Free HVAC Duct Calculator
Use our free HVAC duct calculator to determine the right duct size and estimate airflow volume based on duct type, velocity, length, and number of bends.
Get accurate measurements you can use to estimate the materials and labor—so you can make every job efficient and profitable.
Disclaimer:
The outputs, any associated images or diagrams, any measurements, and any and all other information (collectively, the “Outputs”) are provided for informational purposes only and do not constitute a binding quote or professional advice. The Outputs are based on general assumptions and the information you provided, which may not reflect the actual specifications or condition of the subject of the calculation, particularly in the case of complex structures or inputs. Accuracy may also vary depending on location, as some data may be more complete in certain regions than others. Actual results or outcomes may vary significantly following an onsite inspection and detailed assessment by a qualified professional. Any auto-generated visuals are for illustrative purposes and may not accurately represent the subject of the calculation. The Outputs should not be relied upon as a final result or used as the sole basis for financial, renovation, operational, or strategic decisions. Jobber recommends consultation with a qualified professional before proceeding with any work, quotes, or financial decisions. Jobber and its affiliates make no warranties, express or implied, regarding the accuracy, completeness, or suitability of the Outputs, and to the fullest extent permitted by law, we disclaim all liability for any damages or losses arising from reliance on the Outputs.
How to use Jobber’s free HVAC duct calculator
Follow these steps to use Jobber’s free HVAC duct calculator to build an accurate and profitable estimate for your next HVAC job:
1. Select your duct type.
Specify whether you’re using metal or flex material. This will impact both friction and sizing.
2. Enter the velocity or friction rate.
- Velocity: Enter the air speed in feet per minute (FPM). This is how fast the air moves through the duct, impacting airflow and noise levels.
- Friction rate: Enter the friction loss rate in inches of water gauge per 100 ft (in/wg per 100 ft), which measures airflow resistance.
3. Add in the number of bends
Select how many 45°, 90°, and 180° bends you have. This will affect airflow resistance and factor into the total effective duct length.
4. Enter the duct total effective length (TEL)
Including fittings such as bends and elbows, enter your total effective length in feet.
5. Choose how you want to calculate the airflow rate
Using Jobber’s free HVAC duct calculator, you can estimate airflow based on either airflow rate or dimensions.
- Airflow rate: This is based on the cubic feet per minute (CFM) and duct diameter.
- Dimensions: This is based on duct shape (round duct or rectangular duct) and diameter.
6. Calculate
Once you add your duct details, hit calculate to see results based on your information, including:
- Duct size
- Friction loss
- Velocity
- CFM
Why use an HVAC duct calculator?
A duct size calculator is a handy HVAC tool to keep in your back pocket because it can help:
1. Avoid airflow issues
Airflow issues caused by oversized or undersized ducts lead to inefficient systems, costly utility bills, and unhappy customers. Accurately calculating HVAC ducts ensures consistent and effective airflow.
Not only does this improve energy efficiency, but it also keeps systems running longer by reducing strain on equipment and minimizing wear and tear.
2. Save time and money
Calculating HVAC ducts by hand takes time, and, if not done right, it can lead to human error.
Using a calculator keeps you from having to scratch out calculations on a sheet of scrap paper at a job site. That means fewer installation mishaps, less wasted ductwork, and more accurate supply orders.
3. Provide more accurate estimates
Estimates aren’t just important for your clients, they’re also how you determine your profit on a job.
If you underestimate costs, you could lose money. But if you overestimate, you risk pricing yourself out or charging more than what’s fair.
Using a duct size calculator helps you estimate the materials and labor required more precisely. That way, you can stay profitable, cover your costs, and offer fair, competitive pricing to your customers.
4. Improve system performance
Using an HVAC duct calculator lets you test different duct sizes and configurations to optimize airflow and efficiency. That way, you can find an option that balances airflow and noise levels, resulting in more consistent indoor temperatures and greater comfort for clients.
5. Meet compliance requirements
Accurately calculating airflow, air speed, and velocity helps you meet local building codes and ventilation standards by ensuring you’re meeting regulatory minimums.
For example, some codes specify the required airflow to maintain indoor air quality.
Using an HVAC duct calculator can help you quickly see whether your plan is ready to implement, preventing you from having to redo or adjust your work later and helping pass HVAC inspections.
Duct size formula
The easiest way to calculate the ideal duct size for a project is by using this formula:
Square feet of duct area = CFM / velocity
• CFM is the airflow per cubic foot per minute.
• Velocity is the air speed in feet per minute (FPM).
• Duct area is the size of the duct opening.
For example, if you need to move 400 CFM of air while keeping the air speed around 800 FPM, here’s how you would calculate it:
400 / 800 = 0.5 square foot duct area
To find the duct diameter using this information, estimate the duct area size with this formula. Then use a standard duct sizing chart to convert that area into the right duct diameter for the job.
For example, a 0.5 square foot duct area is roughly equal to a round duct with an eight-inch diameter.
Other relevant formulas:
- Flow rate: CFM = Area × Velocity
- Velocity: Velocity (FPM) = CFM ÷ Area
- Friction loss (metal ducts): Friction = [CFM ÷ (3.12 × Diameter^2.63)]² × (Effective Length ÷ 100)
Frequently Asked Questions
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An HVAC duct calculator is a tool that helps size HVAC ducts based on airflow (CFM), duct length, friction loss, and bends.
By inputting duct materials, diameter, and air velocity or desired friction rate, an HVAC duct size calculator estimates the optimal duct dimensions needed to efficiently and effectively move air through a system.
You can adjust these numbers to accurately estimate project costs, provide better quotes, and determine job requirements.
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Calculating the right duct diameter for each job involves a few different steps:
1. Determine the required airflow in CFM. Find out how much air your HVAC system needs to move based on room size, ventilation requirements, and equipment specifications.
2. Choose a target air velocity in FPM. Determine how fast the air should move through the ducts, which can impact noise and system efficiency.
3. Estimate the duct opening size. Divide the airflow by velocity to find the size of the duct opening. Then use a standard duct sizing chart to convert that number to a diameter. This tells you how big the duct needs to be to properly circulate air.
4. Consider duct length, materials, and bends: Each of these factors impacts friction loss and airflow and can affect the duct size you use.
5. Use a calculator to check duct diameters: A duct size calculator takes the information you gathered and uses it to determine how effective your HVAC system will be based on velocity, airflow, and friction loss. Input different duct diameters to find the sweet spot for optimal airflow, noise, comfort, and efficiency. -
Supply ducts send air out, while return ducts bring air back into an HVAC system.
Because the return airflow typically matches the supply airflow, your CFM, velocity, duct length, bends, and friction values stay the same.
That means you calculate the return air duct size using the same method as the supply air duct size, starting with this formula:
Square feet of duct area = CFM / velocity -
Duct calculators give HVAC professionals a chance to estimate the airflow, friction loss, and velocity for different duct sizes.
That way, they can determine the correct duct size to balance:
• Ideal airflow
• Low friction loss
• System requirements
• Noise levels
• Client comfort
• Overall system efficiency
Using a duct calculator to weigh these factors leads to more accurate estimates, more effective HVAC systems, and more satisfied clients.
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HVAC duct size calculators can be used by:
• HVAC professionals planning ductwork layouts and optimizing system performance
• HVAC contractors providing estimates or troubleshooting airflow issues
• HVAC technicians adjusting or repairing existing HVAC systems
• HVAC service businesses quoting installations, updates, or upgrades