HVAC Duct Sizing Calculator

mechanical

Sizes round and rectangular HVAC ducts using equal friction, velocity reduction, or static regain methods. Calculates friction and fitting losses, total pressure drop, and flags noise concerns. Looks up fitting loss coefficients from ASHRAE reference data.

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Airflow Cfm*

Design airflow rate in cubic feet per minute. Obtain from HVAC load calculation or equipment schedule. Typical: 100-400 CFM per diffuser, 1000-20000 CFM for main ducts.

Method

Duct sizing method. 'equal_friction' (ASHRAE default, 0.08 inWG/100ft typical) maintains uniform friction rate. 'velocity_reduction' caps velocity at a maximum. 'static_regain' sizes for pressure recovery between sections. If omitted, defaults to 'equal_friction'.

Friction Rate Inwg Per 100Ft

Target friction rate in inches of water gauge per 100 ft of duct. Only required when method is 'equal_friction'. ASHRAE recommends 0.08-0.10 for low-velocity systems, 0.15-0.40 for medium-velocity. If omitted, defaults to 0.08 inWG/100ft.

Max Velocity Fpm

Maximum allowable air velocity in feet per minute. Only required when method is 'velocity_reduction' or 'static_regain'. ASHRAE noise limits: 1500 fpm branch, 2000 fpm main duct, 2500 fpm risers. Typical: 600-1200 fpm for branch ducts, 1500-2500 fpm for main ducts. If omitted, defaults to 2000 fpm.

Duct Shape

Duct cross-section shape. Round ducts have lower friction and cost per CFM; rectangular ducts fit tighter ceiling plenums. If omitted, defaults to 'round'.

Aspect Ratio Max

Maximum allowable aspect ratio (width/height) for rectangular ducts. ASHRAE recommends keeping below 4:1 to limit friction penalty; never exceed 6:1. Typical: 4:1 max per SMACNA. Lower ratios reduce friction loss. If omitted, defaults to 4.0.

Duct Length Ft

Straight duct run length in feet for pressure drop calculation. Measure along the duct centerline. Enter total run length from AHU/fan to most remote outlet. If omitted, defaults to 100 ft.

Fittings

List of fitting types along the duct run for loss coefficient lookup. Examples: 'elbow_90_smooth', 'tee_branch', 'transition'. Names must match the fitting loss coefficient table. If omitted, fitting losses are not included in the pressure drop.

Duct Material

Duct material, which determines internal surface roughness. Galvanized steel (0.0003 ft) is standard; fiberglass-lined (0.003 ft) adds acoustic attenuation; flex duct (0.01 ft) has highest friction. If omitted, defaults to 'galvanized'.

Reference data

This calculator pulls live values from the following engineering reference tables in the database.

duct fitting loss coefficients
fan curves

About this calculator

HVAC Duct Sizing Calculator is part of the Engineering Database calculator suite. Each tool runs deterministic calculations against authoritative reference data and code-compliant methods, so results are reproducible and traceable to source standards.

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Reference data

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