VentProof reporting resources
Dryer vent length calculator
Add the straight duct and each elbow to get the equivalent length of a dryer exhaust duct, then compare it with the 35 ft specified length in the International Residential Code.
Calculate the equivalent length
Equivalent length: 0 ft
How the 35 ft rule works
Under the 2021 International Residential Code (section M1502.4.6.1), a dryer exhaust duct may be up to 35 feet long, measured from the connection to the transition duct to the outlet terminal. Each fitting uses up part of that length, by the amount in the table below. The transition duct behind the dryer is not counted.
| Fitting | Counts as |
|---|---|
| 4-inch radius mitered 45° elbow | 2 ft 6 in |
| 4-inch radius mitered 90° elbow | 5 ft |
| 6-inch radius smooth 45° elbow | 1 ft |
| 6-inch radius smooth 90° elbow | 1 ft 9 in |
| 8-inch radius smooth 45° elbow | 1 ft |
| 8-inch radius smooth 90° elbow | 1 ft 7 in |
| 10-inch radius smooth 45° elbow | 9 in |
| 10-inch radius smooth 90° elbow | 1 ft 6 in |
Example: 20 ft of straight duct with two 4-inch radius mitered 90° elbows is 20 + 5 + 5 = 30 ft, inside the limit. The same run with three of those elbows is 35 ft, right at it.
When the manufacturer’s numbers apply
The code puts the dryer manufacturer’s installation instructions first (M1502.4.6.2). Many manufacturers publish their own maximum length and fitting allowances, which can be longer or shorter than 35 ft. The table above is for use when the manufacturer gives no fitting calculation. A listed dryer exhaust duct power ventilator follows its own instructions instead (M1502.4.6.3).
Where the equivalent length is over 35 ft, the code asks for a permanent label or tag with the equivalent length, within 6 ft of the exhaust duct connection (M1502.4.7).
Other duct rules worth checking on site
- Rigid metal duct, 4 inches nominal, at least 0.0157 inch (No. 28 gage), with a smooth inside (M1502.4.1).
- Transition duct: one listed length (UL 2158A), 8 ft at most, never hidden inside walls or ceilings (M1502.4.3).
- Joints fastened and sealed, no screws sticking more than 1/8 inch into the duct, supports at least every 12 ft (M1502.4.2).
- Domestic booster fans are not allowed; listed dryer exhaust power ventilators are a separate product (M1502.4.4, M1502.4.5).
Local codes differ, and your jurisdiction may use another edition with different section numbers. This calculator does arithmetic on the numbers you enter. It does not inspect a vent, certify safety, or decide code compliance.
Record what you measured
Write the measured run, the fittings, and the result into the visit record so the customer can see why a reroute or a power ventilator was recommended. The free report generator turns that into a customer PDF, and the inspection report form works on paper.