Ductwork Sizing Calculator
Room airflows come from a Manual J load calculation. As a rough guide a system moves about 400 CFM per ton.
0.06–0.10 is typical for homes. It is the static pressure the fan can spare for straight duct, divided by the longest run's total effective length × 100.
Main supply trunk
14" round
790 CFM · needs 13.3" · 739 fpm · 0.062 in. w.c./100 ft
Rectangular equivalents: 22 × 8", 18 × 10", 14 × 12".
About 2.0 tons of cooling at 400 CFM per ton.
Branch ducts
| Run | CFM | Round duct | Velocity |
|---|---|---|---|
| Living room | 220 | 9" | 498 fpm |
| Kitchen | 160 | 8" | 458 fpm |
| Primary bedroom | 150 | 8" | 430 fpm |
| Bedroom 2 | 110 | 7" | 412 fpm |
| Bedroom 3 | 100 | 7" | 374 fpm |
| Bathroom | 50 | 5" | 367 fpm |
Sized by the equal-friction method for round galvanized duct, rounded up to the next standard size. Flexible duct has more friction: go up one size, keep it stretched tight and avoid sharp bends. Branch velocities above about 600 fpm (red) tend to be noisy at the register.
A first-pass estimate, not a substitute for an ACCA Manual D design: it does not size returns, account for fittings or check the blower's fan curve.
Size round supply ducts for each room and the main trunk by the equal-friction method from the airflow (CFM) each run needs and a design friction rate, with air velocities and rectangular equivalents for the trunk.
Formula
Worked Example
The trunk carries all 790 CFM and needs a 13.3 in duct at that friction rate, rounded up to the standard 14 in. 790 CFM is about 2 tons of cooling at 400 CFM per ton.
Frequently Asked Questions
What size duct do I need for a given CFM?
At a typical friction rate of 0.08 in. w.c. per 100 ft, about 50 CFM needs a 5 in round duct, 100 CFM a 7 in, 150–220 CFM an 8–9 in and 400 CFM a 12 in. Flexible duct has more friction: go up one size and keep it stretched straight.
How many CFM per ton of air conditioning?
About 400 CFM per ton of cooling is the usual design airflow, from 350 in humid climates (for better dehumidification) to 450 in dry ones. A 3-ton system moves about 1,200 CFM.
What friction rate should I use?
The static pressure the blower can spare for straight duct, after the coil, filter, registers and fittings, divided by the longest run's total effective length and multiplied by 100. For many homes it falls between 0.06 and 0.10 in. w.c. per 100 ft. ACCA Manual D gives the full procedure.
Common Mistakes
- !Sizing ducts from rules of thumb instead of each room's load from a Manual J calculation.
- !Using rigid-duct sizes for flexible duct, which has much more friction, especially when compressed or bent.