Total Effective Length Calculator
Add up straight duct and fitting equivalent lengths on the longest supply and return runs to get the total effective length — and, with the available static pressure, the design friction rate.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Total effective length
- 170ft
- Supply run TEL
- 120ft
- Return run TEL
- 50ft
- Design friction rate
- 0.118in. w.c./100 ft
Total effective length 170 ft (supply 120 ft + return 50 ft). With 0.2 in. w.c. available, size the ducts at about 0.118 in. w.c. per 100 ft.
- Use the longest run (the one with the greatest total effective length), which is not always the longest in feet: a short run with many fittings can be “longer”.
- Share of TEL from fittings
- 50%
Show the working
- Fitting 1 (supply)1 × 10 ft = 10 ft
- Fitting 2 (supply)2 × 15 ft = 30 ft
- Fitting 3 (supply)1 × 20 ft = 20 ft
- Fitting 4 (return)1 × 25 ft = 25 ft
- Supply TEL = straight + fittings60 + 60 = 120 ft
- Return TEL = straight + fittings25 + 25 = 50 ft
- Total effective length = supply + return170 ft
- Design friction rate = ASP × 100 ÷ TEL0.2 × 100 ÷ 170 = 0.118 in. w.c. per 100 ft
What total effective length is
Every fitting resists airflow as if it were a length of straight duct. Total effective length (TEL) is the straight duct plus those equivalent lengths, for the longest supply run plus the longest return run. It is usually dominated by fittings, not straight duct.
Divide the available static pressure by TEL and multiply by 100 to get the design friction rate in inches of water per 100 feet — the method shown in ACCA’s Manual D checklist.
Where the equivalent lengths come from
Equivalent lengths depend on the exact fitting and are published by fitting manufacturers and in design references, some of which are copyrighted. This calculator does not include them: the numbers in the form are placeholders to show how it works. Replace them with the values for your fittings.
Which run is the longest
The run with the greatest total effective length sets the friction rate. A short run with several elbows and a boot can be “longer” than a long straight one, so check the candidates.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Longest supply run: straight duct length
- 60 ft
- Longest return run: straight duct length
- 25 ft
- Number of fitting types
- 4
- Fitting 1: on the
- Supply run
- Fitting 1: how many
- 1
- Fitting 1: equivalent length of each
- 10 ft
- Fitting 2: on the
- Supply run
- Fitting 2: how many
- 2
- Fitting 2: equivalent length of each
- 15 ft
- Fitting 3: on the
- Supply run
- Fitting 3: how many
- 1
- Fitting 3: equivalent length of each
- 20 ft
- Fitting 4: on the
- Return run
- Fitting 4: how many
- 1
- Fitting 4: equivalent length of each
- 25 ft
- Available static pressure for the ducts (optional)
- 0.2 in. w.c.
Result
- Total effective length
- 170 ft
- Supply run TEL
- 120 ft
- Return run TEL
- 50 ft
- Share of TEL from fittings
- 50 %
- Design friction rate
- 0.118 in. w.c./100 ft
Total effective length 170 ft (supply 120 ft + return 50 ft). With 0.2 in. w.c. available, size the ducts at about 0.118 in. w.c. per 100 ft.
Assumptions
- Total effective length = straight length plus the equivalent length of every fitting, for the longest supply run plus the longest return run, as the Manual D checklist describes.
- Design friction rate = available static pressure × 100 ÷ total effective length.
Limitations
- Fitting equivalent lengths are not built in: they come from the fitting manufacturer or your design reference (published tables are copyrighted). The defaults are placeholders only.
Frequently asked questions
Do I add the supply and return together?
Yes. The design friction rate divides the available static pressure by the longest supply run plus the longest return run.
Why are the fitting values just examples?
Published equivalent-length tables are copyrighted and vary by fitting, so the site does not reproduce them. Enter the values from your fitting manufacturer or design reference.
What friction rate should I expect?
It is whatever your available pressure and TEL give. The Manual D checklist’s worked example lands at 0.10 in. w.c. per 100 ft; much lower means very large ducts, much higher means small, fast ones.
Sources
- Manual D calculation checklist (Table 3-1 velocity limits) — ACCA (hosted by AHRI). Friction rate = available static pressure × 100 / total effective length; residential velocity limits (rigid trunk 700 recommended / 900 max; flex 600 / 700).
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