Duct Size Calculator

The airflow in CFM is divided by the design air velocity in feet per minute, which gives the required cross-section in square feet. That area is multiplied by 144 to get square inches, then the round diameter is found with the circle-area formula, diameter equals the square root of four times the area divided by pi.

Roughly 700 to 900 feet per minute is common for residential supply runs, and 600 to 700 keeps return air quieter. Main trunks sometimes run faster and long flexible branches slower. These are rules of thumb, not a replacement for a full Manual D calculation.

Flexible duct and rigid spiral pipe are both sold by their round diameter, so an equivalent diameter tells you exactly what to order. Even when you plan a rectangular duct, the round figure is the standard reference point for comparing sizes.

Match the square-inch cross-section shown here, then choose height and width whose product is close to it, favoring a ratio near 1 to 3 for efficiency. Sheet metal shops use equivalent-diameter tables because a rectangle needs slightly more area than a round duct to move the same air at the same friction.

Yes, moving the same airflow faster needs less cross-section, so the diameter shrinks. The trade-off is that higher velocity raises friction loss and air noise, which is why bedrooms and returns usually stay on the slower end of the range.

The formula returns the exact equivalent diameter, so 400 CFM at 700 fpm lands at 10.24 inches rather than a stocked size. Round up to the next standard duct, such as a 12 inch pipe, because a smaller duct would push velocity and noise higher than planned.

Both the airflow and the velocity must be finite numbers greater than zero. If either is zero, blank, or negative, the calculator shows an input error instead of a result, since dividing by zero has no physical meaning here.

No, it sizes purely from airflow and velocity, so it does not model static pressure, elbows, or long flex runs. Use it for a fast first-pass duct size, then confirm the design with a friction-rate method or Manual D for the whole system.

No. The math runs entirely in your browser, so the airflow and velocity you type never leave your device and nothing is stored on a server.