Duct Size Calculator: What Size Duct Do I Need?

A duct that’s too small makes the blower work harder, cuts airflow to the rooms at the end of the run and whistles. One that’s too big costs more and can leave weak airflow at the registers. Residential ducts are usually sized by the equal-friction method: pick a pressure loss per 100 feet of duct, then size every section so the air loses pressure at that rate. This calculator does that for one duct section and checks the velocity.

How it works

friction (in. w.c. / 100 ft) = 0.109136 × CFM^1.9 ÷ D^5.02
round size D                 = (0.109136 × CFM^1.9 ÷ friction)^(1 ÷ 5.02)
rectangle a × b              = smallest a with 1.30 × (a·b)^0.625 ÷ (a + b)^0.25 ≥ D
velocity (fpm)               = CFM ÷ duct area (ft²)
airflow from tons            = tons × 400 CFM

Friction formula. This is the standard fit for round galvanized steel duct with normal joints, the same relationship behind the duct slide rules HVAC designers use. Flex duct, even fully stretched, has noticeably more friction, so go up one size when you use it.

Standard sizes. The calculator rounds up to the next common round size (4 to 10 inches by the inch, then 12 to 24 by two) and gives the friction at that size.

Rectangular ducts are sized by the equivalent-diameter formula, which finds the rectangle with the same friction as the round duct. Pick a height that fits your joists or soffit and the calculator finds the width.

Velocity. We flag ducts faster than 900 fpm for supply trunks and 700 fpm for branches and returns, common limits for quiet residential systems.

What it doesn’t do: the full Manual D layout, which adds the lengths of every fitting, the blower’s available pressure and the airflow each room needs.

Worked example

A 3-ton system moves about 1,200 CFM through its main supply trunk, designed at 0.08 in. w.c. per 100 ft.

• Round size: (0.109136 × 1,200^1.9 ÷ 0.08)^(1 ÷ 5.02) = 15.6 inches, so a 16-inch round trunk
• At 16 inches, the friction is 0.070 and the air moves at 1,200 ÷ 1.40 ft² = 859 fpm, under the 900 fpm limit
• In an 8-inch-high space, the equivalent rectangle is 28 × 8 inches

For a bedroom branch carrying 100 CFM at the same friction rate, the exact size is 6.1 inches. A 7-inch metal duct is the safe choice, and a 6-inch duct runs at 0.085, which is fine if the run is short. In flex duct, use 7 inches.

Duct airflow reference table

How much air each round duct size carries at three common friction rates.

Tips and common mistakes

Size from the airflow each room needs. Room airflow comes from the room-by-room load. A bedroom with two windows facing west needs more air than an interior one of the same size.

Count the fittings. Elbows, boots and takeoffs add the equivalent of many feet of straight duct. A run with lots of turns needs a lower friction rate, which means bigger ducts.

Stretch flex duct and support it. Compressed or sagging flex duct can have several times the friction of a straight run. Pull it tight, keep runs short and support it every few feet.

Don’t forget the return. Undersized returns are one of the most common problems in houses. The return side has to carry the full system airflow too.

Balance the system after it’s installed. Even well-sized ducts need adjustment. Dampers at the trunk takeoffs let an installer trim airflow to rooms that run warm or cold, and a flow hood reading at each register confirms the design numbers.

Seal and insulate ducts outside the conditioned space. Leaks in an attic or crawlspace waste conditioned air and pull in dusty, humid air. Mastic or approved tape on every joint is cheap and effective.