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Flexible Duct Friction Pressure Loss Calculator

Estimate the total friction pressure loss through a flexible round duct run, using a curve-fit of the ASHRAE duct friction chart with an installation-quality correction factor.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
39.37 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Estimated total friction pressure loss

0.06 in. w.g.

Low confidence

This uses a third-party curve-fit approximation of the ASHRAE Fundamentals duct friction chart, not an ASHRAE-published closed-form equation, plus a user-supplied installation-quality multiplier standing in for the actual flex-duct friction chart. For final duct design, use the specific flex duct manufacturer's published friction chart (required by ACCA Manual D Appendix 3) rather than this screening estimate — a compressed or sagging flex run can have far higher friction than the multiplier ranges shown here suggest.

Base smooth-duct friction rate (per 100 ft)
0.04 in. w.g./100ft

At the values currently entered, the estimated total friction pressure loss works out to 0.06 in. w.g.. Confidence on this run is low: check the result against a supplier quote or a professional before ordering or building. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 0.06 in. w.g. — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Base friction rate (in. w.g. per 100 ft) = 0.109136 × CFM^1.9 ÷ Diameter(in)^5.02, a widely-reproduced curve-fit approximation of the ASHRAE Fundamentals Ch. 21 smooth round galvanized duct friction chart. Flexible duct has significantly higher friction than smooth rigid duct of the same diameter — per ASHRAE-sponsored research (Culp, 'Compression Effects on Pressure Loss in Flexible HVAC Ducts'), a fully-extended flex duct runs roughly 1.5-3x smooth duct friction, while typical field installations with some sag/compression can run 3-4x or considerably higher; ACCA Manual D Appendix 3 and manufacturer-published flex-duct friction charts are the authoritative source for a specific product.

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Frequently asked questions

Why does flexible duct have more friction loss than rigid duct of the same size?
Flex duct's inner wire-helix core creates a corrugated, ribbed inner surface, and any sag or compression along the run adds further disruption — per ASHRAE-sponsored research, a fully-extended flex duct runs roughly 1.5-3x smooth rigid duct friction, and a typical field installation with some sag can run 3-4x or considerably higher.
Which installation-quality multiplier should I use?
Use 1.5-2 for flex duct that is fully extended with no sag, 3-4 for an average field installation with some sag, and 5-10+ if the run is visibly compressed or kinked. These are approximate ranges — the specific product's manufacturer friction chart is the authoritative source.
Is this precise enough for final duct design?
No. This calculator uses a third-party curve-fit approximation of the ASHRAE smooth-duct friction chart plus a user-supplied multiplier, not an ASHRAE-published closed-form equation or the actual flex-duct chart. For final design, ACCA Manual D Appendix 3 requires using the specific flex duct manufacturer's published friction chart — a compressed or sagging run can have far higher friction than these multiplier ranges suggest.