Dryer Vent Equivalent Length Calculator
Enter your duct configuration below. The calculator applies the IRC equivalent length formula — accounting for every elbow and transition section — and tells you instantly whether your run is within the IRC 35-foot limit.
🧮 Duct Configuration
Fill in all fields. Leave a field at 0 if that component is not in your run.
| Rigid duct | 0.0 ft |
| Flex transition | 0.0 ft |
| 90° elbows (×5 ft each) | 0.0 ft |
| 45° elbows (×2.5 ft each) | 0.0 ft |
| Total equivalent | 0.0 ft |
How to Use This Calculator
Measure the rigid duct run
Using a tape measure, measure all sections of rigid metallic duct from the wall stub-out to the exterior termination. Add these together for the total rigid run length in feet.
Measure the flex transition
Measure the semi-rigid flex section connecting the back of your dryer to the wall stub-out. This is typically 4–8 feet. If you have no flex section (dryer connects directly to rigid duct), enter 0.
Count all elbows
Walk the full duct path and count every elbow — both 90° and 45°. Include the elbow at the back of the dryer if it's part of the duct system (not built into the appliance). The elbow where the duct exits through the wall and transitions to the exterior cap typically does not count — but count any elbows inside the wall.
Check your dryer's manual
Your dryer manufacturer's installation manual specifies the maximum duct length for your specific model. If it's less than 35 feet, the manufacturer's specification governs. Common examples: some LG models specify 25 ft, some compact units specify 20 ft.
What If I'm Over the Limit?
If your equivalent length exceeds the limit, you have four options in order of preference:
- Replace a 90° elbow with a 45° — saves 2.5 equivalent feet per elbow converted
- Eliminate an elbow entirely — re-route so the duct takes a more direct path
- Shorten the physical run — find a closer exterior wall exit point
- Install a listed booster fan — only if your dryer manufacturer authorizes it; see the Booster Fan Guide
Calculator FAQ
No. The termination cap's airflow resistance is accounted for in the design of the 35-foot limit — it's built into the assumption. You do not add anything for the cap itself. However, if your cap has a screen or is partially blocked, the effective restriction is greater than the code assumes, which is why screened caps are prohibited.
If you have an elbow inside the wall that redirects the duct from horizontal to vertical (or any other angle change), count it. The physical location of the elbow — inside a wall, in a chase, or in the open — doesn't matter for equivalent length calculation. Every elbow in the duct system counts.
You're within the code limit at 34.5 feet. However, being this close to the limit means any future modifications — adding a section of duct, replacing a 45° with a 90°, or adding a transition section — could push you over. If practical, consider redesigning the run to give yourself more margin. A run at 25–28 equivalent feet is not only code-compliant but performs significantly better — more airflow, less lint accumulation, shorter drying times.