HVAC Duct Sizing Calculator — Free Friction Rate Tool
Size HVAC supply and return ductwork by airflow (CFM), target friction rate, and equivalent length using the ACCA Manual D friction chart method.
- Calculate required CFM Use ~400 CFM per ton of cooling as a starting point; enter actual CFM per branch from your Manual J/D output.
- Choose friction rate Typical residential design uses 0.08–0.10 IWC per 100 ft. Lower friction rates produce larger, quieter ducts.
- Enter duct material Sheet metal vs. flex duct vs. duct board — the calculator uses the matching equivalent diameter.
- Read duct diameter or rectangular size Result is the minimum round duct diameter or rectangular dimensions. Always round up to a standard size and verify with a return-grille velocity check.
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.
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.