CFM Duct Size Calculator

Enter Duct Data

Airflow

CFM

Target Velocity

FPM

Preferred Duct Shape

Rectangular Aspect Ratio

Example: 2 means width is twice height.

Safety Allowance

Straight Duct Length

Fitting Equivalent Length

Planning Friction Rate

in. w.g./100 ft

Air Density

lb/ft³

Formula Used

Design airflow: Design CFM = CFM × (1 + Safety Factor ÷ 100)

Duct area: Area = Design CFM ÷ Velocity

Round diameter: Diameter = √(4 × Area ÷ π)

Rectangular height: Height = √(Area ÷ Aspect Ratio)

Rectangular width: Width = Height × Aspect Ratio

Velocity pressure: VP = (Velocity ÷ 4005)² × (Air Density ÷ 0.075)

Estimated pressure drop: Drop = Friction Rate × Total Equivalent Length ÷ 100

Example Data Table

Airflow	Target Velocity	Required Area	Approx Round Diameter	Example Use
400 CFM	800 FPM	0.50 sq ft	9.6 in	Small branch run
800 CFM	900 FPM	0.89 sq ft	12.8 in	Medium branch run
1600 CFM	1000 FPM	1.60 sq ft	17.1 in	Main supply path
2400 CFM	1100 FPM	2.18 sq ft	20.0 in	Large trunk duct

How to Use This Calculator

Enter the required airflow in CFM.
Choose a target duct velocity in FPM.
Select round or rectangular duct preference.
Add the rectangular aspect ratio when needed.
Enter a safety allowance for design margin.
Add duct length, fitting length, and friction rate.
Press the calculate button.
Review the result above the form.
Download the CSV or PDF file for records.

CFM Duct Size Planning Guide

Why Duct Area Matters

A CFM duct size calculator helps select a duct area that can carry a required air volume at a chosen velocity. It supports early HVAC design, fan checks, diffuser planning, and renovation reviews. The tool uses cubic feet per minute, feet per minute, shape, aspect ratio, and an optional safety allowance. It then estimates area, round diameter, rectangular dimensions, equivalent diameter, velocity pressure, and a simple pressure loss guide.

Balancing Size and Air Speed

Good duct sizing balances comfort, noise, energy use, and available space. High velocity can reduce duct size, but it may increase sound and resistance. Low velocity can reduce noise, but it often needs larger ductwork. This calculator lets you test both choices quickly. You can also compare round and rectangular outputs for the same airflow.

Using Safety Allowance

Use the safety factor when the final air path may include flexible duct, extra elbows, filters, dampers, or future airflow changes. A small allowance gives the design more room. The pressure estimate is only a planning value. Final construction should follow local codes, duct fittings, equipment data, and a full duct design method.

Physics Behind the Calculator

For physics, the key relation is continuity. Airflow equals area multiplied by velocity. Once area is known, a round diameter comes from circle geometry. Rectangular duct height is derived from area and the selected width to height ratio. The hydraulic diameter helps compare rectangular ducts with round duct behavior.

Pressure and Velocity Checks

The calculator also reports velocity pressure. This value is useful because dynamic pressure rises with the square of velocity. A modest velocity increase can create a large pressure rise. That is why duct design should not only chase the smallest size.

Practical Review

Example rows show common CFM values and typical velocities. They are not fixed rules. Supply trunks, returns, branches, and exhaust runs can require different targets. Always check system static pressure, fan capability, grille limits, and noise goals before installation. Use the export buttons to save calculation records for estimates, job notes, or client review.

Final Selection Tip

Record assumptions beside every result. Air density, altitude, lining, leakage, and fitting layout can change real performance. Treat each answer as a design checkpoint, not a final stamp. When values look extreme, adjust velocity, aspect ratio, or allowance, then compare the revised duct size before selecting materials during planning.

FAQs

What does CFM mean in duct sizing?

CFM means cubic feet per minute. It shows how much air volume moves through the duct each minute. The calculator uses CFM with target velocity to estimate duct area and size.

What velocity should I use?

Velocity depends on the duct purpose, noise limits, fan pressure, and building design. Lower values are usually quieter. Higher values can reduce duct size but may increase pressure loss and sound.

Can this calculator size round ducts?

Yes. Select the round option. The calculator estimates the required diameter, then suggests a common rounded duct size that is equal to or larger than the calculated value.

Can this calculator size rectangular ducts?

Yes. Select the rectangular option and enter an aspect ratio. The tool calculates width and height from required area, then rounds the dimensions upward for practical planning.

What is aspect ratio?

Aspect ratio compares duct width to duct height. A value of 2 means the width is twice the height. Very high ratios may create inefficient airflow and harder installation.

What is velocity pressure?

Velocity pressure is dynamic pressure caused by moving air. It rises with the square of air speed. The calculator gives a planning estimate using standard HVAC velocity pressure logic.

Is the pressure drop result final?

No. It is a planning estimate based on the entered friction rate and equivalent length. Final duct design should include fittings, dampers, filters, equipment data, leakage, and local code requirements.

Why add a safety factor?

A safety factor increases design airflow before sizing. It can help when future airflow, extra fittings, flexible duct, or construction uncertainty may affect the final system.