Dryer Vent Length Calculator
Dryer vent effective length vs 35 ft code max.
What it does
Dryer vents choke on bends before length. Each 90-degree elbow eats 5 ft of the 35 ft IRC budget and each 45 eats 2.5, so 20 straight feet with two 90s runs exactly 30 ft effective. Long twisty runs dry slowly and feed lint fires.
The tool above runs IRC M1502 math. Enter straight feet with elbow counts, and it prints effective length, PASS/FAIL, and margin. Defaults pass at 30 ft with 5 ft spare; 25 ft with three 90s and a 45 fails at 42.5 ft, 7.5 over.
Code and energy offices agree. ICC Section M1502 caps developed length at 35 ft with the mitered elbow table, and DOE vent job aids demand short straight rigid metal to daylight with dampers and no screens, adding booster fans past 35 ft. Transition hoses stay outside the count.
Material matters as much as math. Rigid metal beats flex for airflow and lint shedding, and daylight termination with a backdraft damper keeps weather and pests out. Clean the run yearly; length compliance never replaces lint duty.
How to use
- Enter straight duct feet.
- Count 90-degree elbows.
- Count 45-degree elbows.
- Read effective length first.
- Read PASS/FAIL and margin.
- Fix fails by shortening or boosters.
Formula
- straight
- Straight duct feet, 0 to 200.
- 90s
- 90-degree elbows at 5 ft each.
- 45s
- 45-degree elbows at 2.5 ft each.
IRC M1502.4.6.1 (35 ft max; 4-in. mitered 90 = 5 ft, 45 = 2.5 ft) with DOE booster guidance past 35 ft.
Worked example
Example: 20 ft, two 90s
Defaults: 20 ft straight with two 90s.
- Inputs: 20 ft straight, two 90-degree elbows.
- Elbows: 2 x 5 = 10 ft equivalent.
- Effective: 20 + 10 = 30 ft.
- Limit: 35 ft max per IRC M1502.
- Margin: 35 minus 30 = 5 ft. Tool shows 30 ft effective, PASS, 5 ft margin.
Reference table
| Fitting | Each | Example |
|---|---|---|
| 90-degree elbow | 5 ft | Two 90s = 10 ft |
| 45-degree elbow | 2.5 ft | Two 45s = 5 ft |
| 35 ft straight | 0 | At the limit |
| 30 ft + two 90s | 10 ft | 40 ft: FAIL |
Limits
- 4-in. mitered elbows assumed; smooth radius differs.
- IRC residential scope; commercial differs.
- Booster sizing needs manufacturer tables.
- Lint condition unmanaged by math.
Common mistakes
- Counting flex connectors in the 35 ft.
- Ignoring elbow equivalents entirely.
- Screening the outlet terminal.
- Flex duct sags trapping lint.
- Skipping yearly cleaning.
Try it yourself
Quick checks. Answers sit tucked under each question.
25 ft with three 90s and one 45?
25 + 3x5 + 2.5 = 42.5 ft. Over 35: FAIL, 7.5 ft over.
15 ft with one 90 and two 45s?
15 + 5 + 2x2.5 = 25 ft. Under 35: PASS, 10 ft margin.
Questions
How is effective length figured?
Straight feet plus 5 per 90 and 2.5 per 45 (4-in. mitered). Here 20 + 10 = 30 ft.
What is the length limit?
35 ft max from transition connection to outlet per IRC M1502, transition duct excluded.
Pass or fail: how to read?
Thirty feet or less passes; more fails. Here 30 passes with 5 ft margin.
How do I fix a failure?
Shorten the run, cut elbows, or add a listed booster fan per code.
What duct material?
Rigid or semi-rigid metal to daylight with a backdraft damper and no screen, per IRC and DOE.
Does the flex connector count?
No. The transition hose from dryer to wall does not count toward the 35 ft.
Sources
- ICC IRC Section M1502 (35 ft, elbow table)
- DOE: Vent a Clothes Dryer (rigid duct, booster past 35 ft)