Duct Plenum Size Calculator

Size supply and return plenums by airflow, velocity, and shape

Unit System

Quick Presets

Select a common system to pre-fill inputs

Calculator Inputs

Total Airflow (CFM)

Target Face Velocity (FPM)

Plenum Type

Plenum Shape

Aspect Ratio Width:Height

Please enter a valid airflow value greater than 0.

Plenum Sizing Results

Supply Plenum — Rectangular

Required Cross-Section Area

—

sq in

Recommended Dimensions

—

Width × Height

Actual Face Velocity

—

FPM

Velocity Pressure

—

in. w.g.

Calculation Breakdown

Airflow Input—

Target Face Velocity—

Plenum Type—

Plenum Shape—

Required Area (sq ft)—

Required Area (sq in)—

Dimension Calculation—

Velocity Pressure Formula—

Velocity Recommendations by Application

Application	Min FPM	Recommended FPM	Max FPM
Residential Return Plenum	200	300–400	500
Residential Supply Plenum	400	500–700	800
Commercial Return Plenum	300	400–500	600
Commercial Supply Plenum	500	600–800	1000
Industrial Supply Plenum	600	800–1000	1200
Quiet / Noise-Sensitive Area	150	200–300	400

Plenum Sizing by System Tonnage

System Tons	Typical CFM	Supply Area (sq in)	Return Area (sq in)
1 Ton	400 CFM	~80–96	~100–128
1.5 Ton	600 CFM	~96–120	~128–180
2 Ton	800 CFM	~128–160	~160–256
2.5 Ton	1000 CFM	~160–200	~200–300
3 Ton	1200 CFM	~192–240	~240–384
4 Ton	1600 CFM	~256–320	~320–512
5 Ton	2000 CFM	~320–400	~400–640
7.5 Ton (Comm.)	3000 CFM	~480–600	~600–960

Common Air Handler Specifications

Air Handler Model Size	Rated CFM	Supply Opening	Return Opening
AH-018 (1.5 Ton)	600 CFM	10 x 12 in	14 x 14 in
AH-024 (2 Ton)	800 CFM	12 x 12 in	14 x 16 in
AH-036 (3 Ton)	1200 CFM	14 x 14 in	16 x 20 in
AH-048 (4 Ton)	1600 CFM	16 x 16 in	20 x 20 in
AH-060 (5 Ton)	2000 CFM	18 x 18 in	20 x 24 in
AH-090 (7.5 Ton)	3000 CFM	20 x 24 in	24 x 30 in

Supply vs. Return Plenum Sizing Supply plenums need to handle higher velocities (500–800 FPM) to distribute air efficiently. Return plenums should be sized for lower velocities (300–500 FPM) to minimize noise and static pressure loss.

Aspect Ratio and Airflow Balance Keep the W:H aspect ratio between 1:1 and 3:1 for best performance. Extreme ratios create uneven velocity profiles and can cause turbulence inside the plenum, reducing system efficiency.

The supply plenum is basically that box that sits directly on the air handler or the furnace and it pushes conditioned air through the ducts to every room One can consider it as the heart of the HVAC system. You must choose the right size because too small a size causes noise, loss of pressure and bad distribution of air.

Good rule for the plenum depth is to make it about 2.5 times bigger than the diameter of the fan for good flow. The area of the plenum should match or be more than the outlet of the fan or the air handler. Usually, the plenum box has the same size as the supply side of the furnace, and the size of the ducts depend on the need of every zone.

How to Size a Supply Plenum

Every job is different, but when you do that correctly, the result is equal flow of air.

Regarding the proporitons of the box, the width should be around 2 to 3 times bigger than the connecting supply trunk. The best length is twice the width, but it can range between one and three times. A plenum with length of at least 2 feet works well, but 4 feet are even better.

The branch ducts are measured according to the amount of air required in the various rooms; that measurement is key so that each place receive enough air.

Normal supply plenum measures about three feet long if it is on slab coil, and four feet on box coil or air handler. Ceiling return plenums most commonly are three feet high. Plenums can be installed at every end of the furnace or cooling coil, and they are measured to match that opening, usually no longer than 20 or 25 inches.

Out of the plenum exits the trunk duct, and usually there is only one such duct on the return side.

If the supply plenum is too short, that can create problems, depending on how quickly the ducts get smaller later. For instance, you had a supply plenum of 22x19x19, connected to a 5-foot trunk duct of 26×9, that later got smaller to an 18-foot duct of 20×9 to provide seven vents.

The return air commonly is too small, which creates its own problems. In systems with two return ways, every way should not be longer than 30 feet from the return plenum to the filter grille. When you need to bend the duct, metal elbows are better than flex.

Every return way can have up to 180 degrees of total curve, but no more than 90 degrees from that must be flexible duct.

Before you decide the size, you must first count the load needs of the house. Only then you choose the unit size to satisfy that load. Later, you design the supply and return ducts to deliver the air required for the various areas.

Custom plenums can be made in any size to fit the dimensions. The smallest duct used on runs should be six inches. If you use a calculator for ducts, 700 feet each minute at.08 are good for a silent system.