Skip to content
Calculation Machine

Trunk Duct Size Calculator

Main trunk duct size from total CFM.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Round equivalent diameter
19.1 in
Rectangular size at height
28.8 x 10 in
Cross-section area
2 sq ft
Velocity check
800 FPM

Quick answer

Supply trunk ducts are typically sized for 700-900 FPM velocity to balance airflow capacity against noise and friction.

The formula

Area = CFM / velocity

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Total trunk CFM1600 CFM
  • Design velocity800 FPM
  • Trunk height10 in

Result

  • Round equivalent diameter19.1 in
  • Rectangular size at height28.8 x 10 in
  • Cross-section area2 sq ft
  • Velocity check800 FPM

How to use the Trunk Duct Size Calculator

  1. 1Total the CFM of all branches the trunk must feed.
  2. 2Pick a trunk velocity — 700-900 FPM is typical for residential main trunks.
  3. 3Choose an available trunk height and solve for the matching width.

Frequently asked questions

Why not just use branch sizing rules?

Trunks carry the sum of many branches, so they need larger cross-section even though the velocity target is similar.

Should trunk size taper?

Yes, reducing trunk size as branches peel off keeps velocity consistent along the run.

About this calculator