What size duct do I need for a 3-ton AC unit?

A 3-ton system needs roughly 1,200 CFM total. The main trunk typically sizes to 14"×8" or 16" round at 0.08 IWC friction rate, with branch ducts in the 6"–8" round range depending on per-room CFM. Always size from a full Manual D calculation, not by tonnage alone.

Should I use flex duct or sheet metal?

Sheet metal has lower friction loss and longer life, so it is preferred for trunk lines. Flex duct is acceptable for short branch runs (under 10 ft) where it is fully stretched and properly supported — sagging flex duct can lose 50% of its rated capacity.

HVAC Duct Sizing Calculator — Free Friction Rate Tool

What is friction rate in duct design?

Friction rate is the pressure loss per 100 ft of equivalent length, in inches of water column. Lower friction rates produce larger, quieter ducts; higher rates save space but cost more in fan energy and noise.

Run the numbers for your project

Use the HVAC Duct Sizing Calculator — Free Friction Rate Tool to get an exact answer for your inputs.

Open the calculator

What is friction rate in duct design?

Run the numbers for your project

Related questions

Pro launch updates