Duct Sizing Calculator

About the Duct Sizing Calculator

Calculates the cross-sectional area and equivalent round duct diameter needed to carry an airflow at a chosen velocity.

How to use this calculator

1. Enter the airflow in cubic feet per minute, written as \(CFM\).
2. Enter the target air velocity in feet per minute, written as \(velocity_{fpm}\).
3. Use the calculator to find the required duct area in square feet.
4. If needed, convert the area into a round or rectangular duct size that matches your design.

The formula explained

The formula \(A = \frac{CFM}{velocity_{fpm}}\) finds the duct cross-sectional area needed to carry a certain airflow at a chosen velocity. Bigger airflow needs a bigger area, and higher velocity allows a smaller area.

• \(A\) = duct cross-sectional area in square feet
• \(\text{CFM}\) = airflow in cubic feet per minute
• \(\text{velocity}_{fpm}\) = target air velocity in feet per minute

Step by step method

1. Write down the airflow \(CFM\) and the target velocity \(velocity_{fpm}\).
2. Divide \(CFM\) by \(velocity_{fpm}\) to get the area \(A\).
3. Check that the result is in square feet, then use it to select a duct size.
4. If you need actual duct dimensions, convert the area into a shape such as round or rectangular.

Worked example

Problem. Find the duct area for an airflow of \(1{,}200\) \(CFM\) at a target velocity of \(800\) \(fpm\).

1. Use the formula \(A = \frac{CFM}{velocity_{fpm}}\).
2. Substitute the values, \(A = \frac{1{,}200}{800}\).
3. Compute \(A = 1.5\) square feet.

Tips and common mistakes

• A lower target velocity usually means a larger duct, but it can also reduce noise and pressure loss.
• Make sure you use \(fpm\) for velocity and not another unit, because unit mix-ups can give a wrong duct size.

Frequently asked questions