What size flex duct do I need for 100 CFM?

For 100 CFM at standard residential velocity (600-900 FPM) and friction rate (0.08 in. w.g./100 ft), a 6-inch diameter flexible duct is the correct choice. A 6-inch flex duct delivers 100 CFM at approximately 510 FPM with a friction loss of 0.06 in. w.g. per 100 ft when fully extended. A 5-inch duct can technically handle 100 CFM but runs at 730 FPM, which increases noise and pressure drop — it is only appropriate for very short runs under 15 feet.

How far can you run flexible duct?

ACCA Manual D recommends limiting flex duct runs to 25 feet or less for optimal performance. Runs up to 35 feet are acceptable if the duct is fully extended with proper support every 4-5 feet and maximum sag of 0.5 inches per foot of span. Beyond 35 feet, pressure drop accumulates to the point where airflow drops significantly — a 40-foot run of 6-inch flex duct at 100 CFM loses roughly 0.10 in. w.g. of static pressure, which is often more than the system can afford. For runs over 25 feet, consider upsizing the duct one diameter increment or switching to rigid metal duct, which has 40-60% lower friction loss per foot.

Is flex duct sizing the same as rigid duct sizing?

No. Flex duct has significantly higher friction loss per foot than rigid metal duct — typically 1.5x to 3x higher depending on installation quality. A 6-inch rigid duct carrying 100 CFM has a friction rate of approximately 0.03 in. w.g. per 100 ft, while a 6-inch flex duct at the same airflow has 0.06 in. w.g. per 100 ft when fully extended and up to 0.15 in. w.g. if even partially compressed. ACCA Manual D and ASHRAE Handbook provide separate friction charts for flex duct. The general rule of thumb: if replacing a rigid duct run with flex, go one diameter size larger to compensate for the higher friction.

What happens if flex duct is undersized?

Undersized flex duct causes three measurable problems: (1) Reduced airflow — a duct sized one diameter too small delivers 30-50% less CFM than required, resulting in rooms that never reach setpoint. (2) Increased noise — air velocity rises above 900 FPM, generating audible whooshing and turbulence noise that exceeds the NC 35 threshold acceptable for residential spaces. (3) Higher energy bills — the blower motor works harder against increased static pressure, consuming 15-25% more energy. In extreme cases, chronic high static pressure shortens blower motor life by 3-5 years.

Does flex duct need to be fully stretched for sizing to work?

Yes — this is the single most important installation factor for flex duct performance. Flex duct ships compressed to 20-25% of its extended length. If installed without fully stretching, the inner core bunches up and creates internal turbulence that increases friction loss by 2x to 5x compared to the manufacturer-rated values. A 6-inch flex duct compressed to 70% of its extended length has the same pressure drop as a 4-inch fully extended duct. Always stretch flex duct to its full length before cutting and installing, then allow 2-3% extra for slack at connections. Our friction loss data in this guide assumes fully extended installation per ACCA Manual D standards.

Flexible Duct Sizing Chart: CFM & Diameter Guide

Every HVAC system is only as good as its ductwork — and the most common ductwork failure we see as a manufacturer is flex duct that is the wrong diameter for the airflow it carries. An undersized flex duct starves the room of air, creates noise, and forces the blower to work against excessive static pressure. An oversized duct wastes material and ceiling space. The difference between the two is a sizing chart and five minutes of calculation.

This guide gives you the complete flexible duct sizing chart — CFM requirements by room type and size, the correct duct diameter for each CFM value, maximum recommended run lengths, and the static pressure data you need to verify your design against the total external static pressure budget. All friction loss values are based on fully extended flex duct tested per ASHRAE standards. If you are designing with insulated flex duct, the internal diameter and friction rates are the same — insulation affects the outer diameter only.

In This Guide

1. How Flex Duct Sizing Works
2. CFM Requirements by Room Type and Size
3. The Sizing Chart: CFM to Duct Diameter
4. Friction Loss and Static Pressure Data
5. Maximum Recommended Length by Diameter
6. Step-by-Step Sizing Procedure
7. Five Sizing Mistakes That Kill Performance
8. Flex vs Rigid: When to Upsize
9. FAQ

1. How Flex Duct Sizing Works

Duct sizing balances three variables: airflow (CFM), air velocity (FPM), and pressure drop (in. w.g.). The relationship is governed by the duct's cross-sectional area and its internal friction characteristics. For flexible duct, the sizing process follows ACCA Manual D methodology:

Determine the CFM each room needs — based on the heating/cooling load calculation (Manual J) or the equipment manufacturer's airflow specification.
Select a duct diameter — that delivers the required CFM at a velocity between 600-900 FPM (residential) or 1,000-1,500 FPM (commercial), keeping friction loss within the system's static pressure budget.
Verify the total pressure drop — for the entire duct run (including fittings and transitions) does not exceed the available static pressure from the air handler or furnace blower.

