How do I size an AC duct?

Divide the airflow by the air speed you want. That gives the open area the duct needs, which converts to a round diameter or a rectangular size.

What duct size do I need for 400 CFM?

At a normal 700 feet per minute, about a 10 inch round duct, or a rectangular duct of similar area. For a quiet bedroom branch, go larger.

Is round or rectangular duct better?

Round performs better for the same area, because there is less surface for the air to rub against. Rectangular is used because it fits shallow ceiling voids.

Why is my AC duct noisy?

Almost always air moving too fast through a duct that is too small, or a partly closed damper making the air squeeze past. A larger duct at a lower speed is the real fix.

Can flexible duct be used for everything?

It should be short and straight. The ribbed inside creates far more resistance than smooth duct, and a long, bent or sagging flexible run is one of the most common causes of a room that will not cool.

Does duct insulation matter in the UAE?

A great deal. Ceiling voids here get very hot, so an uninsulated duct picks up heat along its whole length. That is cooling you paid for and never received.

AC Duct Size Calculator

Airflow through this duct

CFM

From the CFM calculator, or a design figure.

Air speed

Lower is quieter and needs a bigger duct.

Rectangular shape

Flatter ducts fit better and perform slightly worse.

Ceiling space you have

So the tool can tell you if the duct fits.

Your answer

Round duct

260 mm across

Rectangular option: 330 by 160 mm.

Open area needed: 0.053 m2
Round diameter: 260 mm
Rectangular width: 326 mm
Rectangular depth: 163 mm
Fits your ceiling space: Yes, with room for insulation

Sizes here are for the free area inside the duct. Insulation adds to the outside, so allow more space than the number shown.

AC Duct Size Calculator for Dubai and the UAE

This tool works out how big a duct needs to be to carry a given amount of air. Put in the airflow in CFM and how fast you want the air to travel, and it gives you the size.

It gives a round duct diameter and a couple of rectangular options, because ceiling space usually decides which shape you can use.

The speed you choose matters as much as the airflow. A small duct will carry the air, but you will hear it.

How duct size is worked out

A duct has to have enough open area for the air to pass through at a reasonable speed.

Airflow divided by speed gives the area needed. So 400 CFM at 700 feet per minute needs about 0.57 square feet of open duct, which is a round duct about 10 inches across.

That is the whole calculation. Everything difficult about ducts is in choosing the speed and in fitting the result into a real ceiling.

Round ducts are more efficient than rectangular ones of the same area, because air has less surface to rub against. But round ducts are deep, and ceiling voids are shallow, so most homes end up with flat rectangular ducts or with flexible round duct squashed into a space it does not fit.

Speed is really about noise

The number that decides duct size is air speed, and the reason to keep it low is not efficiency. It is noise.

Air moving fast through a duct hisses. It hisses more at bends, at dampers and at the grille where it comes out.

In a living room with a television on you will not notice 900 feet per minute. In a bedroom at two in the morning you will notice it at 700.

So bedroom branches get bigger ducts than the arithmetic minimum. It is the same air, moved slower, through more space.

The other cost of high speed is resistance. The faster the air, the harder the fan has to push, and the more electricity that takes for the whole life of the system. A slightly larger duct is a one time cost and a permanent saving.

Round, rectangular and flexible

Three kinds turn up in UAE buildings and they are not equal.

Rigid round
The best performing shape for a given area. Least resistance, least noise. Also the deepest, which is why it often does not fit.
Rectangular sheet metal
What most commercial ducting is. Fits shallow ceilings, can be made any shape, and performs a little worse than round for the same area.
Flexible duct
Cheap, fast to fit, and the most commonly abused. The ribbed inside creates far more resistance than smooth duct, so a flexible run should be short and straight.
Pre insulated board
Common in UAE villas. Light, insulated in one piece, and easy to cut. Needs care at joints, because a badly sealed joint leaks cold air into the ceiling.

The mistakes that cost cooling

A duct system loses performance in ways nobody sees, because it is all above the ceiling.

Flexible duct squashed to fit
Bending flexible duct sharply, or crushing it under a pipe, can halve the air through that branch. This is the most common fault in a home here.
Long sagging runs
Flexible duct that has not been pulled taut has ripples along its whole length, and every ripple is resistance.
Leaking joints
Cold air escaping into a ceiling void does nothing for you and you still paid to cool it.
Missing or damaged insulation
In a UAE ceiling void the air around the duct can be very hot. Uninsulated duct gains heat all the way to the room.
Ducts sized for the space, not the air
Sometimes a duct is small because that is what fitted. It carries the air, noisily, and the fan pays for it every hour.
No return path
Air pushed into a room has to get back to the unit. A sealed door with no gap or return grille chokes the whole branch.

What to check in an existing system

If a room is not cooling and the unit is fine, the duct to it is the next place to look.

Start at the grille. Hold a hand to it and feel whether air is actually coming out with force. Then compare it to a room that works. A weak grille with a healthy one down the corridor tells you the problem is in that branch, not in the unit.

Then look above the ceiling if you can. What you are looking for is a flexible duct with a sharp bend in it, a long sag, a crushed section where something crosses it, or a joint that has come apart.

Check the damper too. Every branch should have one, and a damper closed off from a previous adjustment is a common and free fix.

If the grille itself is grey with dust, that is restricting air as well, and it is the easiest thing on the list to sort out.

Questions people ask

How do I size an AC duct?: Divide the airflow by the air speed you want. That gives the open area the duct needs, which converts to a round diameter or a rectangular size.
What duct size do I need for 400 CFM?: At a normal 700 feet per minute, about a 10 inch round duct, or a rectangular duct of similar area. For a quiet bedroom branch, go larger.
Is round or rectangular duct better?: Round performs better for the same area, because there is less surface for the air to rub against. Rectangular is used because it fits shallow ceiling voids.
Why is my AC duct noisy?: Almost always air moving too fast through a duct that is too small, or a partly closed damper making the air squeeze past. A larger duct at a lower speed is the real fix.
Can flexible duct be used for everything?: It should be short and straight. The ribbed inside creates far more resistance than smooth duct, and a long, bent or sagging flexible run is one of the most common causes of a room that will not cool.
Does duct insulation matter in the UAE?: A great deal. Ceiling voids here get very hot, so an uninsulated duct picks up heat along its whole length. That is cooling you paid for and never received.

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