Calculator Inputs
Example Data Table
| Use Case | Airflow | Target Velocity | Length | Bends | Likely Standard Size |
|---|---|---|---|---|---|
| Small bedroom branch | 80 CFM | 650 FPM | 18 ft | 1 | 5 in |
| Living room branch | 160 CFM | 700 FPM | 28 ft | 2 | 7 in |
| Return flex run | 300 CFM | 600 FPM | 35 ft | 3 | 10 in |
Formula Used
Design airflow: Design CFM = Airflow × (1 + Allowance ÷ 100)
Duct area: Area = Design CFM ÷ Target velocity
Round duct diameter: Diameter = 12 × √((4 × Area) ÷ π)
Selected duct velocity: Actual velocity = Design CFM ÷ Selected duct area
Effective length: Effective length = Straight length × Installation factor + Bends × Equivalent length per bend
Pressure loss: Pressure loss = Friction rate × Effective length ÷ 100
How to Use This Calculator
- Enter the airflow needed for one flexible duct run.
- Enter the target velocity for your supply or return branch.
- Add the straight duct length, bend count, and bend equivalent length.
- Use an installation factor above 1 when the duct has sag or compression.
- Enter a friction rate allowance for pressure loss planning.
- Choose auto sizing or test a manual standard diameter.
- Press the calculate button and review the result above the form.
- Download the CSV or PDF file for your job notes.
Practical Flex Duct Planning
Flexible duct looks simple, yet it affects comfort, noise, and system capacity. A small duct raises air speed and friction. A large duct may waste space and cost more. The best size balances airflow, length, bends, and installation quality.
Airflow and Diameter
Good flex duct work starts with the required airflow. Airflow is usually listed in cubic feet per minute. Each room, branch, or diffuser needs its own value. The calculator divides that airflow by a chosen velocity. It then converts the needed area into a round duct diameter. This gives a clean starting point for field planning.
Length, Bends, and Installation
Flexible duct needs extra care because it is not smooth like metal duct. Tight bends, sagging runs, and crushed sections reduce the free air path. They also increase resistance. A short and straight run works best. Long runs with several bends need a larger diameter or a lower airflow target. The installation factor in this tool helps account for those field losses.
Velocity and Noise
Velocity is another important design limit. Higher velocity may move more air through a smaller duct. It can also create noise at grilles and registers. Lower velocity is quieter, but it needs a larger duct. Residential supply branches often use moderate velocities. Return ducts usually need lower velocities because noise and pressure loss are more noticeable.
Pressure Loss Check
Friction rate tells how much static pressure is lost over a measured equivalent length. This calculator uses the entered friction allowance and the calculated effective length. It then estimates total pressure drop. The result helps compare different routes before cutting, hanging, or ordering duct material.
Use the nearest standard size as a practical selection. The exact calculated diameter may not be sold locally. When the standard size is larger, velocity drops and pressure loss often improves. When the selected size is smaller, airflow may suffer. Always compare the velocity warning and pressure estimate.
Field Use
This tool supports construction planning, bidding, and early layout checks. It does not replace a full Manual D design, equipment data, or local code review. Final sizing should be checked by a qualified HVAC professional. Good sealing, proper stretching, wide-radius bends, and strong supports.
FAQs
1. What does this calculator size?
It estimates a round flexible duct diameter from airflow, velocity, length, bends, and installation correction. It also checks selected size velocity and pressure loss.
2. What airflow value should I enter?
Enter the required CFM for one duct run. Use room load data, diffuser schedules, equipment design documents, or a professional airflow plan.
3. Why does flexible duct need a correction factor?
Flex duct can sag, compress, or bend sharply. These issues increase resistance. The correction factor helps reflect less ideal field conditions.
4. What target velocity should I use?
Use the velocity required by your design standard. Lower values reduce noise and pressure loss. Higher values may need careful grille selection.
5. What does effective length mean?
Effective length combines straight run length with bend resistance and installation quality. It is used to estimate duct pressure loss.
6. Can I choose a manual duct size?
Yes. Select a standard diameter from the list. The result will show whether that size meets the calculated diameter and velocity limit.
7. Is this a final HVAC design?
No. It is a planning calculator. Final duct design should follow applicable codes, equipment data, Manual D methods, and professional review.
8. Why are CSV and PDF downloads included?
They help save project assumptions, calculated sizes, and pressure estimates. You can attach them to estimates, notes, or field worksheets.