Ventilation is critical for any garage because it allows air to circulate the area. Without adequate ventilation, you might be unaware of stagnant or polluted air until it has been in the space for a few days.
Garage exhaust fans are an easy and inexpensive method for keeping your garage well-ventilated. However, many people are not sure about what size they should buy for this purpose.
The size needed for a garage ventilation fan depends on the number of air changes required in the garage. For a regular garage, the recommended number of air changes is 6 per hour. If it is used as a workshop, 20 to 30 air changes per hour are recommended.
The recommended size for a garage exhaust fan is 800 CFM for each 1000 sq. ft for a normal garage and 4000 CFM per 1000 sq. ft for a garage used as a workshop.
The rest of this article describes how the above numbers are reached and what you should know about ventilating your garage.
Garage exhaust fan size calculation
Garages differ in size and purpose. Most garages in the USA are built to accommodate two cars. Many also hold bicycles, rakes, lawnmowers, and other tools. It is also common to use garages for tasks like welding and conducting repairs.
To decide the correct garage fan size one needs to know the following;
- The size of the garage in cubic feet (length x width x height)
- Number of air changes needed
Garage size calculation
Most single car garages in the USA are of size 12 x 24 sq.ft. Most garages have a height of 7 or 8 feet. Let’s use 8 in our calculation.
So, the total cubic foot area of the above garage is 12 x 24 x 8 = 2304 cu.ft
Number of air changes needed
Air changes per hour are the number of times the entire volume of the air inside the garage is replaced with fresh air from the outside. This means that for the above garage, a volume of 2304 cu.ft of air need to be replaced for one air change cycle.
As per the recommendations by experts, a garage used as a storage unit requires 4 to 6 air changes per hour, and that used for works requires 20 to 30 air changes. This is because welding and other similar tasks create fumes and gases, which are very dangerous to your health. The fumes may be carbon monoxide; they also include some of the most toxic elements found in nature, like lead or mercury. These gases can cause health problems if not quickly vented outside.
Equations to use:
- Total air needs to be exchanged in an hour = Cubic foot volume of garage x number of air changes per hour.
- CFM of fan = Total air need to be exchanged in an hour/60
For the above garage, we will use 6 air changes per hour. So, the required total amount of air to be exchanged per hour is = 2304 x 6 = 14112 cu ft.
However, the size of a fan is expressed in CFM (Cubic Feet per Minute) ie, the amount of air moved in a minute. Now that we have the required amount of air to be exchanged per hour for our garage, we can calculate the CFM by dividing that number by 60 (minutes per hour).
So, the size required for the garage fan is = total volume of air changed per hour/60 = 14112/60 = 230.4 CFM.
This is the minimum fan size required for a normal, one car garage.
If the above garage is used for works, the volume of air that needs to be moved becomes 2304 x 30 air changes = 69120 cu ft.
Converting that number to CFM, the size needed for the garage fan is 69120/60 = 1152 CFM
How many garage fans do I need?
A garage can have one or more fans. But the total CFM of these fans should meet the minimum ventilation requirements of the space. E.g., if a garage needs ventilation of 1200 CFM, you can use two 600 CFM fans or a single 1200 CFM fan.
How I made the size recommendation
The CFM ratings we found are the exact minimum sizes required for a garage fan. However, to keep it simple, I decided to calculate the size of the fan required for a 1000 sq.ft garage area with a height of 8 feet.
The first step to doing this was to find out how much CFM is required per square foot of the garage using the above values. As you know, the garage area we considered is 288 sq.ft (12 x 24). So I divided the CFM values we got with the area to determine how many CFM is required per sq.ft.
CFM per sq.ft = CFM required for the garage/Area of the garage = 230.4/288 = 0.8 CFM per sq.ft for a normal garage.
For a workshop garage, it is 1152/288 = 4 CFM per Sq.ft.
So for a garage of 1000 sq. ft, the CFM ratings become 800 and 4000 respectively.
Garage exhaust fan size guide
For ease of use, here is a fan size guide for garages of different sizes. These are the average size of one car, two-car, and three-car garages in the US and the minimum CFM the garage fan should have.
|Number of Cars Garage Holds||Average Garage Size in Feet (Width x Length)||Fan Size for Regular Garage||Fan Size for Workshop|
|One car||12x 22||211.2 CFM||1056 CFM|
|One car||14 x 22||246.4 CFM||1232 CFM|
|One truck||14 x 24||268.8 CFM||1344 CFM|
|Two cars||18 x 20||288 CFM||1440 CFM|
|Two cars||20 x 20||320 CFM||1600 CFM|
|Two cars||20 x 22||352 CFM||1760 CFM|
|Two trucks||22 x 22||387.2 CFM||1936 CFM|
|Three cars||32 x 22||563.2 CFM||2816 CFM|
|Three cars||36 x 25||720 CFM||3600 CFM|
|Three trucks||38 x 26||790.4 CFM||3952 CFM|
Garage exhaust fan recommendations
The exhaust fan you choose for your garage should be ideal for its purpose. If the garage is used as a storage unit, an exhaust fan is the best choice for ventilation. However, if you use it for anything else, a wall fan or a floor fan is a must.
You can have both types of fans; an exhaust fan for ventilation and a floor fan for cooling while working. I also recommend using a variable speed fan because it gives you more control over airflow.
Here are some of the garage exhaust fans we recommend, listed in no prticular order;
Lasko 20 inch high velocity fan
This is a floor mount fan by Lasko. It has a blade span of 20 inches and can deliver a maximum airflow of 3460 CFM. It is an overkill for a regular garage but an excellent choice for a large garage. It comes with 3-speed settings, the lowest delivering 3160 CFM.
It comes with a quick mount system, so it can be easily installed on the garage wallas well.
iLiving 16 inch exhaust fan
This is an apt choice for garages that are solely used as storage. It can move 1200 CFM of air and is suitable for garages up to 1800 sq.ft in size. However, it wouldn’t provide a breeze effect, so it is not recommended for fast cooling.