How the Dryer Vent Length Checker works
Add up your straight duct and elbows and see whether the run is inside the 35-foot limit of the 2021 IRC, before you close up the wall.
How this is calculated
The 2021 International Residential Code gives three ways to set the maximum length of a dryer exhaust duct (M1502.4.6). This tool works the first two.
The 35-foot method. The duct may be up to 35 feet, measured from the connection to the transition duct out to the outlet terminal. Each fitting in the run uses up part of that allowance: the code gives every elbow type an equivalent length that is deducted from the 35 feet.
equivalent length = straight duct + sum of (elbows × allowance for that elbow)
the run passes when equivalent length ≤ limit| Fitting, as named in the code table | Counts as |
|---|---|
| 4" radius mitered 90° elbow | 5' 0" |
| 4" radius mitered 45° elbow | 2' 6" |
| 6" radius smooth 90° elbow | 1' 9" |
| 6" radius smooth 45° elbow | 1' 0" |
| 8" radius smooth 90° elbow | 1' 7" |
| 8" radius smooth 45° elbow | 1' 0" |
| 10" radius smooth 90° elbow | 1' 6" |
| 10" radius smooth 45° elbow | 0' 9" |
This is why elbow choice matters so much. Two 4-inch-radius mitered 90° elbows use 10 ft of the allowance; two 10-inch-radius smooth 90° elbows use 3 ft.
The manufacturer's method. The size and maximum length may instead be taken from the dryer's installation instructions. The tool offers this two ways, and you choose which describes your manual. If the manual has a chart of maximum length by number of elbows, you enter the figure for your number of elbows and the tool compares your straight duct with it, deducting nothing more. That figure is tied to the number of elbows: change the elbows and the tool asks you to read the chart again. If the manual gives a single maximum and no elbow figures, the tool deducts the code's elbow table from that maximum, because the code says its table is used when the manufacturer gives no fitting figures. When the manual is used, the code requires a copy of the installation instructions to be given to the code official at the concealment inspection.
An equivalent length over 35 ft has to be written on a permanent label or tag within 6 ft of the duct connection (M1502.4.7). The tool works that out with the code's elbow table, whichever method sets the limit.
Scope. This tool uses the 2021 edition; the 2015 edition has the same limit and table under section M1502.4.5. Other editions and state or local amendments can set different limits. The third method, a dryer exhaust duct power ventilator, follows that product's instructions and is not calculated here. Check which code your jurisdiction has adopted.
What this calculator is tested against
This tool cannot be published unless it reproduces every example below. 10 were worked by hand from the cited rule.
| Example | Expected | Basis |
|---|---|---|
| 10 ft straight with one standard (4" radius mitered) 90° elbow | elbow counts as 5 ft; 15 ft equivalent, passes | By hand 2021 IRC M1502.4.6.1 and Table M1502.4.6.1 |
| 10 ft straight with one 10" radius smooth 45° elbow | elbow counts as 9 in; 10 ft 9 in equivalent | By hand 2021 IRC M1502.4.6.1 and Table M1502.4.6.1 |
| Two standard 90° elbows | 25 ft of straight duct allowed (35 − 2 × 5) | By hand 2021 IRC M1502.4.6.1 and Table M1502.4.6.1 |
| One standard 90° and one standard 45° | 27 ft 6 in of straight duct allowed (35 − 5 − 2.5) | By hand 2021 IRC M1502.4.6.1 and Table M1502.4.6.1 |
| 20 ft straight with three standard 90° elbows | 35 ft equivalent — exactly at the limit, passes | By hand 2021 IRC M1502.4.6.1 and Table M1502.4.6.1 |
| 30 ft straight with two standard 90° elbows | 40 ft equivalent — 5 ft over, fails | By hand 2021 IRC M1502.4.6.1 and Table M1502.4.6.1 |
| Same 30 ft run using 10" radius smooth 90° elbows instead | 33 ft equivalent — passes with 2 ft to spare | By hand 2021 IRC M1502.4.6.1 and Table M1502.4.6.1 |
| Manual gives one maximum of 64 ft and no elbow figures; 40 ft straight, two standard 90° elbows | 50 ft equivalent by the code table against 64 ft — passes, and needs a permanent length label | By hand 2021 IRC M1502.4.6.2 and M1502.4.7 |
| Manual’s chart gives 44 ft for two elbows; 50 ft straight, two standard 90° elbows | 50 ft against the chart’s 44 ft — 6 ft over, fails | By hand 2021 IRC M1502.4.6.2 and M1502.4.7 |
| Manual’s chart gives 44 ft for two elbows; 40 ft straight, two standard 90° elbows | 40 ft against the chart’s 44 ft — passes; 50 ft by the code table, so it needs a label | By hand 2021 IRC M1502.4.6.2 and M1502.4.7 |
Sources
- 2021 International Residential Code, Section M1502.4.6 and Table M1502.4.6.1As published by UpCodes (Texas adoption of the 2021 IRC). The 35 ft limit, the fitting table, the manufacturer’s-instructions method and the labelling rule (M1502.4.7) used here were read from this text.
- 2015 International Residential Code, Section M1502.4.5 and Table M1502.4.5.1Excerpt hosted by In-O-Vate Technologies. Same limit and the same table under the earlier section numbers.