HVAC Duct Calculator
Enter airflow in CFM and a friction rate to get the correct round duct diameter, equivalent rectangular size, air velocity, and aspect ratio check using the industry-standard ACCA Manual D equal-friction equations. Free, instant, runs in your browser.
Recommended Duct Size
Duct Performance at a Glance
Round Diameter (calc.)
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Standard Size (round up)
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Rectangular Equivalent
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Air Velocity
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Actual Friction Rate
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Cross-Section Area
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Aspect Ratio
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Velocity Rating
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What Size HVAC Duct Do You Actually Need?
That question is what this calculator was built to answer precisely, not approximately. Get the duct size wrong by even one step and the consequences are real: undersized ducts create high static pressure, rushing noise at registers, and equipment that can never reach its rated capacity. Studies from NIST and Lawrence Berkeley National Laboratory document that 30 to 40 percent of residential cooling capacity is commonly lost to duct system problems, undersizing being one of the leading causes. This free HVAC duct calculator removes the guesswork by applying the same math a licensed HVAC designer uses: the ACCA Manual D equal-friction method.
You enter the required airflow in CFM and your target friction rate. The calculator returns the exact round duct diameter, rounds up to the nearest standard sheet-metal size, converts to an equivalent rectangular option, and checks the resulting air velocity against recommended limits for your application (supply trunk, branch run, or return). A live performance visual shows you immediately whether the duct is in the quiet efficient zone or headed toward problems.
The Math Behind the Calculation
The core equation used here is the ASHRAE Handbook of Fundamentals friction formula for galvanized round duct, as adopted in ACCA Manual D:
ASHRAE / ACCA Manual D Equal-Friction Equation
D = ( 0.0992 × CFM^1.9 / friction )^( 1 / 5.02 )
Where D is the required round duct diameter in inches, CFM is the required airflow, and friction is the target pressure loss per 100 feet in inches of water column (in. w.g.).
Velocity (FPM) = CFM ÷ ( π × (D/24)² ) then rounded up to the nearest standard size from the SMACNA stock list.
Rectangular equivalent: D_eq = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25 the Huebscher formula, ASHRAE Chapter 21.
For flexible duct, the calculator automatically adds 1.5 inches to the calculated diameter before rounding up to account for the higher internal friction from flex duct's corrugated inner liner. For ductboard, it adds 1 inch. This matches the SMACNA and manufacturer recommendation that flex duct must be upsized 1 to 2 inches versus rigid metal for equivalent performance.
Three Duct Types: When to Use Each
Rigid Metal (Galvanized)
Lowest friction, best airflow, longest lifespan. Use for trunk lines and any run longer than 10 feet. The baseline material in ACCA Manual D sizing.
Flexible Duct
Higher friction from corrugated liner size up 1 to 2 inches vs rigid. Best for short final connections from rigid trunk to register box. Must be fully stretched; a 15% compression can cut airflow by 50%.
Fiberglass Duct Board
More friction than metal, less than flex. Provides built-in insulation. Size up 1 inch vs rigid. Common in attic installations where insulation and acoustics matter.
Recommended Air Velocity by Application
Air velocity is the key quality check. These are the ASHRAE and ACCA Manual D recommended ranges for US residential and light commercial systems:
| Application | Target Velocity (FPM) | Notes |
|---|---|---|
| Supply main trunk (residential) | 700 – 900 | Quiet and efficient; standard design target |
| Supply branch runs | 600 – 700 | Low noise at registers; branches are closer to living spaces |
| Return air ducts | 500 – 600 | Lower than supply to reduce return noise; undersized returns are the most common duct error |
| Commercial main supply | 1,000 – 1,500 | Higher velocity acceptable in mechanical rooms and above ceilings |
| Over 1,500 FPM (residential) | Too high size up | Expect audible noise, increased static pressure, and reduced equipment life |
HVAC Duct Sizing Rules You Should Know
- 400 CFM per ton: most US residential systems move approximately 400 CFM per ton of cooling. A 3-ton system = 1,200 CFM total, divided across all supply runs.
- Standard friction rate: 0.08 in. w.g./100 ft is the ACCA Manual D default for residential equal-friction design. Use 0.10 for shorter, simpler systems.
- Aspect ratio max 4:1: rectangular ducts should not exceed a 4:1 width-to-height ratio. Beyond that, the Huebscher formula loses accuracy and friction climbs sharply. A 4" × 16" duct is at the limit; a 3" × 16" is a real problem.
- Return ducts are usually undersized: return trunk must handle 100% of system CFM. A 3-ton system (1,200 CFM) needs at least a 20" round return trunk or equivalent rectangular. Most residential installs undersize this badly.
- Flex duct must be stretched: even 15% compression of flex duct can reduce its effective CFM by 50%. Always use rigid metal for trunk lines and limit flex to short final connections under 6 feet.
- Round is always more efficient: for the same cross-section area, round duct has less surface area and therefore less friction than rectangular. Use round where space allows.
Worked Example: Sizing a 3-Ton Residential Supply Trunk
Given: 3-ton system, 1,200 CFM total, equal-friction design at 0.08
Step 1 Total system CFM: 3 tons × 400 CFM/ton = 1,200 CFM at the air handler discharge.
Step 2 Size the trunk: D = (0.0992 × 1200^1.9 / 0.08)^(1/5.02) ≈ 18.4 inches → round up to 20" round trunk.
Step 3 Check velocity: V = 1200 ÷ (π × (10/12)²) ≈ 825 FPM → within the 700–900 FPM quiet trunk range. ✅
Step 4 Branch example (one bedroom at 200 CFM): D ≈ 9.8" → round up to 10" round branch.
This is the same calculation a Manual D software would run. Enter your values above for your specific runs.