Duct Size Calculator
Convert airflow (CFM) into a practical round diameter or rectangular duct size. Choose supply/return/exhaust presets, adjust target velocity, and compare common duct sizes.
On this page: Calculator · Duct size chart · Velocity guide · Sizing tips · FAQ
Size ductwork from airflow
This is a sizing helper for typical residential/light commercial airflow. Final duct design should consider equivalent length, fittings, static pressure, balancing, and equipment blower capability.
Duct sizing tips (quick)
- Start with airflow (CFM). If you only know AC size, estimate CFM from tons as a starting point.
- Lower velocity is usually quieter. Returns and flex duct often benefit from lower FPM.
- Round is usually more efficient. Use rectangular where space is limited, and keep aspect ratio reasonable.
- Don’t ignore fittings. Elbows, transitions, and long runs add pressure drop—use a pro duct design if needed.
Sizing equipment next? Use: BTU calculator · AC size · Heat pump · Furnace
CFM to duct size chart (round duct)
Use this as a quick starting point. Actual sizing depends on your target velocity (FPM), duct material, and pressure drop. For the most accurate match to your situation, use the calculator above and adjust the velocity preset.
| Airflow (CFM) | Typical round duct | Notes |
|---|---|---|
| 80–120 | 6" | Common small branch / bath exhaust (short runs). |
| 130–180 | 7" | Branch runs where noise is a concern. |
| 190–260 | 8" | Typical medium branch / small return. |
| 270–360 | 9"–10" | Often used for larger branches or short trunk segments. |
| 370–500 | 10"–12" | Common trunk or larger return sections. |
| 510–700 | 12" | Large trunk/return; keep velocity reasonable for comfort. |
| 710–950 | 14" | Typical for higher system airflow or main trunks. |
| 960–1,250 | 16" | Large main ducts; verify static pressure and fittings. |
| 1,260–1,600 | 18" | Light commercial / larger homes; pro design recommended. |
Looking for a specific case? Search within this page for: "What size duct for 400 CFM" or "1000 CFM" in the FAQ below.
Tools for better duct performance
Once you have your duct sizes, these tools help measure and improve system performance:
Energy monitor ($20–30) — measure actual power draw and identify inefficiencies. Smart thermostat ($25–250) — scheduling and runtime reports help identify airflow problems. Get a professional HVAC quote — includes duct evaluation and Manual D design for new or problem systems.
Recommended duct velocity (FPM)
Velocity (FPM) affects noise and pressure drop. Lower is usually quieter and easier on the blower, especially for returns and flex duct.
| Application | Metal duct (typical) | Flex duct (typical) |
|---|---|---|
| Supply trunk (main) | 700–900 FPM | 500–700 FPM |
| Supply branch (room run) | 500–700 FPM | 400–600 FPM |
| Return duct | 500–800 FPM | 400–600 FPM |
| Exhaust / ventilation | 400–700 FPM | 300–500 FPM |
Tip: If you’re chasing quiet comfort, try a lower velocity setting (e.g., 600–800 FPM) and compare sizes.
How to estimate airflow (CFM)
Duct sizing starts with airflow. If you don’t know CFM, these methods help you get close enough to use the calculator.
- From system size (tons): a common starting point is about 400 CFM per ton (often 350–450 depending on comfort goals and humidity).
- From equipment data: some air handlers list airflow tables by static pressure and blower speed.
- From commissioning/testing: a contractor can measure airflow and balance registers for each room.
Not sure what size system you need? Start with the BTU calculator and then check equipment sizing via AC size or heat pump.
Drafty home? Air leakage can increase required airflow and capacity — estimate an air-leakage penalty. Sizing HRV/ERV ventilation ducts? Use the HRV & ERV sizing calculator for required CFM, then return here to size the ductwork.
Round vs rectangular duct sizing
Round duct is usually the most efficient shape for a given airflow. Rectangular duct is helpful when space is limited (joist bays, soffits, chases). For rectangular ducts, keep the aspect ratio reasonable to limit noise and friction.
- Prefer round when possible: less perimeter for the same area often means lower friction.
- Cap aspect ratio: 3:1 is a practical target for many installs; 2.5:1 is even better if space allows.
- Watch the largest side: a 10×20 can fit where a large round duct can’t.
Use the calculator’s rectangular mode to generate several “best fit” options that meet your aspect-ratio and max-dimension limits.
FAQ
Why does flex duct often need larger sizes?
Flex duct has higher friction than smooth metal, especially if it’s compressed or kinked. A slightly larger size and lower velocity can reduce noise and improve airflow.
Should I size returns differently than supplies?
Returns are often sized for lower velocity to reduce noise. If your return is undersized, it can increase static pressure and reduce airflow.
Can I use this for ventilation ducts (HRV/ERV)?
Yes for a rough starting point. Ventilation systems often target lower velocities for quiet operation and to meet manufacturer duct requirements.
What size duct for 400 CFM?
It depends on your target velocity. As a rough guide, many systems land around a 10" round duct for ~400 CFM at moderate velocities. Use the calculator to match your supply/return preset and compare options.
What size duct for 1,000 CFM?
For many residential main ducts, ~1,000 CFM often lands near a 14" round (or a comparable rectangular size) depending on your chosen velocity and duct type. Use the calculator and keep velocity lower if you want quieter airflow.
What aspect ratio should rectangular duct be?
Lower is usually better. A practical target is 3:1 or lower (e.g., 8×24). Very wide/flat ducts can increase friction and noise compared with more balanced shapes.
Does a larger duct always improve performance?
Not always. Oversizing can reduce velocity too much at grilles/registers and may affect distribution. The goal is a duct size that meets airflow with acceptable noise and pressure drop, then balance the system.
Why do my calculated sizes seem bigger than what I have?
Existing ductwork is often undersized, especially returns. Long runs, many elbows, restrictive filters, and flex duct can all increase pressure drop. If comfort or airflow is poor, a pro duct evaluation can confirm where the restriction is.
Last reviewed: 2026-03-04