Use this duct size calculator CFM tool to find the right round or rectangular duct size from your required airflow and target velocity. You get the next standard size up, the actual velocity and a noise rating.
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Enter the airflow you need to move in CFM (cubic feet per minute) and the air velocity you want in FPM (feet per minute). Choose Supply or Return so the noise rating uses the right threshold. Use the Round Duct tab for a diameter, the Rectangular Duct tab for width × height at your chosen aspect ratio, or Size Comparison to see round and rectangular options together. You get the required cross-section area, the next standard size up, the actual velocity at that size and a comfort rating. The diagram above updates with your result.
Duct sizing is one of the most consequential, and most often botched, decisions on an HVAC job. Undersized ducts push air too fast. That causes whistling, high static pressure, weak airflow at the far registers and a blower working harder than it should. Oversized ducts cost more in sheet metal, eat up joist space and can leave air moving too slowly to mix well.
Take a 2,000 sq ft house with a 4-ton system moving roughly 1,600 CFM (about 400 CFM per ton). Most rooms need only 50–200 CFM. A 150 CFM bedroom branch on a 4" round duct would run near 1,720 FPM and be clearly audible. A 7" round carries the same air at about 560 FPM. If a quote shows 5" branches feeding 200 CFM rooms, that works out to roughly 1,470 FPM, and you can spot the problem in seconds.
HVAC contractors, remodelers, energy auditors and homeowners adding a room or chasing a noisy register all run a duct size calculator CFM check before buying duct and fittings.
The tool uses the continuity equation, with velocity guidance in line with common ACCA Manual D and SMACNA residential practice:
CFM = Area (ft²) × Velocity (FPM)
Area (in²) = CFM ÷ FPM × 144
Round duct: D (inches) = 2 × √(Area ÷ π), which is the same as 24 × √(CFM ÷ (π × FPM)). The result is rounded up to the next standard diameter, never down, so the final velocity is never higher than the target.
Rectangular duct: for an aspect ratio R = W ÷ H, H = √(Area ÷ R) and W = H × R. Both dimensions are rounded up to standard sizes. The equivalent round diameter uses the ASHRAE formula De = 1.3 × (a×b)0.625 ÷ (a+b)0.25. It matches friction loss, not cross-section area, so a rectangular duct is not simply the same area as its equivalent round.
Velocity ratings: supply is Quiet at 900 FPM or less and Commercial up to 1,600 FPM. Return is Quiet at 700 FPM or less and Commercial up to 1,200 FPM. Anything higher is flagged as noisy. Friction rate, fitting losses and total external static pressure are not included.
A 200 sq ft bedroom needs about 200 CFM of supply air, and you want a quiet 800 FPM.
Step 1: Required area = 200 ÷ 800 = 0.25 ft² = 36.0 in².
Step 2: Exact diameter = 2 × √(36.0 ÷ π) = 2 × 3.385 = 6.77".
Step 3: Round up to the next standard size: 7" round duct.
Step 4: Actual area at 7" = π × 3.5² = 38.5 in² (0.2673 ft²). Actual velocity = 200 ÷ 0.2673 = 748 FPM, which is in the Quiet range for a supply duct.
A common rule of thumb is about 1 CFM per square foot of conditioned floor area with 8-foot ceilings, so a 200 sq ft bedroom needs roughly 200 CFM. Most rooms land between 50 and 200 CFM. A Manual J load calculation gives a more exact number based on insulation, windows and climate.
Above about 900 FPM in supply ducts and 700 FPM in return ducts, occupants usually start to notice airflow noise. For bedrooms and other quiet spaces, aim lower, around 600–700 FPM. Commercial spaces tolerate up to about 1,600 FPM.
Use the ASHRAE equivalent diameter formula: De = 1.3 × (a×b)^0.625 ÷ (a+b)^0.25, with a and b in inches. The result is a round duct with the same friction loss as the rectangular duct, not the same cross-section area. This calculator shows the equivalent diameter on the Rectangular Duct tab.
Both. A residential main trunk typically runs about 700–900 FPM on the supply side, and branch ducts around 600 FPM or less, so the air slows as it leaves the trunk. Returns should run lower, around 500–700 FPM. Trunk-and-branch systems step the trunk down after each takeoff. Velocities above 1,000 FPM are normally kept for commercial or unoccupied runs.
No. It applies the standard area and velocity relationship only. A full Manual D design also accounts for friction rate, fitting losses, available static pressure and the actual layout. Use a licensed HVAC contractor for final, permit-ready designs.
The formula is identical, but return ducts are sized for lower velocities (about 500–700 FPM) because return grilles are often in hallways and living areas where noise is noticed. Switch the Duct Application selector to Return and the ratings use the stricter thresholds.
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