The Exhaust Length Calculator is a practical engineering tool designed to determine the total effective length of an exhaust duct system. It accounts for both the straight sections of ductwork and the added resistance caused by bends or elbows, which reduce airflow efficiency. This tool is crucial for accurately sizing exhaust fans, maintaining airflow standards, and ensuring proper ventilation performance in systems such as kitchen hoods, HVAC ducts, and industrial ventilation.
Accurately calculating the total exhaust length is essential for minimizing backpressure and selecting the correct fan power. Professionals such as mechanical engineers, contractors, and facility managers use this calculator during planning and installation to meet code requirements and avoid performance issues.
Formula of Exhaust Length Calculator
Exhaust Length = Straight Duct Length + (Number of Bends × Bend Equivalent Length)
Where:
- Straight Duct Length is the total linear length of straight ductwork (in feet or meters)
- Number of Bends is the count of direction changes (typically elbows) in the system
- Bend Equivalent Length is the airflow resistance added by each bend, converted to its straight-length equivalent
Average Values for Bend Equivalent Length:
- 90° Elbow = 7.5 feet (approximate average)
- 45° Elbow = 3.5 feet (approximate average)
Full Expanded Formula:
Exhaust Length = Straight Length + (Elbows₉₀ × 7.5) + (Elbows₄₅ × 3.5)
These values may vary slightly depending on duct diameter and material, but the formula gives a reliable estimate for residential and commercial ventilation systems.
Common Exhaust Configurations Reference Table
The table below shows frequently searched exhaust setups with estimated total lengths. It helps users make quick decisions during planning and equipment selection.
| Straight Duct (ft) | 90° Elbows | 45° Elbows | Total Equivalent Length (ft) |
|---|---|---|---|
| 10 | 2 | 0 | 25.0 |
| 15 | 1 | 2 | 29.0 |
| 20 | 3 | 1 | 45.5 |
| 25 | 2 | 2 | 48.0 |
| 30 | 4 | 0 | 60.0 |
This reference is ideal for builders, HVAC estimators, or homeowners verifying fan compatibility with duct layouts.
Example of Exhaust Length Calculator
Let’s say you are installing a kitchen exhaust system. You measure the straight duct run at 18 feet and plan to use 2 standard 90-degree elbows and 1 angled 45-degree elbow.
Step 1:
Use the formula:
Exhaust Length = 18 + (2 × 7.5) + (1 × 3.5)
Step 2:
Calculate:
Exhaust Length = 18 + 15 + 3.5 = 36.5 feet
So, the total effective length of the exhaust system is 36.5 feet. This number should be compared to the maximum supported length by the exhaust fan or hood manufacturer to ensure efficient airflow and compliance.
Most Common FAQs
This tool belongs to the HVAC and ventilation planning calculators category. It is widely used in residential, commercial, and industrial building design.
Each bend or elbow in the ductwork causes resistance to airflow. Instead of calculating resistance with complex fluid dynamics, these bends are converted to an equivalent straight duct length to simplify planning.
Yes. This calculator is especially useful for appliances like dryer exhausts and kitchen hoods where the duct length and number of bends directly impact performance and code compliance. Just be sure to follow specific manufacturer limits when interpreting the results.