A duct sizing calculator determines the correct duct diameter for a given airflow (CFM) using the ASHRAE equal friction method. Properly sized ducts keep air velocity within quiet, efficient limits — typically 700–900 FPM for supply and 500–700 FPM for return — while maintaining a friction rate of 0.08–0.10 in. w.c. per 100 feet for residential systems.
Duct Parameters
Cubic feet per minute
Feet per minute
Round Duct
Rectangular Equivalents
Same cross-sectional area — choose based on available ceiling/wall space
| Width × Height | Area (sq in) | Aspect Ratio | Note |
|---|
Adjacent Standard Sizes
Sizes based on equal friction method per ASHRAE. Round up to next standard duct size for field installation. Consult a licensed HVAC contractor for full system design.
Typical Room CFM Reference
Use these as starting points — actual values come from a Manual J load calculation.
| Room Type | CFM Range | Typical Round Duct |
|---|---|---|
| Bedroom (small) | 80–120 | 5–6 in |
| Bedroom (master) | 120–200 | 6–7 in |
| Living Room | 200–400 | 8–10 in |
| Kitchen | 150–250 | 7–8 in |
| Bathroom | 50–100 | 4–5 in |
| Home Office | 100–200 | 6–7 in |
| Dining Room | 150–300 | 7–9 in |
| Main Trunk | 800–2,000 | 12–18 in |
How to Use the Duct Sizing Calculator
Correct duct sizing is essential for a quiet, efficient HVAC system. Ducts that are too small create high velocity, causing noise and restricting airflow. Ducts that are too large waste material and can cause condensation issues. This duct sizing calculator uses the ASHRAE equal friction method — the industry standard for residential and light commercial systems — to find the right round duct diameter and rectangular equivalents for any airflow rate.
Step 1: Enter Your CFM
CFM (cubic feet per minute) is the airflow you need to deliver to a space. This number comes from your HVAC system's Manual J load calculation or from the equipment specifications provided by your contractor. If you are sizing individual branch ducts, use the CFM for each room. For the main trunk line leaving the air handler, use the total system CFM (typically 400–600 CFM per ton of cooling capacity).
Step 2: Select Duct Type
Supply ducts carry conditioned air from the air handler to rooms and operate at 700–900 FPM for branch runs, up to 1,200 FPM for main trunks. Return ducts carry room air back to the air handler and should run quieter at 500–700 FPM — noisier returns are a common comfort complaint. Select the appropriate type and the calculator sets the default velocity within that range.
Step 3: Adjust Velocity and Friction Rate
The equal friction method targets a consistent pressure drop per 100 feet of duct — typically 0.08 in. w.c./100 ft for residential. The velocity and friction rate inputs work together: a lower target velocity produces a larger, quieter duct with less friction loss per foot. If you are routing through a tight joist bay, use the velocity slider to find a size that fits while staying within acceptable limits.
Step 4: Review Round Diameter and Rectangular Options
The calculator returns the exact calculated diameter and the nearest standard round duct sizes (4 in. through 24 in. in 1-inch increments). Always round up to the next standard size in the field. The rectangular equivalents table shows multiple width × height combinations with the same cross-sectional area, letting you choose whichever fits your framing cavities. Keep the aspect ratio under 4:1 — wide, flat ducts increase friction and are harder to seal.
Tips for Accurate Duct Sizing
Use the room CFM reference table as a sanity check, but get actual CFM numbers from a Manual J calculation before purchasing materials. Add 50–100% to your calculated duct run length to account for fittings, elbows, and transitions, which all add friction resistance. Flex duct has higher friction than sheet metal — size up one diameter when using flex for long runs. Always balance the system after installation by measuring and adjusting airflow at each register.
FAQ
Is this duct sizing calculator really free?
Yes, completely free with no signup required. All calculations run locally in your browser using JavaScript — no data is ever sent to a server. Use it as many times as you need to plan your duct layout.
Is my data safe when using this tool?
Absolutely. Everything runs entirely in your browser. No CFM values, room data, or personal information is stored or transmitted anywhere. You can even use this calculator offline once the page has loaded.
What is the equal friction method for duct sizing?
The equal friction method sizes ducts so that friction loss per 100 feet of duct is the same throughout the system. ASHRAE recommends 0.08–0.10 inches of water column (in. w.c.) per 100 feet for residential systems. This keeps airflow balanced and avoids noisy high-velocity runs.
What is the correct duct velocity for supply vs. return air?
ASHRAE guidelines recommend 700–900 FPM for supply air ducts and 500–700 FPM for return air ducts in residential systems. Higher velocities cause noise (rumble, whoosh) and increase system pressure. Lower velocities require oversized ducts that waste space. Main trunk lines can handle up to 1,000 FPM.
How do I convert round duct to rectangular duct equivalent?
A rectangular duct is equivalent to a round duct when both have the same cross-sectional area. For example, a 7-inch round duct has about 38.5 sq. in. of area, which equals approximately a 4×10-inch or 5×8-inch rectangular duct. This calculator shows multiple rectangular options for each round diameter.
What CFM does each room typically need?
Typical residential CFM ranges: bedroom 100–150, living room 200–400, kitchen 150–250, bathroom 50–100, home office 100–200. These are rough guidelines — actual CFM should come from a Manual J load calculation or the HVAC equipment specifications for each zone.
What happens if my duct velocity is too high?
Duct velocities above 900 FPM in supply branches cause audible noise — rumbling, whooshing, or vibration. High velocity also increases static pressure, reducing airflow and making the blower motor work harder. If velocity is too high, upsize to the next duct diameter or split into two smaller ducts.
Does this calculator account for duct fittings and transitions?
This calculator sizes straight duct runs using the equal friction method. Fittings like elbows, tees, and transitions add equivalent lengths of resistance. As a rule of thumb, add 50–100% to straight-run friction loss estimates to account for fittings when planning a full duct system.