What is a normal pressure drop for flexible duct?

For properly sized and fully extended flexible duct, the design friction rate is typically 0.06–0.10 in. w.g. per 100 feet. ACCA Manual D recommends designing to a total available static pressure budget of 0.08 in. w.g./100 ft for residential systems. In practice, a 20-foot run of 8-inch flex duct at 200 CFM should drop about 0.015–0.020 in. w.g. If your measurement shows more than 0.05 in. w.g. on a run that short, the duct is either compressed, kinked, or undersized.

How much does a 90-degree bend add to flexible duct pressure drop?

A 90-degree bend in flexible duct adds the equivalent of 10–20 feet of straight duct run, depending on bend radius. A tight bend with radius equal to the duct diameter (R/D = 1.0) adds approximately 20 equivalent feet. A gradual bend with R/D = 1.5 adds about 10 equivalent feet. This is why ACCA Manual D requires a minimum centerline bend radius of 1.0 × the duct diameter for flex duct — going tighter than that effectively creates a near-complete blockage in the inner liner and can reduce airflow by 30–50%.

Why does compressed flex duct have such high pressure drop?

Flex duct ships compressed to about 20–25% of its extended length. When installed without fully stretching, the inner wire coil bunches into tight corrugations that act like dozens of sharp bends in series. A 6-inch flex duct compressed to 70% of its extended length has a friction rate 3–5 times higher than the fully extended value — equivalent to reducing the effective diameter by 1.5–2 inches. This is the single most common cause of "low airflow" complaints in flex duct systems. Always pull the duct fully taut before securing ends, then allow 1–2 inches of slack at each connection.

What is the maximum CFM for 6-inch flex duct?

A 6-inch flexible duct can carry 100–125 CFM at the standard residential design velocity of 600–900 FPM and friction rate of 0.08 in. w.g./100 ft. At 100 CFM, a fully extended 6-inch flex duct has a friction rate of approximately 0.06 in. w.g./100 ft. At 125 CFM, it rises to 0.10 in. w.g./100 ft and air velocity hits 900 FPM — the upper limit for noise-free operation. Beyond 125 CFM in a 6-inch duct, noise becomes audible (NC 35–45 range) and pressure drop accumulates quickly, especially if runs exceed 15 feet.

How do you measure static pressure drop in flex duct?

Use a digital manometer with 0.001 in. w.g. resolution and two static pressure taps — one at the beginning of the duct run (downstream of the takeoff) and one at the end (at the register boot). Drill two ⅛-inch holes, insert Schrader valve taps or straight static pressure probes, and connect to the high and low ports of the manometer with tubing. Run the system at design airflow (verify with a flow hood at the register) and read the differential pressure. Compare to the calculated value from the duct friction chart. A reading significantly higher than calculated indicates compression, kinking, or an undersized duct.

Flexible Duct Pressure Drop: Charts & Fixes

Low airflow in a flex duct system has one root cause more than any other: pressure drop that was never calculated. A duct run that looks right on the blueprint — correct diameter, reasonable length — can deliver 40% less air than designed if it is compressed, kinked, or loaded with tight bends. Understanding how pressure drop accumulates in flexible ductwork is the difference between a system that balances on the first try and one that requires two return visits to fix.

This guide covers the friction loss data for flexible duct by diameter, the equivalent length of bends and fittings, the dramatic effect of duct compression, and a step-by-step procedure for calculating the total pressure drop on any run. All values are based on manufacturer test data per ASHRAE Handbook – Fundamentals and ACCA Manual D methodology, with fully extended duct as the baseline.

In This Guide

1. How Pressure Drop Works in Flex Duct
2. Friction Loss Chart: CFM vs Pressure Drop by Diameter
3. The Compression Effect: Why Installed vs Rated Performance Differs
4. Bend & Fitting Losses: Equivalent Length Tables
5. Step-by-Step Pressure Drop Calculation
6. Troubleshooting Low Airflow with Pressure Measurements
7. FAQ

1. How Pressure Drop Works in Flex Duct

Every HVAC system has a total external static pressure (TESP) budget — the pressure the blower must overcome to push air through the entire duct system and out the registers. Residential systems typically have a TESP budget of 0.50 in. w.g. After subtracting the pressure drop across the air handler coil (0.20–0.30 in. w.g.) and filter (0.08–0.15 in. w.g.), a residential system may have only 0.10–0.20 in. w.g. left for the supply and return ductwork combined.

That is a tight budget. Pressure drop in flexible duct comes from three sources:

Friction loss along the duct length — caused by air rubbing against the corrugated inner liner. Flex duct has 1.5x–3x higher friction than rigid metal duct of the same diameter because the corrugated liner creates turbulence.
Local losses at bends and fittings — each 90-degree bend, offset, or takeoff adds the equivalent of extra feet of straight duct. These add up quickly on complex runs.
Compression losses — if the duct is not fully stretched, friction multiplies dramatically. This is the most common and most preventable problem on real job sites.

2. Friction Loss Chart: CFM vs Pressure Drop by Diameter

The table below shows friction rate (in. w.g. per 100 feet) for fully extended flexible duct at common CFM values. Data is at standard air conditions (70°F, sea level) per ASHRAE Handbook – Fundamentals Chapter 21 flex duct friction data.

