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Flex Duct Compression Calculator

Flexible duct that is not pulled tight loses far more pressure than the charts suggest. Enter its stretched and installed lengths to see the penalty, from Lawrence Berkeley National Laboratory’s tests.

Method reviewed October 26, 2026. Version 1.0.0. How it works

This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.

Your inputs

Unit system

The length of the inner liner pulled tight — for an uncut piece, usually the length on the box.

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

Result

Pressure drop multiplier (PDCF)
4.24
Pressure drop as installed
0.062in. w.c.
Compression
15%

At 15 % compression this 8-inch flex duct loses about 4.2 times as much pressure as it would pulled taut (0.062 vs 0.015 in. w.c.).

  • Pull flex duct taut, cut off the excess and support it so it does not sag — compression is the cheapest pressure loss to remove.
Pressure drop if pulled fully taut
0.015in. w.c.
Extra loss caused by compression
0.047in. w.c.
Coefficient a used
21.61
Older size-independent factor (LBNL fit to ASHRAE 2001 chart)
2.48
Show the working
  1. Compression ratio r = 1 − installed length ÷ stretched length
    1 − 21.25 ÷ 25 = 0.15
  2. Coefficient a for 8 in (LBNL Table 3)
    a = 21.61
  3. Pressure drop correction factor PDCF = 1 + a × r
    1 + 21.61 × 0.15 = 4.242
  4. Fully stretched loss of the same duct (Darcy–Weisbach + Colebrook)
    0.0581 in. w.c./100 ft × 25 ft ÷ 100 = 0.0145 in. w.c.
  5. Compressed loss = PDCF × stretched loss
    4.242 × 0.0145 = 0.0616 in. w.c.

What LBNL measured

In 2002, Abushakra, Walker and Sherman at Lawrence Berkeley National Laboratory tested 6, 8 and 10-inch flexible ducts fully stretched and compressed by up to about 30 %. Moderate compression, typical of field installations, raised the pressure drop by about four times; more compression raised it by nearly ten times. Smaller ducts suffered more.

They fitted a simple model: the pressure drop correction factor PDCF = 1 + a × r, where r is the compression ratio and a was 25.35, 21.61 and 16.18 for the 6, 8 and 10-inch ducts. For other sizes they proposed a = 106 × helix pitch ÷ diameter.

How to measure compression

The compression ratio is 1 minus the installed length divided by the fully stretched length of the inner liner. For an uncut piece, the stretched length is usually the length on the box. A 25-foot duct installed over 21 feet is compressed by 16 %.

The calculator multiplies the loss of the same duct pulled taut — from the standard friction method with ASHRAE’s roughness range for fully extended flex — by the correction factor.

Limits of the model

  • Tested on straight ducts of one construction, up to about 30 % compression
  • Sags, kinks and tight bends add more loss on top
  • The older size-independent factor (an LBNL fit to the ASHRAE 2001 chart) is shown for comparison; LBNL found it underestimated compressed losses

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Nominal flex duct size
8 in (tested by LBNL)
Length of the duct when fully stretched
25 ft
Installed (end-to-end) length
21.25 ft
Airflow
150 CFM
Duct material
Flexible duct, fully stretched (ASHRAE low end)
Site elevation
0 ft

Result

Pressure drop multiplier (PDCF)
4.24
Pressure drop as installed
0.062 in. w.c.
Compression
15 %
Pressure drop if pulled fully taut
0.015 in. w.c.
Extra loss caused by compression
0.047 in. w.c.
Coefficient a used
21.61

At 15 % compression this 8-inch flex duct loses about 4.2 times as much pressure as it would pulled taut (0.062 vs 0.015 in. w.c.).

Assumptions

  • LBNL’s model: PDCF = 1 + a·r, where r = 1 − installed length ÷ fully stretched length and a = 25.35 (6 in), 21.61 (8 in) or 16.18 (10 in) from their tests; for other sizes a = 106 × helix pitch ÷ diameter.
  • The fully stretched loss uses ASHRAE’s “rough” range for fully extended flexible duct (0.0035–0.015 ft) with the Colebrook friction factor.
  • PDCF compares the same piece of duct compressed and pulled tight, so it multiplies the loss of the duct at its stretched length.

Limitations

  • Tested on straight ducts of one construction up to about 30 % compression; sags, bends and kinks add more loss.
  • LBNL found PDCF roughly constant with airflow; individual products may differ.

Frequently asked questions

How much does compression increase flex duct pressure drop?

In LBNL’s tests, about 15 % compression roughly quadrupled the pressure drop of a 6-inch duct, and about 29 % compression raised it more than eightfold. Larger ducts were affected less.

Is a little slack really that bad?

Yes. The inner liner crumples and the effective roughness rises steeply, so even modest compression multiplies the loss. Pull flex tight, cut off the excess and support it so it does not sag.

Does this apply to my duct brand?

The tested ducts were typical wire-helix flex with an R-4.2 jacket. Other constructions may differ, but the direction and rough size of the penalty are the same.

Sources

  1. Compression Effects on Pressure Loss in Flexible HVAC Ducts — B. Abushakra, I. S. Walker & M. H. Sherman, Lawrence Berkeley National Laboratory (2002). ASHRAE classes fabric-and-wire flexible duct as “rough”, 0.0035–0.015 ft (1.0–4.6 mm) fully extended; compression raises effective roughness far above that range.
  2. Exhaust Duct Design (ASHRAE Distinguished Lecture; friction per ASHRAE 2021 Handbook p. 21.7) — P. J. Brooks, SMACNA, for ASHRAE (hosted by ETEK). Duct friction loss in the ASHRAE 2021 Handbook uses Darcy–Weisbach with the Colebrook friction factor; absolute roughness is determined experimentally.

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