Calculator
Formula Used
This tool estimates Equivalent Vent Length by adding your straight duct length to penalties for bends and selected accessories.
Remaining Allowance = Maximum Allowed Length − Equivalent Length. If remaining is negative, the run exceeds your chosen limit.
How to Use This Calculator
- Select your units and enter the maximum allowed length from your dryer documentation.
- Measure straight duct segments along the route and enter the total.
- Count 90° and 45° elbows, then keep or adjust the default deductions.
- Tick any terminations or accessories you will use and set their additions.
- Press Calculate to see equivalent length, remaining allowance, and status.
- Use CSV or PDF to save results while planning improvements.
Example Data Table
Sample scenarios to compare common routing choices. Values are illustrative.
| Scenario | Straight (ft) | 90° | 45° | Accessories | Equivalent (ft) | Status vs 35 ft |
|---|---|---|---|---|---|---|
| Short, direct wall exit | 12 | 1 | 0 | Wall hood | 19.5 | Within |
| Longer route with turns | 22 | 3 | 0 | Wall hood | 39.5 | Over |
| Gentler bends, same distance | 22 | 1 | 4 | Wall hood | 34.5 | Within |
Tip: Replacing a 90° elbow with two 45° elbows often reduces equivalent length.
Understanding maximum allowable vent length
Dryer makers specify a maximum exhaust length to ensure adequate airflow and safe moisture removal. That limit is based on duct diameter, fan capability, and the expected resistance of standard fittings. Enter the limit from your manual, or use a conservative placeholder while planning a route for a laundry nook, shed, or garden workspace.
How equivalent length is calculated
Equivalent length converts bends and accessories into a single comparable measure. The calculator adds your straight duct length to penalties for each 90° and 45° elbow, then adds fixed allowances for items like a wall hood, roof cap, flexible transition, or booster. The result represents the total “effective” run your dryer fan must overcome.
Using elbows and offsets without losing airflow
A 90° elbow generally costs more than a 45° elbow because the turn is tighter and creates more turbulence. If you must shift around framing, consider two 45° elbows with a short straight section between them. Keep the duct smooth, sealed, and as direct as possible, especially where the vent passes near potting benches or storage racks.
Termination choices and outdoor placement
Outdoor terminations protect the duct opening, but they also influence performance. Dampers should open freely, and screens should be avoided when they trap lint. Place the outlet away from shrubs, mulch, and irrigation spray so lint does not mat against wet surfaces. Maintain clearance so the flap can swing fully and exhaust can dissipate.
Interpreting results and planning maintenance
If the remaining allowance is positive, your plan is within the chosen limit and has capacity for minor routing changes. If negative, reduce elbows, shorten the path, or reconsider the exit location. After installation, clean the lint filter every load and inspect the hood monthly; rising dry times or humidity often signal restrictions that increase equivalent length in practice. For garden sheds, check for rodent nests, winter ice, and wind-blown debris; a quick seasonal inspection keeps airflow steady and prevents damp, moldy corners indoors.
FAQs
What is “equivalent vent length”?
It is the straight-duct length that would create similar airflow resistance as your full setup, including elbows and terminations, so different routes can be compared fairly.
Why do 90° elbows count more than 45° elbows?
A tighter turn creates more turbulence and pressure loss, which reduces airflow. That added resistance behaves like extra duct length.
Should I use the defaults or my dryer’s manual?
Use your manual whenever available. Defaults are common planning values, but manufacturers may allow different limits based on duct diameter, materials, and termination style.
Does flexible duct change the result?
Yes. Flexible duct typically increases friction and can sag, trapping lint. If you must use it, keep it short, fully stretched, and supported.
What if the calculator shows “Over limit”?
Shorten the route, remove elbows, relocate the exit, or upgrade to smoother ducting. Treat booster fans as a last step and follow local codes.
How often should I inspect the vent system?
Check the hood flap monthly and clean the duct at least yearly, or sooner if drying time increases, odors appear, or lint collects outside.