must pass through it, and how fast you are willing to let that air travel.
1. Work Out the Airflow (CFM)
Residential systems move about 400 CFM per ton of cooling, so a 3-ton system moves roughly 1,200 CFM. Individual rooms typically need 100–200 CFM each. If you don't know your tonnage, size the equipment first with our AC Tonnage Calculator.
2. Choose the Target Velocity (FPM)
This is the part most DIY guides skip. Air moving too fast is noisy; too slow and the duct is needlessly large and expensive. Standard residential targets:
- Supply trunk: 700–900 FPM
- Supply branch: 400–600 FPM (keeps registers quiet)
- Return trunk: 600–800 FPM
- Return branch: 400–600 FPM
- Range hood / exhaust: 1,000–1,500 FPM (noise matters less, and higher speed carries grease)
3. Convert to a Duct Size
Duct area = CFM ÷ velocity. Convert that area to a round diameter, then round up to the nearest size that keeps velocity inside the limit. For rectangular duct, the equivalent diameter formula De = 1.30 × (a×b)0.625 ÷ (a+b)0.25 gives the round size a rectangular duct behaves like — a rectangular duct always needs more area than the round duct it replaces, because it has more wall surface creating friction.
4. Add the Flex Duct Penalty
Flexible duct has a ribbed inner liner with far more friction than smooth metal. As a rule, go one size larger (about +1 inch) when using flex, keep runs short, pull them taut, and avoid tight bends — a compressed or sagging flex run can lose more airflow than a whole extra size gains.
Frequently Asked Questions
What size duct do I need for 400 CFM?
What size duct do I need for a 3 ton AC?
How many CFM per ton of air conditioning?
Can I use flexible duct instead of rigid sheet metal?
What velocity should supply and return ducts be?
Why are my vents noisy or airflow weak?
Do I need a Manual D calculation?
Related tools from Refcon HVAC
- AC Tonnage Calculator
- Commercial RTU Calculator (outputs CFM)
- BTU Load Calculator
- Furnace Size Calculator
Last updated: 2026. Reviewed by the Refcon HVAC engineering team using ACCA Manual D and ASHRAE duct-sizing guidance. Planning estimate — confirm with a licensed HVAC contractor. Tell us about an error.
Estimates vary by contractor. Compare free quotes from licensed local pros before you commit. Request a quote →
High-efficiency systems may qualify for utility rebates and federal or state incentives. Find current programs at ENERGY STAR & DSIRE.
Figures are reviewed by the Refcon HVAC engineering team using U.S. DOE, ENERGY STAR & ACCA Manual J data. Estimates only — confirm with a licensed pro.