Duct Size Calculator

Note: Duct Dimensions are always Inner Dimensions

Ductulator

Instructions:

Review the methodology to ensure it aligns with your project's requirements.
You must fill in every highlighted field, except Duct Height (Optional)
- Select English or Metric Units
- Select Material Type from the dropdown menu.
  
  (The button next to the selection field will bring you to a specification sheet for the material type you selected)
- Enter the air volume in CFM [l/s].
- Select a Design Criterion:
  - Air Velocity
  - Friction Loss
  - Round Duct Dia.
  - Rect. Duct, W x H
- Enter your Design Criterion value in the units displayed.
- Optional: Enter a constraining rectangular duct dimension in inches [cm]
Click the Calculate button (not always necessary).
The results are displayed in the results table.

HVAC Duct Sizing Calculator Methodology

Overview

This duct size calculator estimates duct dimensions for HVAC systems based on airflow requirements and friction loss constraints using the Equal Friction Method. Similar to a traditional Ductulator, this digital duct sizer provides quick, accurate duct sizing estimates consistent with ASHRAE (American Society of Heating, Refrigeration, and Air-Conditioning Engineers) standards for residential and commercial applications.

Calculation Methods

1. Round Duct Sizing (Darcy-Weisbach Equation)

The duct size calculator uses the Darcy-Weisbach equation adapted for HVAC ductwork to estimate round duct diameter. This duct sizer method is the same principle used in traditional Ductulator tools:

Formula: ΔP/L = (f × ρ × V²) / (2 × D)

Where:

ΔP/L = Pressure drop per unit length (in. wg/100')
f = Friction factor (Darcy-Weisbach)
ρ = Air density (typically 0.075 lb/ft³ at standard conditions)
V = Air velocity (ft/min)
D = Duct diameter (ft)

Simplified HVAC Form:

D = [0.109 × Q^1.9 / (ΔP/L)]^0.199

Where:

D = Diameter (inches)
Q = Airflow rate (CFM)
ΔP/L = Friction loss (in. wg/100')

Reference: ASHRAE Fundamentals Handbook, Chapter 21: Duct Design (2021)

2. Air Velocity Calculation

Once duct diameter is determined, air velocity is calculated using the continuity equation:

Formula: V = Q / A

Where:

V = Air velocity (fpm)
Q = Airflow rate (CFM)
A = Cross-sectional area (ft²)

For round ducts: A = π × (D/12)² / 4

Reference: ASHRAE Fundamentals Handbook, Chapter 21 (2021)

3. Rectangular Duct Equivalent Sizing

Rectangular ducts are sized using the Equal Friction Method with equivalent diameter calculations. This ductsize calculator uses the Huebscher equation for equivalent rectangular duct sizing, providing estimates comparable to manual Ductulator calculations:

Formula: D_eq = 1.3 × [(a × b)^0.625] / [(a + b)^0.25]

Where:

D_eq = Equivalent round duct diameter (inches)
a = Width of rectangular duct (inches)
b = Height of rectangular duct (inches)

The duct sizer estimates rectangular dimensions that provide equivalent friction loss and similar airflow characteristics to the round duct.

Reference: ASHRAE Fundamentals Handbook, Chapter 21 (2021); Huebscher, R.G. (1948)

4. Friction Loss Adjustment

The friction loss factor accounts for duct material roughness:

Absolute Roughness Values:

Galvanized steel (Metal): ε = 0.0003 ft
Flexible duct: ε = 0.00775 ft
Duct board: ε = 0.0019 ft
Fabric duct: ε = 0.024 ft

The Colebrook-White equation relates friction factor to roughness: 1/√f = -2 × log₁₀[(ε/3.7D) + (2.51/(Re×√f))]

Reference: ASHRAE Fundamentals Handbook, Chapter 21 (2021)

Design Criteria

Recommended Velocity Ranges (ASHRAE Standard 62.1-2022)

Supply ducts: 800-1,500 fpm
Return ducts: 600-1,000 fpm
Main supply trunks: 1,000-2,000 fpm

Friction Loss Guidelines

Low-velocity systems: 0.08-0.10 in. wg/100'
Medium-velocity systems: 0.10-0.20 in. wg/100'
High-velocity systems: 0.20-0.50 in. wg/100'

Calculation Example

Given Input Values for Ductsize Calculator:

Air Volume (Q): 2,000 CFM
Friction Loss (ΔP/L): 0.08 in. wg/100'
Material: Galvanized steel (Metal)

Step 1: Estimate Round Duct Diameter (Ductulator Method)

D = [0.109 × 2000^1.9 / 0.08]^0.199 D = [0.109 × 505,964,426 / 0.08]^0.199 D = [689,404,382]^0.199 D ≈ 18.46 inches

Step 2: Calculate Duct Area

A = π × (18.46/12)² / 4 A = π × (1.538)² / 4 A = 1.858 ft²

Step 3: Calculate Air Velocity

V = Q / A V = 2,000 / 1.858 V ≈ 1,076 fpm

Step 4: Estimate Equivalent Rectangular Duct Using the Huebscher equation with aspect ratio optimization (duct size calculator method):

For square duct (a = b): 16.36 × 16.36 inches Area = (16.36)² / 144 = 1.857 ft² ≈ 1.858 ft² ✓

Estimated Results from Duct Sizer:

Round Duct: 18.46 in. diameter
Air Velocity: 1,076 fpm (within recommended range)
Rectangular Duct: 16.36 × 16.36 in.
Pressure Drop: 0.080 in. wg/100'

References

ASHRAE (2021). ASHRAE Handbook—Fundamentals, Chapter 21: Duct Design. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
ASHRAE Standard 62.1-2022. Ventilation for Acceptable Indoor Air Quality. Atlanta: ASHRAE.
Huebscher, R.G. (1948). "Friction Equivalents for Round, Square and Rectangular Ducts." ASHVE Transactions, 54: 101-118.
SMACNA (2006). HVAC Systems Duct Design, 4th Edition. Sheet Metal and Air Conditioning Contractors' National Association.
Moody, L.F. (1944). "Friction Factors for Pipe Flow." Transactions of the ASME, 66(8): 671-684.

This methodology follows industry-standard practices for HVAC duct sizing and is consistent with ASHRAE guidelines for equal friction duct design. This digital duct size calculator provides estimates equivalent to traditional Ductulator tools, offering convenient online duct sizing for HVAC professionals and contractors.