For a given year, the IF of a scientific journal is the average number of citations of each article published in this journal (citations made in other journals as well as itself) during the period of the previous 2 years

FI = citation received in 2019 from articles published in 2017 and 2018 / Number of articles published in 2017 and 2018

Here is example:

A = Number of citation received by the journal in 2018 to articles published in 2016 and 2017, e.g. 400;

B = Number of citable articles published by journal in 2016 and 2017, e.g. 40;

C = The journal's impact factor = A/B = 10

Just a simple calculation...

Impact Factor = citation received in 2014 from articles published in 2012 and 2013 / Number of articles published in 2012 and 2013

I would just like to highlight the followings to support the above answers:

Answers given above define journal impact factor (IF) based on two-year citation window. In addition to this, one can also find IF based on five-year citation window, defined in the similar fashion except the time frame. However, two-year frame is the most popular measure.

As Anton says, it is important to consider the number of 'Citable articles' for the denominator of the equation. Not all articles published in a journal are citable. For example, 'erratum' published in a journal cannot be considered as a citable document. So, they must be excluded from the calculations.

check the given link for calculating the impact factor of technical journal

https://researchguides.uic.edu/if/impact

you can check journals impact factor in this site https://en.wikipedia.org/wiki/Impact_factor