Getting your flex duct CFM right is the foundation of a well-performing HVAC system. Too little airflow and the room never reaches setpoint temperature. Too much and you get noise, drafts, and wasted energy. The calculation itself is straightforward -- this guide walks through every step with real numbers so you can size flex duct branches for any residential or light commercial project.
The Core Formula
The relationship between CFM, duct area, and air velocity is:
CFM = Area (sq ft) x Velocity (fpm)
or equivalently: Area = CFM / Velocity
Where CFM is cubic feet per minute of airflow, Area is the cross-sectional area of the duct in square feet, and Velocity is the air speed in feet per minute (fpm). For round duct, the area is pi x r2, or pi x (diameter/2)2 / 144 to convert from inches to square feet.
To find the required duct diameter when you know the CFM and target velocity:
Diameter (inches) = sqrt(CFM x 144 x 4 / (pi x Velocity))
That formula looks complex, but in practice you just use a lookup table. Let us build one.
Step 1: Determine Room CFM Requirements
The most accurate method is ACCA Manual J, which calculates heating and cooling loads based on window area, insulation, orientation, climate zone, and more. But for estimating purposes, use these rules of thumb:
| Room Type | CFM per sq ft | Example: 150 sq ft room |
|---|---|---|
| Standard bedroom | 1.0 | 150 CFM |
| Living room | 1.0 - 1.2 | 150 - 180 CFM |
| Kitchen | 1.2 - 1.5 | 180 - 225 CFM |
| Sun room / south-facing room | 1.5 | 225 CFM |
| Bathroom | 1.0 | 50 CFM (or 1 CFM/sq ft, min 50) |
| Bonus room over garage | 1.5 | 225 CFM |
| Commercial office | 1.0 - 1.2 | 150 - 180 CFM |
Step 2: Select Target Velocity
ASHRAE recommends these velocity ranges to balance airflow performance against noise:
- Residential supply duct: 600-900 fpm. Going above 900 fpm in flex duct creates audible noise at registers.
- Residential return duct: 400-600 fpm. Return grilles are typically larger to keep velocity (and noise) low.
- Commercial supply: 700-1,200 fpm in rigid metal, 600-900 fpm in flex duct.
- Trunk lines (rigid metal): 900-1,500 fpm. Higher velocities are acceptable on trunk lines because they are further from occupied spaces.
For flex duct specifically, ACCA recommends not exceeding 900 fpm in residential branch runs to keep noise below NC-30 (barely perceptible in a quiet room).
Step 3: Calculate Duct Diameter
With CFM and velocity determined, use this quick-reference table to find the right flex duct diameter. CFM values are at 900 fpm (residential supply) and 700 fpm (quiet applications):
| Flex Duct Diameter | Max CFM at 900 fpm | Max CFM at 700 fpm | Room Size (at 1 CFM/sq ft) |
|---|---|---|---|
| 4" | 79 CFM | 61 CFM | Up to 80 sq ft |
| 5" | 123 CFM | 95 CFM | 80 - 120 sq ft |
| 6" | 177 CFM | 137 CFM | 120 - 175 sq ft |
| 7" | 240 CFM | 187 CFM | 175 - 240 sq ft |
| 8" | 314 CFM | 244 CFM | 240 - 315 sq ft |
| 10" | 491 CFM | 382 CFM | 315 - 490 sq ft |
| 12" | 707 CFM | 550 CFM | 490 - 700 sq ft |
For a complete chart covering all sizes from 3 inches to 24 inches, see our flex duct CFM chart.
Step 4: Account for Flex Duct Friction
Flex duct has higher friction than rigid metal duct due to its corrugated inner wall. ACCA Manual D applies a correction factor that effectively increases the equivalent length of flex duct runs. The impact:
- Runs under 10 feet: Friction correction is minimal. Use the standard CFM table values above.
- Runs 10-15 feet: The friction loss is noticeable but usually within the system's available static pressure. Stay within the table values.