The critical difference between flex duct and rigid metal duct: flex duct has 1.5x to 3x higher friction loss per foot due to the corrugated inner core. This means flex duct sizing charts produce larger diameters than rigid duct charts for the same CFM. Our HVAC CFM and duct sizing calculator covers both types — this article focuses specifically on flex duct.

2. CFM Requirements by Room Type and Size

Before you can size a duct, you need to know how much air the room requires. The table below gives typical CFM values based on room function and floor area. These are planning-level estimates based on 1 CFM per square foot for standard residential spaces (8-foot ceilings, moderate climate) and higher rates for kitchens, server rooms, and commercial spaces. Always use a Manual J load calculation for final design.

Room Type	Typical Size (sq ft)	CFM Required	Recommended Flex Duct Diameter
Small Bedroom	100-120	80-100	6"
Standard Bedroom	120-180	100-150	6"-7"
Master Bedroom	200-350	150-300	7"-8"
Bathroom	40-80	50-80	5"-6"
Home Office	100-150	100-130	6"-7"
Living Room	200-400	200-400	8"-10"
Kitchen	150-250	200-350	8"-10"
Dining Room	120-200	120-200	7"-8"
Basement / Bonus Room	300-600	250-500	8"-12"
Garage Workshop	400-600	300-500	10"-12"
Commercial Office (per zone)	200-500	250-600	8"-14"
Server / IT Room	100-200	400-800	10"-16"

CFM estimates based on 1 CFM/sq ft residential standard (ACCA Manual J baseline). Kitchen and server rooms use higher rates due to elevated heat loads. Always perform a load calculation for final design.

3. The Sizing Chart: CFM to Duct Diameter

This is the core reference table. For each standard flex duct diameter, it shows the recommended CFM range, the resulting air velocity, and the friction loss per 100 equivalent feet. All values assume fully extended flex duct installed per ACCA Manual D — compressed or sagging duct will have significantly higher friction loss (see Section 7).

Duct Diameter	Cross-Section Area (sq in)	Recommended CFM Range	Velocity at Mid-Range (FPM)	Friction Loss (in. w.g./100 ft)
4"	12.6	20-50	400	0.04
5"	19.6	40-80	530	0.06
6"	28.3	75-140	540	0.06
7"	38.5	110-200	590	0.06
8"	50.3	150-300	640	0.07
9"	63.6	200-400	680	0.07
10"	78.5	300-550	780	0.08
12"	113.1	500-850	860	0.08
14"	153.9	700-1,200	900	0.08
16"	201.1	1,000-1,600	930	0.08
18"	254.5	1,300-2,000	940	0.07
20"	314.2	1,600-2,500	940	0.07

Friction loss values for fully extended flex duct at the mid-range CFM, based on ASHRAE friction chart data for corrugated flexible duct. Residential velocity target: 600-900 FPM. Values assume 0.08 in. w.g./100 ft design friction rate per ACCA Manual D.

How to read this chart: Find your required CFM in the third column. The corresponding diameter in the first column is your minimum flex duct size. If your CFM falls near the top of a range, size up to the next diameter — running at the top of the range increases noise and pressure drop.

Quick reference example: A 200 sq ft master bedroom needs ~200 CFM. The chart shows 8-inch flex duct handles 150-300 CFM. An 8-inch duct is the right size. If the run is over 25 feet or goes through an unconditioned attic, consider 9-inch to reduce pressure drop and ensure the room gets full airflow.

4. Friction Loss and Static Pressure Data

Friction loss is the pressure that air loses as it moves through the duct — it is measured in inches of water gauge (in. w.g.) per 100 feet of equivalent duct length. This is the single most important number for verifying that your duct design will actually deliver the required CFM. If the total friction loss across all duct runs exceeds the blower's available external static pressure, airflow drops and the system underperforms.

The table below shows friction loss at specific CFM values for the most common flex duct diameters. These values are for fully extended, properly supported flex duct — the baseline condition that ACCA Manual D assumes. Compressed or sagging duct will have 2x-5x higher friction loss.

