Duct Size Calculator

ASHRAE-based duct sizing with multiple design criteria. Real-time unit conversion, material selection, and graphical analysis.

Affects roughness for friction loss.
For total pressure drop
Advanced: Roughness Coefficient
1.0 = metal baseline
1000 CFM @ 800 fpm 2000 CFM @ 0.1" wg/100ft 500 CFM, 12" round 1500 CFM, 24"x12"
Calculating...

Understanding Duct Sizing

Duct sizing is based on the continuity equation Q = A × V and friction loss correlations. The calculator supports multiple design criteria:

  • Air Velocity: Enter desired velocity → calculates round diameter and rectangular dimensions (using aspect ratio).
  • Friction Loss: Enter allowable pressure drop per 100ft → solves for diameter.
  • Round Diameter: Enter diameter → calculates velocity and friction loss.
  • Rectangular Dimensions: Enter width and height (or height via aspect ratio) → calculates equivalent round diameter, velocity, and friction.

Key Formulas:

  • Round area: A = πD²/4
  • Rectangular equivalent diameter (Huebscher): D_eq = 1.3 (ab)0.625 / (a+b)0.25
  • Friction loss (ASHRAE approximation for metal): ΔP = 0.109136 (Q/60)1.9 / D5.02 (in.wg/100ft, Q in CFM, D in in)

Recommended Design Values

Application Velocity (fpm) Friction Rate (in.wg/100ft)
Main ducts (low velocity) 800–1200 0.08–0.15
Branch ducts 600–900 0.06–0.10
Return ducts 400–700 0.04–0.08

Frequently Asked Questions

Aspect ratios up to 4:1 are common; higher ratios increase friction and cost. Square or 2:1 offers best efficiency.

Use equivalent diameter formulas. Our calculator does it automatically for you.

0.08 to 0.1 in.wg/100ft is typical for residential; commercial may use 0.1–0.2.