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Duct Size Calculator

Round and rectangular duct size from CFM and target velocity.

Your measurementsEvery assumption editable01

Higher velocity means smaller ducts and more noise.

For the rectangular equivalent — the depth your joist bay allows.

Duct sizeCalculated live02

Result

10in round

10 in round — or 10.3 × 8 in

Cross-sectional area400 CFM at 700 ft/min
82.3 sq in
Exact round diameter
10.24 in
Nearest standard sizeround duct comes in 2 in steps
10 in
Rectangular equivalentsame cross-sectional area
10.3 × 8 in

This sizes by velocity, which controls noise. The other method is equal friction, which sizes by pressure drop per hundred feet and is what a full duct design uses — the two agree closely on short residential runs and diverge on long ones.

A rectangular duct of equal area does not carry quite the same airflow as its round equivalent. The more elongated the rectangle, the more surface area per unit of flow and the higher the friction, so avoid aspect ratios beyond about 4:1.

Velocity is the noise control. Above roughly 900 ft/min in residential supply ducts, air movement becomes audible in quiet rooms, and above 1,200 it is obvious.

Enter the airflow the duct has to carry and the velocity you want it to travel at. Velocity is the noise control — above roughly 900 feet per minute air movement becomes audible in quiet rooms, which is why it is an input rather than a constant.

Why use this tool?What it does differently03

Why use this tool?

Velocity is the input

Because velocity controls noise. Sizing by velocity is what keeps a bedroom supply quiet.

Round and rectangular

The exact diameter, the nearest standard size, and a rectangular equivalent for the depth your joist bay allows.

Standard sizes

Round duct comes in two-inch steps, so the exact figure is matched to what you can actually buy.

Aspect ratio guidance

Explains why an elongated rectangle does not carry the same air as its round equivalent.

How this worksThe method04

How this duct size calculator works

Cross-sectional area is airflow divided by velocity — CFM over feet per minute gives square feet, converted to square inches. The round diameter follows from that area, and is matched to the nearest standard size in two-inch steps.

The rectangular equivalent divides the same area by the depth you enter. That works, but only to a point: the more elongated the rectangle, the more surface area per unit of flow and the higher the friction, so aspect ratios beyond about four to one should be avoided.

This sizes by velocity. The other method is equal friction, which sizes by pressure drop per hundred feet and is what a full duct design uses. The two agree closely on short residential runs and diverge on long ones.

How to use itStep by step05

How to use it

  1. Step 1: Determine the airflow

    CFM the duct must carry. Roughly 400 CFM per ton for a whole system, or use the CFM calculator for a room.

  2. Step 2: Choose a velocity

    500 for bedrooms, 700 for general residential supply, 900 for a main trunk, 1,200 only where noise does not matter.

  3. Step 3: Enter the available depth

    For a rectangular duct, the depth your joist bay or ceiling void allows.

  4. Step 4: Check the aspect ratio

    Keep rectangular ducts under about 4:1. Beyond that friction rises sharply.

Example usageWorked figures06

Example usage

A 400 CFM branch
At 700 ft/min that needs 82.3 square inches — a 10.2 inch exact diameter, so a 10 inch round duct, or 10.3 × 8 inches rectangular.
The same run, quieter
Dropping to 500 ft/min raises the area to 115.2 square inches and the duct to 12 inches round. Two inches of extra diameter is the price of a quiet bedroom.
A main trunk
1,600 CFM at 900 ft/min needs 256 square inches — an 18 inch round, or 32 × 8 rectangular, which is right at the 4:1 aspect limit.
Frequently asked questionsCommon questions07

Frequently asked questions

What size duct do I need for 400 CFM?

About 10 inches round at a typical residential velocity of 700 feet per minute. Quieter installations at 500 ft/min need 12 inches; a noisier commercial velocity would allow 8.

How do I convert round duct to rectangular?

Match the cross-sectional area, then check the aspect ratio. A rectangle of equal area carries slightly less air than its round equivalent because it has more surface area per unit of flow.

What velocity should residential ducts run at?

Around 700 feet per minute for supply branches and up to 900 for main trunks. Above 900 air movement becomes audible in quiet rooms; above 1,200 it is obvious.

How much CFM per ton of cooling?

Roughly 400 CFM per ton is the standard rule, and it is the figure a technician measures against. Systems designed for high humidity sometimes target slightly less.

Does duct length matter?

For sizing by velocity, no. For pressure, very much — use the static pressure calculator, since a long run can consume the whole external static budget of an air handler.

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