The Duct Calculator is an interactive utility designed for quick field estimates of duct airflow capacity. You can select duct types and sizes, compile multiple duct runs to estimate total system airflow, and compare findings against an optional system target.

[!NOTE] This calculator is for quick reference and preliminary field assessments only. It does not replace professional Manual D duct design, physical airflow balancing, or manufacturer specifications.

1. Opening the Duct Calculator

To access the calculator:

Open the ACLogics app.
Navigate to Tools in the main menu and tap Duct Calculator.

2. Setting a System Airflow Target (Optional)

You can configure a target airflow to compare your duct runs against the system’s total capacity:

Tap Set System Size.
Select the equipment capacity in Tons (e.g., 2.5 Tons).
The app will calculate and display a baseline target airflow (CFM).

For example, a typical 2.5 Ton split system will establish a baseline target of approximately 1,000 CFM (calculated at 400 CFM per Ton).

[!IMPORTANT] This target represents a standard baseline. Actual airflow requirements vary depending on climate humidity, blower speed configurations, and manufacturer design standards.

3. Selecting a Duct Type and Size

Choose the duct material and dimensions that match the run you are evaluating:

Duct Type	Performance Characteristics	Field Considerations
Flex (Flexible Ducting)	Higher resistance to airflow.	Extremely sensitive to sagging, sharp bends, compression, and installation tension.
Round (Rigid Metal)	Smooth interior surfaces provide optimal airflow.	Requires secure joint sealing to prevent static pressure loss.
Rect (Rectangular Metal)	High volume capacity, fits in joists.	Aspect ratio affects resistance; requires proper bracing.

Select the exact diameter or rectangular dimension in inches. Note that actual physical damage, compression, or severe bends in the field will reduce real airflow below the calculated estimate.

4. Managing Duct Runs

ACLogics allows you to compile multiple registers or duct runs to estimate cumulative system airflow:

Add a Run: Once you configure the duct type and size, tap Add to include that run in the cumulative list.
Review Cumulative Airflow: The app automatically adds the CFM estimates of all compiled runs and displays the total system airflow.
Remove a Run: Tap the Delete icon next to any run in the list to remove it and automatically recalculate the total.

5. Understanding “May Be Undersized” Alerts

If you configure a system target and the cumulative airflow estimate of your added runs falls below that target, the app will display a “May be undersized” alert.

What it means: The currently configured duct runs do not have sufficient calculated capacity to handle the required CFM for the selected system size.
Next Steps: Inspect the system for restrictive duct runs, look for crushed runs, or evaluate if additional return/supply runs are needed.

6. Variables Affecting Real-World Airflow

Calculators represent ideal conditions. In the field, actual airflow is heavily influenced by:

Fittings and Turns: Elbows, tees, and registers add significant equivalent length and resistance.
Flex Duct Compression: Even minor sagging or compression can reduce flex duct airflow capacity by over 30%.
System Static Pressure: High external static pressure (crushed coils, dirty filters) reduces total blower output.
Climate Adjustments: Humid climates often prioritize lower CFM per ton for enhanced moisture removal, while dry climates may use higher CFM targets.

Use the Duct Calculator