CFM	5" Flex	6" Flex	7" Flex	8" Flex	10" Flex	12" Flex
50	0.03	0.01	—	—	—	—
75	0.06	0.03	0.01	—	—	—
100	0.10	0.06	0.03	0.01	—	—
150	0.20	0.10	0.05	0.03	0.01	—
200	0.35	0.16	0.08	0.05	0.02	0.01
300	—	0.33	0.16	0.09	0.03	0.01
400	—	—	0.26	0.14	0.05	0.02
500	—	—	—	0.20	0.07	0.03
700	—	—	—	—	0.12	0.05
1,000	—	—	—	—	0.22	0.08

Friction loss in inches of water gauge per 100 equivalent feet. Data for fully extended flex duct per ASHRAE friction charts. "—" indicates the velocity exceeds 900 FPM (residential limit) or the diameter is oversized for that CFM. Equivalent length includes straight duct plus fitting equivalents.

Practical application: Most residential furnaces and air handlers have 0.50 in. w.g. of total external static pressure available. After subtracting pressure drops for the filter (~0.10), coil (~0.15), and supply/return fittings (~0.10), you typically have 0.10-0.15 in. w.g. remaining for the longest supply duct run. If your longest run is 25 feet with 100 CFM through a 6-inch flex duct, the friction loss is 0.06 × (25/100) = 0.015 in. w.g. — well within budget. Add equivalent lengths for any bends or transitions using the fitting tables in our duct fitting guide.

5. Maximum Recommended Length by Diameter

One of the most common questions we receive from contractors: "How long can I run this flex duct before I need to go up a size?" The answer depends on the CFM, the available static pressure, and the installation quality — but the table below gives practical maximum lengths based on a 0.08 in. w.g. available static pressure budget for the branch run (a typical residential budget after accounting for trunk, filter, and coil losses).

Duct Diameter	CFM (mid-range)	Max Length — Fully Extended	Max Length — Typical Install (85% stretched)	Pressure Drop at Max Length
5"	60	25 ft	15 ft	0.08 in. w.g.
6"	100	35 ft	20 ft	0.08 in. w.g.
7"	150	40 ft	25 ft	0.08 in. w.g.
8"	200	40 ft	25 ft	0.08 in. w.g.
10"	400	45 ft	30 ft	0.08 in. w.g.
12"	650	50 ft	30 ft	0.08 in. w.g.

Max lengths are for straight runs with no fittings. Each 90-degree flex duct bend adds 15-25 equivalent feet. Each 45-degree bend adds 8-12 equivalent feet. Based on 0.08 in. w.g. available static pressure for the branch run.

Notice the dramatic difference between "fully extended" and "typical install (85% stretched)" — the partially compressed duct has 35-40% less usable length before hitting the pressure budget. This is why proper installation technique matters as much as correct sizing. See our duct sealing and installation guide for the full installation procedure.

6. Step-by-Step Sizing Procedure

Here is the procedure we recommend to HVAC contractors and mechanical engineers for sizing flex duct on residential and light commercial projects. This follows ACCA Manual D methodology with flex-duct-specific adjustments.

Step 1: Determine room CFM

Use the Manual J load calculation to get the heating and cooling BTU requirements for each room. Convert to CFM using the equipment's temperature rise (heating) or temperature drop (cooling). For a quick estimate, use 1 CFM per square foot for standard residential rooms, 1.5 CFM/sq ft for kitchens with cooking appliances, and 2-4 CFM/sq ft for server rooms and data centers.

Step 2: Select duct diameter from the sizing chart

Find your required CFM in the sizing chart (Section 3). Choose the diameter where your CFM falls in the lower two-thirds of the recommended range — this keeps velocity and noise within residential limits (under 900 FPM, under NC 35). If the CFM falls in the upper third, go up one diameter size.

Step 3: Calculate total equivalent length

Measure the straight-line duct run from the supply plenum or branch takeoff to the ceiling diffuser or register. Add equivalent lengths for each fitting in the run: 15-25 ft for each 90-degree bend, 8-12 ft for each 45-degree bend, and 5-10 ft for each transition or reducer.

Step 4: Check friction loss against static pressure budget

Multiply the friction loss from the chart (per 100 ft) by your total equivalent length divided by 100. Compare this number against your available branch static pressure — typically 0.08-0.15 in. w.g. for residential systems. If the friction loss exceeds the budget, either upsize the diameter or shorten the run by relocating the branch takeoff.

