Duct Sizing Calculator

Calculate the correct duct size for your HVAC airflow requirements using the equal friction method. Compare duct materials and estimate costs for your duct run.

Duct Shape

Airflow Requirements

Required CFM

Duct Run Length (ft)

Number of Elbows

Friction Limit (in w.g./100ft)

Material Pricing per Linear Foot (edit to match local rates)

Galvanized Sheet Metal ($/ft)

Flex Duct ($/ft)

Fiberglass Duct Board ($/ft)

Round Duct Diameter

Calculated: 6.4"

Air Velocity

1497 fpm

Feet per minute

Effective Length

55 ft

25 ft + 2 elbows

Best Value

Flex Duct

$130.00

Purchase Option	Quantity	Est. Cost
Galvanized Sheet Metal	7" dia / 25 ft + 2 fittings	$244.00
Flex DuctBest value	8" dia / 25 ft + 2 fittings	$130.00
Fiberglass Duct Board	7" dia / 25 ft + 2 fittings	$188.00

Recommended: Flex Duct ($130.00)For 400 CFM over a 25 ft run with 2 elbow(s), use 8" diameter Flex Duct. Total material cost including 7 joints of tape/sealant and 2 fittings.

Long flex duct runs reduce efficiencyFlex duct has higher friction loss than rigid ductwork. For runs over 50 ft of effective length, consider galvanized sheet metal for better airflow performance and lower static pressure drop.

This calculator provides estimates using simplified ASHRAE equal friction methods. Actual duct sizing depends on system design, equipment static pressure requirements, and local code. Always consult an HVAC engineer or contractor for final duct layout and sizing.