- Runs 15-25 feet: Consider sizing up one diameter. A room that needs 140 CFM on a 20-foot flex run should use a 7-inch duct instead of 6-inch.
- Runs over 25 feet: Flex duct runs this long should be avoided if possible. If required, size up one or two diameters and verify the total system friction budget.
Critical: Add Equivalent Length for Bends
Every 90-degree bend in flex duct adds approximately 15 equivalent feet to the run length. A 12-foot branch run with two 90-degree bends has an effective length of 42 feet (12 + 15 + 15). This dramatically increases friction and reduces CFM delivery. Minimize bends in your routing, and use the effective length -- not the physical length -- in your sizing calculations.
Worked Example: 2,000 sq ft Home
Let us size the flex duct branches for a typical 2,000 sq ft, 4-bedroom home with a 3-ton AC system (1,200 CFM total) in Houston, TX:
| Room | Size (sq ft) | CFM Need | Run Length | Bends | Duct Size |
|---|---|---|---|---|---|
| Master Bedroom | 250 | 250 | 12 ft | 1 | 8" |
| Bedroom 2 | 150 | 150 | 15 ft | 1 | 6" |
| Bedroom 3 | 120 | 120 | 20 ft | 2 | 7" |
| Bedroom 4 | 100 | 100 | 8 ft | 0 | 5" |
| Living Room | 300 | 300 | 10 ft | 1 | 8" |
| Kitchen | 150 | 180 | 6 ft | 0 | 6" |
| Dining | 130 | 130 | 14 ft | 1 | 6" |
Design notes: Bedroom 3 was sized up to 7-inch because the 20-foot run plus two bends creates an effective length of 50 feet -- too much friction for a 6-inch duct at 120 CFM. The kitchen gets a higher CFM factor (1.2) due to cooking heat load. The living room uses two 8-inch runs if needed, or a single 10-inch run split at a tee.
Step 5: Verify System Balance
After sizing individual branch runs, verify the total system:
- Sum all branch CFM. The total should approximately equal the system's rated airflow (in our example, 1,200 CFM for a 3-ton unit). If the total is significantly higher, the system will not deliver adequate airflow to each room. If significantly lower, the system has excess capacity.
- Size the trunk line. The trunk line must carry the total system CFM from the air handler to the first branch takeoff, then reduce in size as branches split off. A 1,200 CFM system needs at least a 14-inch trunk.
- Check total static pressure. The sum of all friction losses (trunk, branches, fittings, registers, filters) must not exceed the air handler's rated external static pressure -- typically 0.4-0.8 in. w.g. for residential systems.
- Size return air. The return air system must handle at least 90% of the supply CFM. A single central return for a 3-ton system needs a 20-inch duct or equivalent rectangular size.
Common CFM Calculation Mistakes
- Using total system CFM as branch CFM. A 3-ton system delivers 1,200 CFM total, not per room. Each branch gets a fraction proportional to the room's load.
- Ignoring friction in flex duct. Flex duct has approximately 1.5x the friction of rigid metal at full stretch. On long runs or runs with multiple bends, this compounds significantly. Always calculate effective length including bend equivalents.
- Not accounting for equipment blower limitation. The air handler has a maximum external static pressure rating. If the duct system friction exceeds this, the blower cannot deliver the rated CFM. Total system friction must stay under the equipment limit.
- Same duct size for every room. A 100 sq ft bedroom and a 300 sq ft living room have very different CFM needs. Using 6-inch duct for everything means some rooms are starved while others get excess airflow.
- Not using flex duct at full stretch. A flex duct installed at 70% of its stretched length has approximately 2-3x the friction of the same duct at full stretch. Always measure and cut to full-stretch length.
Want an interactive calculator? Our online ductulator lets you input CFM and velocity to instantly calculate the required duct diameter, with presets for common residential and commercial applications. For complete size charts, see our flex duct sizes guide.