Step 5: Verify velocity for noise

Calculate actual velocity: Velocity (FPM) = CFM / Area (sq ft). For bedrooms and living spaces, keep velocity under 700 FPM. For utility rooms, laundry, and garages, up to 900 FPM is acceptable. Above 900 FPM, the flex duct inner core generates turbulence noise that is audible in most residential environments. Our noise control guide covers mitigation strategies when velocity cannot be reduced.

7. Five Sizing Mistakes That Kill Performance

We manufacture millions of feet of flex duct every year, and we see the same five mistakes on jobsite callbacks. Every one of them is a sizing or installation error — not a product defect.

Mistake 1: Using rigid duct sizing charts for flex duct

This is the most expensive mistake. A 6-inch rigid duct at 100 CFM has a friction rate of 0.03 in. w.g. per 100 ft. A 6-inch flex duct at the same CFM has 0.06 — double the friction. Contractors who size flex duct using rigid duct tables end up with runs that are one diameter too small, resulting in 30-50% airflow reduction at the register. Always use flex-specific friction charts or add one size when converting from a rigid duct design.

Mistake 2: Not accounting for compression

Flex duct ships compressed to 20-25% of its extended length. If installed at even 85% of full extension, the friction loss increases by 50-75% compared to the rated value. At 70% extension, it doubles. Our sizing charts assume 100% extension. On every run, fully stretch the duct before cutting, then allow 2-3% slack for connection flexibility — no more. See our air balancing guide for measuring delivered airflow after installation.

Mistake 3: Ignoring bend equivalent lengths

A single 90-degree bend in flex duct adds 15-25 equivalent feet of friction loss. A typical residential branch run with two 90-degree bends and one 45-degree bend adds 40-60 equivalent feet on top of the straight run. A 15-foot straight run with two turns becomes a 55-75 foot equivalent run — and many contractors size only for the 15 feet. Always calculate total equivalent length including all bends and fittings.

Mistake 4: Excessive sag between supports

When flex duct sags between support points, the bottom compresses and the cross-section deforms from round to oval. A 6-inch duct with 2 inches of sag effectively becomes a 5-inch duct at the sag point — cutting airflow by 30%. ACCA Manual D limits sag to 0.5 inches per foot of support span. Support flex duct every 4-5 feet with 1.5-inch-wide nylon straps. Never use wire or zip ties — they cut into the insulation jacket and create a thermal and vapor barrier break.

Mistake 5: Sizing for cooling only and forgetting heating

In heating mode, supply air temperature is 110-140 degF (higher than the 55 degF cooling supply). The warmer air is less dense, so the same CFM requires slightly higher velocity. If you sized the duct at the absolute minimum for cooling CFM, heating mode may deliver 5-10% less actual mass airflow. The safest practice: size for cooling CFM (which is typically higher than heating CFM for the same room) and verify that heating velocity stays under 900 FPM.

8. Flex vs Rigid: When to Upsize

If your HVAC design was done with rigid metal duct diameters, converting those runs to flex duct requires adjustment. The general rule: go up one diameter size when switching from rigid to flex to compensate for the higher friction loss. Here is the conversion table:

Rigid Duct Diameter	Equivalent Flex Duct Diameter	CFM Capacity (both)	When Same-Size Flex Is Acceptable
5" rigid	6" flex	60-80	Runs under 10 ft, fully extended
6" rigid	7" flex	80-140	Runs under 15 ft, fully extended
7" rigid	8" flex	140-200	Runs under 15 ft, fully extended
8" rigid	9"-10" flex	200-300	Runs under 10 ft, fully extended
10" rigid	12" flex	300-550	Short connector runs only (<6 ft)
12" rigid	14" flex	550-850	Short connector runs only (<6 ft)

Upsizing rule applies to runs over 15 feet. For short connector runs (branch takeoff to diffuser, typically 3-6 feet), same-size flex is acceptable if fully extended. For a detailed comparison of all duct materials, see our HVAC ducting materials comparison.

When runs exceed 25 feet or include multiple bends, consider using rigid spiral duct for the trunk portion and flex duct only for the last 6-10 feet as a connector to the diffuser or register. This hybrid approach gives you the low friction of rigid duct for the long run and the flexibility of flex duct for the connection — the best of both materials.