MLS Maker Label Studio
Cited standard/category: Construction, Trades & MEP

HVAC Duct Size Calculator

Size round and equivalent rectangular air ducts from CFM using target velocity or equal-friction rate assumptions.

Inputs

Use project design criteria and verify final sizing against the current regulation, code, and shop standards.

Supply or return airflow in CFM.
The calculator always shows comparable rectangular options.
Sizing method
Feet per minute (fpm). Common comfort duct ranges vary by system type.
in. w.g./100 ft. Default design range is typically 0.08-0.10.
Formula basis: area (sq ft) = CFM / velocity (fpm); round duct diameter = sqrt(4 x area / pi) x 12 (in). Friction-rate mode is an equal-friction ductulator estimate.

Results

Calculated from the current inputs.

Ready
Required area
-
sq ft
Round diameter
-
inches
Velocity check
-
fpm
Friction estimate
-
in. w.g./100 ft
Recommended starting point Enter inputs to calculate a duct size.
Shape preference: -
area = CFM / velocity

Equivalent rectangular options

Option Size Area Equivalent round Velocity Check
No result yet.

Calculation self-tests

Runs golden tests against the pure area and round-diameter functions.

About the HVAC Duct Size Calculator

HVAC designers, installers, and energy auditors use a duct size calculator to convert airflow requirements into round or rectangular duct dimensions based on target velocity. Enter CFM and velocity limits to compare equivalent sizes before layout. It gives a quick sizing check for comfort, noise, and airflow planning while leaving final friction and static-pressure design to proper methods.

How it works

  1. Enter the required airflow in CFM for the room, branch, or trunk.
  2. Choose a target velocity suitable for the duct location.
  3. Calculate round diameter and rectangular size options.
  4. Check aspect ratio, available space, and noise expectations.
  5. Verify the final design with friction rate and total external static pressure.

Frequently asked questions

How is HVAC duct size calculated from CFM?

Basic sizing divides airflow by target velocity to find duct area, then converts area into a round diameter or rectangular dimensions. Final design should also evaluate friction loss and pressure balance.

Is bigger ductwork always better?

No. Oversized ducts can waste space and cost, while undersized ducts can create noise, high static pressure, and low airflow. Good design balances velocity, friction, layout, and equipment performance.

Can rectangular and round ducts carry the same airflow?

Yes, if they have equivalent hydraulic performance, but shape affects friction, fittings, leakage, fabrication, and available space. Very flat rectangular ducts can perform poorly even with enough area.

Should flex duct be sized the same as metal duct?

Not automatically. Flex duct has higher resistance when compressed, sagging, kinked, or poorly supported. Use the applicable duct design method and manufacturer data.

Does this replace ACCA Manual D?

No. A quick velocity calculator is a screening tool. Residential duct design should account for room loads, equipment airflow, friction rate, fittings, registers, and total external static pressure.

References