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Engineering Calculators

Duct Size Calculator

Note: Duct Dimensions are always Inner Dimensions

Instructions:

  1. Review the methodology to ensure it aligns with your project's requirements.

  2. 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]

  3. Click the Calculate button (not always necessary).

  4. The results are displayed in the results table.

Round duct diameter

in. (cm)

Also calculated
Length ft (m)
Air volume cfm (l/s)
Air velocity fpm (m/s)
Pressure drop in. wg/100' (Pa/30 m)
Rect. duct width in. (cm)
Rect. duct height in. (cm)

Equal-friction duct sizing. Pick the one quantity you know from the design criteria list and enter it in the field below it; the units for that field follow both the dropdown and the Units selection. Equivalent length is fixed at 100 ft (30 m).

Unit labels follow the Units dropdown: the first unit shown on each row applies under US, the one in brackets under Metric.

Entering a duct height alongside a round diameter or a friction loss gives the equivalent rectangular duct of that height; leaving it blank gives the equivalent square duct.

Corrected 2026-09: the rectangular duct width for the round-diameter and friction-loss rows was wrong in the previous version. The workbook cell that sizes the width against a given height (N19) holds its own formula as literal text rather than as a formula, so Excel and the published build both skipped it. This version evaluates it, which the calculator's own neighbouring rows (N15 and N16) show was the intent.

Also corrected 2026-09: with a round duct diameter entered and the duct height left blank, the previous version reported the rectangular width as infinite. A blank height now reads as no height given, which is what the workbook does and what the height row beside it already did.

How to use the duct size calculator

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

  1. ASHRAE (2021). ASHRAE Handbook—Fundamentals, Chapter 21: Duct Design. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

  2. ASHRAE Standard 62.1-2022. Ventilation for Acceptable Indoor Air Quality. Atlanta: ASHRAE.

  3. Huebscher, R.G. (1948). "Friction Equivalents for Round, Square and Rectangular Ducts." ASHVE Transactions, 54: 101-118.

  4. SMACNA (2006). HVAC Systems Duct Design, 4th Edition. Sheet Metal and Air Conditioning Contractors' National Association.

  5. 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.

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