Enter room dimensions and type to get required CFM, round duct diameter, rectangular duct options, air velocity in FPM, and a quiet/loud rating — based on ACCA Manual D and ASHRAE standards.
Free Tool · CFM · Round & Rect Duct · FPM Velocity · Flex vs Sheet Metal
Room & CFM Parameters
📐
Use Room Size + ACH for a quick estimate. Use Manual if your HVAC contractor has already calculated CFM via Manual J.
↔ft
↕ft
↑
🏠
Room type sets the recommended Air Changes per Hour (ACH). More heat or moisture = higher ACH = more CFM needed.
💨CFM
Enter the room's calculated CFM from your HVAC load calculation report. This is the most accurate input.
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Example: house = 2,000 sq ft, this room = 240 sq ft → room share = 12%.
Duct Configuration
🔧
Supply ducts carry conditioned air to rooms. Return ducts pull room air back. Returns should be sized 25% larger than supply for same CFM.
🏭
Flex duct has higher friction due to internal ridges. Always fully stretch flex duct — compressed installation can reduce CFM by 50%.
⚡
Lower velocity = larger duct + quieter. ACCA Manual D recommends ≤900 FPM for residential supply ducts, ≤700 FPM for return ducts near living areas.
Results
Required CFM
—
cubic feet per minute
Round Duct Diameter
—
next standard size up
Air Velocity
—
feet per minute (FPM)
Friction Loss
—
in. wg per 100 ft
Air Velocity Rating
04007009001,200+ FPM
Quiet (<700)
Acceptable (700–900)
Loud (>900)
Velocity Check
Duct Size Options
Round Duct
—
diameter
Actual Velocity
—
FPM at this duct size
Rectangular Duct Equivalents
These rectangular sizes deliver equivalent airflow to the round duct. Keep aspect ratio under 4:1. All use Huebscher equivalent diameter formula (ASHRAE Ch. 21).
Detailed Breakdown
Room & CFM Calculation
CFM method used
—
Room volume
—
ACH applied
—
Required CFM (raw)
—
Material derating
—
Design CFM (derated)
—
Duct Sizing
Target velocity
—
Required duct area
—
Calculated diameter
—
Standard round duct size
—
Actual velocity at std. size
—
Friction loss (approx.)
—
Inputs Used
Duct type
—
Material
—
Room type
—
How HVAC Duct Sizing Works
Duct sizing is a two-step process: (1) determine how much air (CFM) a room needs, then (2) size the duct to carry that CFM at an acceptable velocity. Too small = high velocity, noise, and pressure drop. Too large = low velocity, poor air mixing, wasted sheet metal. This calculator uses the ACH method for CFM estimation and the equal-friction method for duct sizing — the same approach used by ACCA Manual D and ASHRAE Fundamentals Chapter 21.
1 CFM from Room Size
Multiply room volume (L × W × H) by the target Air Changes per Hour (ACH), then divide by 60 to get CFM. ACH varies by room type — kitchens need 7–9 ACH; bedrooms only need 5–6 ACH.
Volume (ft³) =
L × W × Ceiling H
CFM =
Volume × ACH ÷ 60
Bedroom 14×12×8 ft:
= 1,344 ft³ × 6 ÷ 60
= 134 CFM
2 Duct Area → Diameter
Divide CFM by target velocity (FPM) to get required duct area in sq ft. Convert to sq inches, then use the circle area formula to find the required diameter. Round up to the next standard duct size.
Area (sq ft) =
CFM ÷ Velocity (FPM)
Area (sq in) =
Area × 144
Diameter (in) =
√(4 × Area ÷ π)
Standard sizes:
4,5,6,7,8,9,10,12,14,16 in
3 Rectangular Equivalent
Use the Huebscher equation (ASHRAE 1948, still standard today) to find rectangular duct dimensions that have the same hydraulic resistance as the round duct. Keep aspect ratio under 4:1 for best airflow behavior.
D_eq = 1.3 ×
[(W × H)^0.625] ÷
[(W + H)^0.25]
Where D_eq = round duct
equivalent diameter.
Solve for W given H
or iterate common sizes.
4 Friction Loss & Velocity
Friction loss (in. wg per 100 ft) is the pressure the blower must overcome. Target 0.08–0.10 in. wg/100 ft for residential equal-friction design. Velocity determines whether you'll hear the air moving.
1. Return ducts need to be larger than supply ducts. Size the return for ≤600 FPM velocity (vs 700–800 FPM for supply) to reduce return noise near living areas. Undersized returns are the #1 cause of high static pressure.
2. Flex duct must be fully stretched. Flex duct compressed or kinked by even 15% can lose 50% of its rated CFM. Always use rigid metal for trunk lines; flex duct works best only for final short branch connections.
3. Aspect ratio matters. Keep rectangular ducts under 4:1 (4×16 is acceptable; 2×16 is terrible). High aspect ratios have more perimeter per unit of cross section, increasing friction dramatically.
