Every attic should be well insulated and well ventilated. We have covered the recommended attic R insulation values here. Here we are going to look at the ventilation part, specifically, what size attic fan you need in terms of CFMs.
Attic fans come in all sizes; from the smallest 80 CFM attic fan to over 2,000 CFM attic fans. How do you figure out which one is adequately sized for your attic?
To help you out with these calculations, we have created these 2 useful resources you can freely use:
- Attic Fan Size Calculator. This calculator will tell you how many CFM should an attic fan produce. You just insert the attic square footage, specificy if you have a steep roof or dark shingles, and the calculator will immediately determine the attic fan CFMs needed.
- Attic Fan Size Chart (CFMs). We have calculated how big an attic fan you need for 100 sq ft to 2,000 sq ft attics, and summarized the results in a neat chart you will find below the calculator.
Now, let’s clear up how you can adequately calculate the attic fan size:
In HVAC, we use the rule of thumb put forward by the Home Ventilation Institute (HVI) that states the attic fan should be big enough to ventilate the whole volume of the attic at least 10 times per hour. We are talking about 10 air changes per hour or 10 ACH.
For the majority of attics, you can use these two attic fan sizing rules of thumb that stem from the 10 ACH HVI rule:
- For standard attics, multiply the attic’s square footage by 0.7 to get the attic fan size in terms of CFMs.
- For dark roofs or steep roof attics, multiply the attic’s square footage by 0.805 to get the attic fan size in terms of CFMs. This is a 15% increase in size because the dark roof attic is hotter and needs to be ventilated more frequently, and because steep slope attics have more air that needs to be ventilated per square foot.
Note: We measure the size of an attic fan in CFMs. CFM stands for Cubic Feet per Minute; this is the airflow volume an attic fan is capable of producing in 1 minute.
The attic fan size formulas are quite simple. They look like this:
Standard Attic Fan Size = Square Footage × 0.7
Dark Roof or Steep Roof Attic Fan Size = Square Footage × 0.85
Example: Let’s say you have a 1,000 sq ft standard attic. What size attic fan do you need? Just input the square footage in the 1st equation since we have a standard roof:
Standard 1,000 Sq Ft Attic Fan Size = 1,000 Sq Ft × 0.7 = 700 CFM
We see that for a standard 1,000 sq ft attic we need a 700 CFM attic fan.
Alright, to make these calculations even simpler, we designed an easy-to-use attic fan size calculator. You just input the square footage and specify if you have a standard roof or you have a dark or steep roof, and the calculator will tell you the size of attic fan required automatically. Below the calculator, you will also find a chart specifying how big an attic fan you need for attics ranging from 100 sq ft to 2,000 sq ft:
Attic Fan Size Calculator
Here is one example that illustrates how this calculator works:
Let’s say we have an 800 sq ft attic with a steep roof. You just slide the square footage slider to ‘800’, and select ‘Yes’ on
Here is the result: An 800 sq ft attic with a steep roof required a 644 CFM attic fan or higher.
This is just one example. We have calculated fan sizes for standard and non-standard roof sizes ranging from 100 sq ft to 2,000 sq ft, and summarized the results in this chart:
Attic Fan Size Chart (CFMs)
| Attic Size: | Fan Size (Standard Attic): | Fan Size (Attic With Steep Roof or Dark Shingles): |
| 100 Sq Ft | 70 CFM | 80 CFM |
| 200 Sq Ft | 140 CFM | 161 CFM |
| 300 Sq Ft | 210 CFM | 241 CFM |
| 400 Sq Ft | 280 CFM | 322 CFM |
| 500 Sq Ft | 350 CFM | 403 CFM |
| 600 Sq Ft | 420 CFM | 483 CFM |
| 700 Sq Ft | 490 CFM | 563 CFM |
| 800 Sq Ft | 560 CFM | 644 CFM |
| 900 Sq Ft | 630 CFM | 725 CFM |
| 1,000 Sq Ft | 700 CFM | 805 CFM |
| 1,100 Sq Ft | 770 CFM | 886 CFM |
| 1,200 Sq Ft | 840 CFM | 966 CFM |
| 1,300 Sq Ft | 910 CFM | 1,046 CFM |
| 1,400 Sq Ft | 980 CFM | 1,127 CFM |
| 1,500 Sq Ft | 1,050 CFM | 1,207 CFM |
| 1,600 Sq Ft | 1,120 CFM | 1,288 CFM |
| 1,700 Sq Ft | 1,190 CFM | 1,368 CFM |
| 1,800 Sq Ft | 1,260 CFM | 1,449 CFM |
| 1,900 Sq Ft | 1,330 CFM | 1,530 CFM |
| 2,000 Sq Ft | 1,400 CFM | 1,610 CFM |
As you can see from the chart, you have to add:
- 70 CFM for every 100 sq ft of standard attic area.
- 80.5 CFM for every 100 sq ft of a steep roof or dark roof attic area.
For smaller attic spaces up to 1,000 sq ft, we would thus use attic fans with up to 700 CFM (standard attic)s or up to 805 CFM (steep roof or dark roof attics).
For big attics, ranging from 1,000 sq ft to 2,000 sq ft, we use attic fans with 700 CFM to 1,400 CFM airflow generation (standard attics) or from 805 CFM to 1,610 CFM (steep roof or dark roof attics).
Example: If you want to ventilate a 1,500 sq ft standard attic, you can just use a 1,050 CFM gable-mounted attic fan.
By using the attic fan CFM calculator and this chart, you now have all you need to correctly determine the size of your attic fan. Hopefully, you now have a good idea of how big a fan you need for your attic. If you have any questions or would like for us to do a bit of math for you, you can use the comment section below, give us some figures, and we’ll help you out.