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The actual measured straight length of the duct run.
This is the physical run length before adding any fitting-contributed equivalent length.
The total count of 90° elbow fittings in the duct run.
Count every 90° elbow along the run being analyzed for pressure drop.
The straight-duct length that produces the same friction loss as one 90° elbow.
This value depends on duct size, shape, and elbow radius/type — read it from the SMACNA HVAC Duct Design Manual table or the fitting manufacturer's data for your exact fitting.
The total count of tee or branch fittings in the duct run.
Count every branch takeoff or tee fitting along the run being analyzed.
The straight-duct length that produces the same friction loss as one tee or branch fitting.
Tees typically have a higher equivalent length than elbows of the same duct size because of the more abrupt flow disruption at the branch.
Total equivalent duct length
119 ft
Equivalent length per fitting varies by fitting type, radius/angle, and duct size — always use the specific SMACNA HVAC Duct Design Manual table entry or fitting manufacturer's data for your exact fitting, not an assumed default.
- Fitting-contributed equivalent length
- 70 ft
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HVAC Duct Fitting Equivalent Length Calculator: 119 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Total equivalent length = straight duct length + Σ(fitting count × equivalent length per fitting), the standard SMACNA/ASHRAE method for estimating a duct run's total frictional resistance as a single equivalent straight-duct length. Equivalent length per fitting type/size is read from SMACNA HVAC Duct Design Manual tables or the fitting manufacturer's data — this calculator applies the per-fitting equivalent lengths you supply from that source. This is the ductwork-specific counterpart to the site's pipe-fitting-equivalent-length-calculator, which covers hydronic/plumbing pipe fittings under a different set of tables.
Inputs used
- Straight Duct Run Length
- 49 ft
- Number of 90° Elbows
- 3
- Equivalent Length per 90° Elbow (from SMACNA Table)
- 15 ft
- Number of Tees/Branches
- 1
- Equivalent Length per Tee (from SMACNA Table)
- 25 ft
Intermediate steps
- Fitting-contributed equivalent length
- 70 ft
Confidence note: Equivalent length per fitting varies by fitting type, radius/angle, and duct size — always use the specific SMACNA HVAC Duct Design Manual table entry or fitting manufacturer's data for your exact fitting, not an assumed default.
What this calculation does not cover
- Two fitting types is not a duct system. Transitions, offsets, balancing and fire dampers, filters, coils, flexible connectors, and the entry and exit losses at the plenum and at the grille all carry equivalent lengths of their own — and on a short branch, the register and its damper can outweigh every elbow in the run. What comes back here is a floor under the real resistance, not the resistance.
- An equivalent length is not a pressure. To get the static the fan has to make, this total is multiplied by the friction rate for that duct at that airflow, the figure quoted per 100 ft or per 100 m on the friction chart, and it is that pressure the fan is selected against. The same 40 m of equivalent length is a trivial loss in a generously sized trunk and a fan-stalling one in an undersized branch.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Total equivalent length = straight duct length + Σ(fitting count × equivalent length per fitting), the standard SMACNA/ASHRAE method for estimating a duct run's total frictional resistance as a single equivalent straight-duct length. Equivalent length per fitting type/size is read from SMACNA HVAC Duct Design Manual tables or the fitting manufacturer's data — this calculator applies the per-fitting equivalent lengths you supply from that source. This is the ductwork-specific counterpart to the site's pipe-fitting-equivalent-length-calculator, which covers hydronic/plumbing pipe fittings under a different set of tables.
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