Duct Calculator Overview
Duct Calculator
Get instant, accurate duct sizing for any HVAC system. This free duct calculator sizes round or rectangular ductwork from your airflow and velocity targets, then checks the friction rate, so you can confirm a duct size on the job in seconds.
Calculate Duct Size
Why You'll Love Our HVAC Duct Calculator
Whether you're a seasoned HVAC professional or a DIY enthusiast, this tool will speed up your duct design and improve the precision of your calculations. With real-time, accurate results, you'll save time and reduce errors.
For rectangular ducts, you can specify your preferred aspect ratio (width:height) to ensure the duct dimensions match your space constraints. For example, a 2:1 ratio means the width will be twice the height, helping you design ducts that fit perfectly in your available space.
How to Use the HVAC Duct Calculator
How to Calculate HVAC Duct Size?
1. Determine Airflow Requirements
First, calculate the required airflow (CFM) for the space, considering temperature and ventilation needs.
2. Choose a Sizing Method
Select from the velocity, equal friction, or static regain method based on your project.
3. Calculate Duct Velocity
Use airflow and duct area to determine the required velocity for your ducts.
4. Determine Duct Size
Find the duct size that fits your required airflow and velocity while considering space and system compatibility.
5. Account for Friction Loss
Factor in the friction caused by airflow resistance to ensure the duct system is efficient.
6. Check & Revise
Verify that the calculated duct size meets your system's needs and adjust if necessary.
Worked example: sizing a supply run
Say you need to move 600 CFM through a 50-foot round trunk and want to keep velocity under 900 FPM with a friction rate no higher than 0.10 in. wg per 100 ft. Enter those numbers, pick a 14-inch round duct, and calculate.
- Duct area: 1.07 ft²
- Velocity: 561 FPM — within range
- Friction rate: 0.037 in. wg per 100 ft — within your 0.10 limit
- Total static loss over 50 ft: 0.018 in. wg
Both checks pass, so a 14-inch round duct is a sound choice. If the velocity had come back high, you would step up to a larger diameter and recalculate — that is how the equal-friction method balances airflow against static pressure.
From load calc to a scheduled, invoiced job
Field Promax helps HVAC contractors turn a sized duct into scheduled work, dispatched crews, and paid invoices — all from one platform. Trusted by 100,000+ contractors across the USA and Canada.
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Frequently asked questions
How does this duct calculator size ductwork?
This duct size calculator takes your airflow (CFM), duct shape, and dimensions, then computes cross-sectional area, air velocity, and the friction rate in inches of water per 100 feet. It uses the standard equal-friction method that most HVAC duct sizing follows, so you get a defensible duct size in seconds instead of reaching for a manual ductulator wheel.
What is a good duct velocity for residential HVAC?
For most residential supply trunks, 700 to 900 FPM keeps noise and static pressure in check; branch runs are usually kept below 600 FPM. Commercial mains can run higher, often 1,000 to 1,500 FPM. Enter your target as the maximum velocity and the duct velocity calculator flags whether your selected size stays within range.
How do I calculate duct size from CFM?
To calculate duct size, divide the airflow (CFM) by your target velocity (FPM) to get the required cross-sectional area in square feet, then convert that area to a round diameter or a rectangular width and height. This ductwork sizing calculator does the arithmetic for you and checks the friction rate at the same time.
Is this the same as a Manual D duct calculation?
This tool uses the same equal-friction physics that underpins ACCA Manual D, so it is a fast field check for hvac duct sizing. A full Manual D design also accounts for fittings, total effective length, and every run in the system. Field Promax pairs quick checks like this with the job scheduling and invoicing your team runs every day.
Round or rectangular duct — which should I use?
Round duct moves air with the least friction and is easiest to seal, so use it wherever space allows. Rectangular duct fits tight framing and chases. This ductwork sizing calculator converts your rectangular width and height to an equivalent round diameter, so you can compare both shapes at the same airflow before you cut metal.