Flex Duct Calculator Online

Advanced Flex Duct Calculator

Example Data Table

Formula Used

Duct area: A = π × D² / 4

Airflow relation: Q = A × V

Diameter from airflow: D = √(4Q / πV)

Reynolds number: Re = ρVD / μ

Laminar friction factor: f = 64 / Re

Turbulent friction factor: f = 0.25 / [log10(ε / 3.7D + 5.74 / Re⁰·⁹)]²

Pressure drop: ΔP = f × (L / D) × (ρV² / 2) × sag multiplier

The compression input reduces effective area. Bend count adds equivalent length. Sag adds a planning multiplier to pressure loss.

How to Use This Calculator

Select the calculation mode first. Enter airflow, diameter, or velocity according to that mode. Add the duct length, bends, fitting allowance, sag, and compression values. Keep default air conditions if you do not know them. Press submit. Review velocity, friction rate, Reynolds number, and pressure drop. Use CSV or PDF export for records.

Flex Duct Calculator Guide

Why flexible duct sizing matters

Flexible duct systems are common in air distribution work. They are easy to route. They also react strongly to poor installation. A small bend, long run, or compressed liner can raise pressure loss. This calculator helps you estimate those effects before choosing a duct size. It uses airflow, area, velocity, equivalent length, and a Darcy based friction model. The result is not a replacement for a listed duct chart. It gives a structured check for early design and comparison.

Airflow and velocity balance

Good sizing starts with the required airflow. The airflow may come from a room load, a supply register, or a return path. Once airflow is known, the duct diameter controls velocity. Higher velocity can move more air through a small duct. It can also create noise and extra fan load. Lower velocity is quieter, but it needs more space. A practical design balances comfort, pressure, noise, and available ceiling space.

Installation effects

Flex duct performance depends on shape. A straight and fully stretched run performs best. A sagging run behaves like a rougher, longer duct. Tight bends add equivalent length. Compression reduces the effective area. That is why this tool includes sag, compression, fittings, and bend entries. These inputs let you compare a clean layout with a crowded layout. The difference can be large on long branch runs.

Calculation method

The calculator first finds duct area from diameter. It then links flow and velocity. If you know airflow and target velocity, it sizes an estimated round duct. If you know diameter and velocity, it estimates airflow. Then it converts the final values into SI units for friction work. Reynolds number and roughness help estimate the friction factor. Pressure loss is reported for the whole run and for one hundred feet.

Practical checking

Use the answer as a planning guide. Round the diameter to a real duct size. Check local codes, manufacturer data, and accepted HVAC design methods. Keep flex duct stretched. Support it often. Avoid sharp bends. Use short runs when possible. After installation, verify airflow with field instruments. Better layout usually saves energy. It also makes balancing easier. For final selections, compare nearby sizes. A larger branch may reduce loss. A smaller branch may fit better. Always confirm diffuser neck limits too. before cutting materials onsite.

FAQs

What does this flex duct calculator estimate?

It estimates airflow, diameter, velocity, equivalent length, Reynolds number, friction factor, and pressure drop. It also gives quick notes about velocity and loss level.

Can I use this for final HVAC design?

Use it for planning and comparison. Final duct design should follow local codes, manufacturer duct data, balancing rules, and accepted HVAC design methods.

What is equivalent length?

Equivalent length converts bends and fittings into extra straight duct length. It helps include added resistance from turns, boots, dampers, and other duct parts.

Why does compression affect the answer?

Compression reduces the working area of the duct. Smaller area raises velocity. Higher velocity usually increases pressure drop and can increase noise.

What is a good velocity for flex duct?

Many comfort branch ducts work best at moderate velocity. The calculator flags low, common, high, and very high velocity ranges for quick review.

Why is sag included?

Sag can make flexible duct behave like a rougher and less efficient path. The calculator adds a pressure loss multiplier for sag severity.

What roughness value should I use?

A roughness near 0.006 inches is a common planning value for flexible duct. Use manufacturer data when available for better accuracy.

Can I download the result?

Yes. Use the CSV button for spreadsheet records. Use the PDF button after a result appears to save a simple report.