How to Use This HVAC Duct Calculator
Quickly calculate the right duct size for your HVAC system. Enter a few details to quickly estimate airflow, duct size, and pressure drop.

HVAC Duct Calculator

Recommended duct size

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Air velocity

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Total pressure drop

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Equivalent length

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How This HVAC Duct Calculator Works

This calculator estimates duct size using airflow, duct length, and air velocity to determine the right balance for efficient performance.

Step 1: Airflow

This tool uses your airflow (CFM) or converts system size into airflow using a standard estimate: 1 ton is approximately 400 CFM.

Step 2: Duct Length & Bends

Duct length and elbows are combined into an equivalent length, accounting for added resistance from fittings.

Step 3: Duct Size & Pressure

Using airflow and velocity, the calculator determines:

  • Recommended duct size
  • Air velocity
  • Estimated pressure drop

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Features

How to Choose the Right Duct Size

How to Choose the Right Duct Size

Proper duct sizing is essential for airflow, efficiency, and comfort. Ducts that are too small can restrict airflow and increase pressure, while oversized ducts may reduce air velocity and system performance. This calculator helps you find the right balance by estimating duct size based on airflow, length, and system conditions, giving you a reliable starting point for planning or quoting HVAC work.
When to Use an HVAC Duct Calculator

When to Use an HVAC Duct Calculator

Use this tool when:
  • Planning a new HVAC installation
  • Replacing or modifying ductwork
  • Estimating duct sizes during a quote
  • Troubleshooting airflow issues
  • Validating system performance

HVAC Duct Glossary

CFM (Cubic Feet per Minute)

Measures the volume of air flowing through a system. Higher CFM means more airflow.

Duct Velocity (FPM)

The speed of air moving through the duct. Higher velocity can increase noise and pressure loss.

Friction Rate

A measure of resistance to airflow inside the duct. Higher friction means more energy is needed to move air.

Equivalent Length

The total effective duct length, including added resistance from fittings like elbows and bends.

Pressure Drop

The loss of air pressure as it travels through ductwork. Excessive pressure drop can reduce system efficiency.

Duct Diameter

The size of a round duct required to carry airflow efficiently.

Airflow

The movement of air through the HVAC system, typically measured in CFM.

Static Pressure

The resistance to airflow within the system, caused by ducts, filters, and components.

Return Duct

Ductwork that carries air back to the HVAC system for reconditioning.

Supply Duct

Ductwork that delivers conditioned air (heated or cooled) into a space.

HVAC System Tonnage

A measure of heating or cooling capacity. One ton equals 12,000 BTUs per hour.

BTU (British Thermal Unit)

A unit of heat energy used to measure heating and cooling capacity.

Air Balancing

The process of adjusting airflow to ensure consistent temperatures and proper system performance.

Ductwork

The system of ducts used to distribute air throughout a building.

Frequently Asked Questions

1. What does this HVAC duct calculator do?

It estimates the recommended duct size, airflow velocity, and pressure drop based on a few simple inputs.

2. Do I need to know CFM to use this tool?

No. You can enter system size in tons, and the calculator will estimate airflow automatically.

3. What is a good duct velocity?

Most systems operate between 600-900 FPM, depending on design goals and noise considerations.

4. How accurate is this calculator?

This tool provides a practical estimate based on common HVAC assumptions. For detailed system design, additional calculations may be required.

5. What happens if my duct is too small?

Airflow may be restricted, increasing pressure and reducing system efficiency.

6. What happens if my duct is too large?

Air velocity may drop too low, which can reduce system effectiveness and air distribution.

7. What software can help with HVAC calculations and estimates?

After sizing ducts and systems, Service Fusion's HVAC software can help you turn those calculations into accurate estimates and proposals. With Service Fusion, you can manage quoting, scheduling, and invoicing, all in one place.

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