Duct Calculator (HVAC Ductwork Sizing & Cost Estimation)

Last updated: 06 Apr 2025 | Author: Brij | Review By: Irshad

Share on :

Easily calculate HVAC duct size and estimate installation cost based on airflow requirements and duct dimensions.

Airflow (CFM):

CFM

Cubic Feet per Minute

Air Velocity:

FPM

Feet per Minute (600-1200 recommended)

Duct Shape:

Material:

Total Length:

feet

Duct Sizing Results

Duct Size:

Pressure Drop:

Material Cost:

Equivalent Diameter:

Surface Area:

Velocity Pressure:

Proper ductwork design is crucial for efficient HVAC system performance. Our Duct Calculator simplifies complex airflow calculations to help you size ducts correctly. This guide explains the engineering principles behind the tool, complete with formulas, LaTeX representations, and practical examples.

Understanding Key Ductwork Calculations

1. Duct Sizing Fundamentals

The primary function of ductwork is to deliver air at the proper velocity while minimizing pressure drop.

Airflow Equation (CFM to Duct Size)

Formula:

$\text{Duct Area (ft²)} = \frac{\text{CFM}}{\text{Velocity (FPM)}}$

Example:

For 1200 CFM at 900 FPM:

$\frac{1200}{900} = 1.33 \text{ ft}^2$

2. Round Duct Diameter Calculation

Converts area to diameter for round ducts.

Formula:

$\text{Diameter (in)} = \sqrt{\frac{\text{Area (in²)} \times 4}{\pi}}$

Example:

Convert 1.33 ft² (192 in²) to diameter:

$\sqrt{\frac{192 \times 4}{3.1416}}$ $= 15.6\ \text{inches}$

3. Rectangular Duct Conversion

Maintains equivalent airflow with different aspect ratios.

Formula (1:2 Aspect Ratio):

$\text{Width (in)} = \sqrt{2 \times \text{Area (in²)}}$

$\quad \text{Height (in)} = \frac{\text{Width}}{2}$

Example:

For 192 in²

$\text{Width} = \sqrt{2 \times 192} = 19.6\ \text{inches}$

$\text{Height} = \frac{\text{Width}}{2} = 9.8\ \text{inches}$

4. Pressure Drop Estimation

Predicts system resistance using simplified Darcy-Weisbach.

Formula:

$\Delta P = L \times 0.05 \times \left(\frac{V}{1000}\right)$

Where:

ΔP = Pressure drop (in WC)
L = Duct length (ft)
V = Velocity (FPM)

Example:

50 ft duct at 900 FPM:

$50 \times 0.05 \times \left(\frac{900}{1000}\right) = 2.25... \text{ in WC}$

Practical Example: Residential HVAC System

Project Parameters

Airflow: 1800 CFM
Velocity: 1000 FPM
Duct Shape: Round
Material: Galvanized Steel
Length: 75 ft

Step-by-Step Calculation

Calculate Duct Area:

$\frac{1800}{1000} = 1.8 \text{ ft²} = 259.2 \text{ in²}$

Determine Round Duct Diameter:

$\sqrt{\frac{259.2 \times 4}{\pi}} = 18.2 \text{ inches}$

Estimate Pressure Drop:

$75 \times 0.05 \times 1 = 3.75 \text{ in WC}$

Calculate Material Cost (@ $2.50/ft):

$75 \times \$2.50 = \$187.50$

Conclusion

This calculator implements industry-standard formulas from:

ACCA Manual D (Residential Duct Design)
ASHRAE Fundamentals (Commercial Systems)

For complex projects, consult an HVAC engineer.

FAQs

1. What is a duct in HVAC systems?

A duct is a passage or conduit used in heating, ventilation, and air conditioning (HVAC) systems to deliver and remove air. It ensures proper air circulation, temperature regulation, and indoor air quality.

2. What are the different types of duct shapes?

Ducts typically come in three shapes:

Rectangular
Circular (Round)
Oval

Each shape has its advantages in terms of space, airflow efficiency, and ease of installation.

3. How is duct volume calculated?

Duct volume is calculated using the formula:

For Rectangular Ducts:

$\text{Volume} = \text{Width} \times \text{Height} \times \text{Length}$

For Circular Ducts:

$\text{Volume} = \pi \times \left(\frac{\text{Diameter}}{2}\right)^2 \times \text{Length}$

The result is usually expressed in cubic feet (ft³).

4. Why is duct sizing important?

Proper duct sizing ensures efficient airflow, reduces energy consumption, and minimizes noise. Undersized or oversized ducts can lead to poor HVAC performance and increased utility costs.

5. What materials are commonly used for ducts?

Ducts are made from materials like:

Galvanized steel
Aluminum
Flexible plastic
Fiberglass board

The material choice depends on the application, budget, and installation conditions.

Check out 3 similar Duct Calculator (HVAC Ductwork Sizing & Cost Estimation) :

Duct Volume Calculator

Duct Calculator (HVAC Ductwork Sizing & Cost Estimation)

Duct Noise Calculator - Measure Regenerated, Breakout, and Final Noise

Help Improve This Calculator

Your suggestions help us make better tools for everyone.