Field tools · Calculator

Airflow (CFM) Calculator

Square footage × velocity, or 400 × tons. Cross-checks against residential velocity bands and the Rule-of-400 sanity target, with a friction-rate reference table for common duct sizes.

US units · inH₂O · CFM · FPM

Pick the duct shape, drop in the area and your measured FPM (or tons), and ChillFlow resolves CFM against the standard residential sanity bands.

Width (in)

step ±1 (·10 hold)

Height (in)

step ±1 (·10 hold)

Velocity (fpm)

step ±50

Computed airflow

600CFM

Velocity in typical residential range (400–900 FPM).

Cross-section area: 1.000 sqft
Equivalent round: 13.1" dia.

Reference

Friction-loss reference

Typical hairpin friction rates — ACCA Manual D / SMACNA ballparks, not a substitute for system design.

Size	CFM (low–high)	Friction (inH₂O/100 ft)	Velocity (FPM)	Notes
6"	60–120	0.12	600	Branch run-out, low CFM.
6"	121–200	0.20	900	Branch run-out, design upper end.
8"	90–180	0.06	500	Branch run-out, typical.
8"	181–280	0.10	700	Branch run-out, design mid.
8"	281–400	0.18	900	Typical trunk transition.
10"	180–320	0.06	500	Small trunk / branch.
10"	321–500	0.09	700	Trunk mid.
12"	300–500	0.04	450	Residential trunk, low end.
12"	501–800	0.07	650	Residential trunk, typical.
12"	801–1100	0.10	800	Residential trunk, upper end.
14"	500–800	0.04	500	Large trunk / lead duct.
14"	801–1200	0.06	700	Large trunk, design mid.
16"	800–1200	0.03	500	Lead / trunk, low end.
16"	1201–1600	0.05	700	Lead / trunk, design mid.

Round and rectangular rows are for straight galvanized steel run-out / trunk. Flex duct runs fully extended (≥ 4 ft) — kinked or coiled flex roughly doubles the friction rate.

Friction rates are ACCA Manual D / SMACNA-residential ballparks — always verify against the system's design specs and your airflow probe before adjusting the blower.