Impact Factor (IF), or Journal Impact Factor (JIF)
The journal impact factor (JIF) measures the number of times an average article was cited in a given year based on the journal it was cited in. Where does this number come from?
JIF is calculated through adding all the citations for “citable items” in the journal for the last two years from the target year, divided by the number of articles published in the journal for the two years prior to the target year. Let's look at how that would look:
So for example, finding Journal X's impact factor for 2018 would look like this:
IF 2018 = (Publications 2016 + Publications 2017) / Citations 2018
Pretending Journals X's IF for 2018 is 41.577 it would be:
41.577 = (880 + 902) / 74090
Generally speaking, high impact factor journals are usually considered more prestigious then lower impact journals. A journal must be in publication for a minimum of 3 years for an impact factor to be gained. Remember, IF is in regards to the journal not and individual article. There are cases where very popular articles alter the JIF. Please keep in mind this is just one form of academic and scholarly measurement.
For more information on JIF, I recommend the below sites. Some do have some bias towards or against IF but they help give a better picture of what it is and what it means to the academic community.
Where do you find a journals impact factor?
Frequently, a journal will list their most recent impact factor somewhere on their homepage or their about page. However, if it is a new journal it will not have an IF until it it is in its 3rd year of publication at least.
For example, this is the journal homepage of Medicine & Science in Sports & Exercise and looked for the journal IF (highlighted in yellow). Here, they even show how they came to the conclusion.
You can also look at these free websites to help confirm your findings - but you should manually find the journal IF.
Scimago can also be looked at for some of the statistical information. If you need a list of places you can get article citations from, check out this page.
These websites help explain journal impact factor and may help you calculate it as well. Please manually check all calculators.