4. Plan for 400 CFM per ton. If your system delivers 1,200 CFM (3 ton) total, all your room CFMs must add up to no more than 1,200 CFM — or you'll starve the system.
Example Calculations — 3 Real-World Scenarios
These worked examples show the calculator across three common U.S. residential HVAC scenarios.
Round: 6 in · Velocity: 681 FPM Rect options: 4×10, 5×8, 6×6
A 6-inch round supply duct is the standard choice for bedrooms up to ~200 sq ft. Actual velocity of 681 FPM is right in the "quiet" zone. Friction loss ≈ 0.08 in. wg/100 ft — ideal for a long branch run.
Round: 8 in · Velocity: 770 FPM Rect options: 6×10, 8×8
Kitchens need more airflow than bedrooms due to heat from appliances and cooking moisture. An 8-inch round duct at 770 FPM is quiet enough for a kitchen where ambient noise masks air sound.
Round: 12 in · Velocity: 815 FPM Rect options: 8×12, 10×10
Main trunk lines carry full system CFM before branching. Running at 900 FPM is acceptable in an unconditioned attic. Drop to 700 FPM on all branch takeoffs serving living areas — noise travels through smaller ducts.
These velocity limits are for residential and light commercial applications. Higher velocities may be acceptable in commercial settings with background noise.
Duct Location / Type
Max Velocity (FPM)
Recommended (FPM)
Noise Level
Notes
Bedroom Supply Quiet Zone
700
500–700
Very quiet
Sensitive sleeping area
Living Area Supply
800
600–800
Quiet
Standard residential
Kitchen / Bath Supply Higher Load
900
700–900
Acceptable
Ambient noise masks duct sound
Main Trunk (Attic)
1,000
800–1,000
Acceptable
Unconditioned space, noise not heard
Return Duct (near room) Size Up
600
400–600
Quiet
Return noise more noticeable than supply
Return Trunk (main)
800
600–800
Quiet
Central return, away from rooms
Flex Duct (any) Derating
700
500–700
Quiet
15% less capacity than metal; higher friction
Bedroom Supply
Max velocity700 FPM
Target500–700 FPM
NoiseVery quiet
Living / Kitchen
Max velocity900 FPM
Target700–900 FPM
NoiseAcceptable
Return Duct
Max velocity600 FPM
Target400–600 FPM
NoteSize up vs supply
Frequently Asked Questions
CFM stands for Cubic Feet per Minute — the volume of conditioned air delivered to a room each minute. The most accurate method is a full Manual J load calculation performed by a certified HVAC contractor. For a quick estimate, use the ACH method: multiply the room's cubic volume by the recommended ACH for that room type, then divide by 60. Bedrooms typically need 5–6 ACH; kitchens need 7–9 ACH. As a rule of thumb, residential supply systems deliver 400 CFM per ton of cooling capacity — so a 2-ton system moves 800 CFM total across all rooms.
At 700 FPM target velocity: 150 CFM needs approximately a 6-inch round duct (actual velocity ≈ 764 FPM — slightly over target; use 7-inch for quieter 567 FPM). For 300 CFM at 700 FPM: required area = 300 ÷ 700 = 0.429 sq ft = 61.7 sq in → diameter = √(4×61.7 ÷ π) ≈ 8.9 inches → use 9-inch round duct. At 800 FPM target: 300 CFM fits a 8-inch round duct. Use this calculator to instantly size for any CFM and velocity combination.
Flexible duct has a corrugated inner liner with ridges that create turbulence, increasing friction and reducing effective CFM by about 15% compared to smooth sheet metal of the same diameter. This worsens dramatically if the flex duct is not fully extended — a compressed or kinked flex duct can lose 40–50% of its rated capacity. Industry guidelines recommend limiting flex duct runs to 14 feet or less, fully stretching all sections, and minimizing bends. Rigid sheet metal should be used for trunk lines; flex duct works best only for final short branch connections to registers.
Friction loss is the pressure drop that air experiences as it moves through ductwork, measured in inches of water gauge (in. wg) per 100 feet of duct. Your blower must overcome this pressure drop to deliver rated CFM. A typical residential blower has 0.5 in. wg of total static pressure available. If your longest duct run uses 0.5 in. wg of friction, the blower is at its limit. The equal-friction design method sizes all ducts to the same friction rate — typically 0.08–0.10 in. wg per 100 ft — ensuring balanced pressure across all branches.
Yes — return ducts should be sized for lower velocity (400–600 FPM near rooms vs 700–800 FPM for supply), which means they are physically larger than the equivalent supply duct for the same CFM. Return air must overcome filter resistance in addition to duct friction, so lower velocity reduces static pressure load. Many homes are significantly undersized in returns — a system rated for 1,200 CFM may only deliver 900 CFM because the return ducts create too much resistance. Every room should ideally have a dedicated return vent or a transfer grille to the main return pathway.