Diameter	50 CFM	75 CFM	100 CFM	150 CFM	200 CFM	300 CFM	Velocity @150 CFM
4 inch	0.35	0.70	1.15	—	—	—	—
5 inch	0.12	0.24	0.38	0.75	—	—	1,110 FPM
6 inch	0.04	0.08	0.06	0.14	0.23	—	764 FPM
7 inch	0.02	0.04	0.06	0.08	0.14	—	562 FPM
8 inch	0.01	0.02	0.03	0.06	0.09	0.18	430 FPM
10 inch	<0.01	0.01	0.01	0.03	0.04	0.08	275 FPM
12 inch	<0.01	<0.01	0.01	0.01	0.02	0.05	191 FPM

Values in in. w.g./100 ft for fully extended flexible duct. Green = within design range (≤0.10), Amber = high (0.10–0.25), Red = excessive (>0.25). Per ASHRAE Handbook – Fundamentals flex duct friction data.

Key takeaway: A 4-inch or 5-inch duct carrying anything over 75 CFM is operating at extreme pressure — use 6 inch minimum for any supply run delivering over 75 CFM. A 12-inch flex duct has 20x lower friction than a 6-inch duct at the same airflow.

3. The Compression Effect: Why Installed vs Rated Performance Differs

The values in the table above assume fully extended flex duct. This is how the manufacturer tests the product and how it appears on data sheets. On real job sites, flex duct is rarely installed at full extension — and the performance penalty is severe.

Extension Level	Compression %	Friction Multiplier	Effective Diameter	Airflow at 0.08 in. w.g.
Fully extended (100%)	0%	1.0x	8 inch	200 CFM
Slightly loose (90%)	10%	1.5x	~7.5 inch eff.	173 CFM (−14%)
Loose (75%)	25%	2.5x	~6.5 inch eff.	148 CFM (−26%)
Compressed (50%)	50%	5x	~5 inch eff.	98 CFM (−51%)
Highly compressed (30%)	70%	10x+	~4 inch eff.	62 CFM (−69%)

8-inch flex duct example. Compression multipliers per ACCA Manual D Table 3-2 and DOE Flex Duct Performance Characterization Study.

The data is stark: a 50%-compressed 8-inch flex duct delivers half the designed airflow. This is why rooms at the end of a long run with any compression deliver barely perceptible airflow even with a high-capacity blower. The field fix is always the same — pull the duct fully extended and re-support.

4. Bend & Fitting Losses: Equivalent Length Tables

Every change of direction in a flex duct run adds pressure drop. We express this as "equivalent length" — how many feet of straight duct a fitting is worth in pressure drop terms. Add all equivalent lengths to your measured straight duct length before calculating friction loss.

Fitting Type	Description	Equiv. Length (6")	Equiv. Length (8")	Equiv. Length (10")
90° bend, R/D = 1.0	Tight, min code radius	15 ft	20 ft	25 ft
90° bend, R/D = 1.5	Good practice radius	8 ft	10 ft	12 ft
45° bend	Half of 90° loss	5 ft	6 ft	7 ft
Wye branch	Flex to flex junction	10 ft	13 ft	16 ft
Register boot	End-of-run floor/ceiling boot	10 ft	12 ft	15 ft
Takeoff from plenum	Start of supply run	5 ft	7 ft	9 ft

Equivalent length values per ACCA Manual D Table 2-2 and SMACNA HVAC Systems Duct Design, 4th Edition. Values for fully extended flex duct with proper bend radius.

A single 90-degree bend at minimum code radius in an 8-inch duct adds the equivalent of a 20-foot straight run. A run with two tight 90-degree bends and a register boot adds 52 equivalent feet before you measure an inch of straight duct. On a 20-foot straight run, that triples the effective duct length to 72 feet — and triples the pressure drop.

5. Step-by-Step Pressure Drop Calculation

Here is the calculation procedure for a single flex duct supply run using ACCA Manual D methodology:

Determine the CFM requirement for the room (from Manual J load calculation or equipment design data).
Select a duct diameter from the friction loss chart above that keeps friction rate at or below 0.08 in. w.g./100 ft at your design CFM.
Measure the actual duct run length in feet (straight sections only).
Add equivalent lengths for all bends and fittings using the table above.
Calculate total equivalent length (TEL) = straight length + sum of fitting equivalent lengths.
Calculate pressure drop = (friction rate from chart) × (TEL / 100).
Check against your static pressure budget. Supply side budget is typically 40–50% of available static pressure after subtracting coil and filter losses.

Worked Example

Bedroom supply run: 150 CFM, 8-inch flex duct, 18 feet straight, one 90° bend (R/D = 1.5), one register boot.

Straight length: 18 ft
90° bend equivalent (8"): +10 ft
Register boot equivalent (8"): +12 ft
Total equivalent length: 40 ft
Friction rate at 150 CFM in 8" duct: 0.06 in. w.g./100 ft
Pressure drop: 0.06 × (40/100) = 0.024 in. w.g.
Within budget ✓ (typically 0.10–0.12 in. w.g. per supply run)

6. Troubleshooting Low Airflow with Pressure Measurements

When a room has inadequate airflow, the pressure drop measurement tells you what is wrong before you open the ceiling:

Measured pressure drop much higher than calculated: Duct is compressed or kinked. Pull it straight and re-measure.
Pressure drop at the expected value but airflow still low: The duct was sized correctly for the stated CFM, but the blower is not delivering the rated airflow (check filter, coil, or blower speed).
Very low pressure at both ends: There may be a disconnect or leak in the duct run — no pressure means no air is actually flowing through.
Pressure drop exactly 3–5x calculated: Classic compression signature. The duct is installed at roughly 50–60% of its extended length.

A digital manometer costs $150–$300 and can diagnose most flex duct problems in 15 minutes. The measurement-based approach eliminates guesswork and prevents the second callback. For a deeper dive into how bends specifically affect boot performance, see our duct boot selection guide and complete flex duct sizing chart.