Suppose one journal has 100 papers published in year 2011-12.

These 100 paper together have got the total citations of 150 in 2013 (even some got 0 citation).

IF (2013)= 150/100=1.5

This is an example from the journal of pharmacological research:

Cites in 2008 to items published in 2007 (= 492) + 2006 (= 389 )= total 881

Number of items published in 2007 (= 132) + 2006 (= 136) = total 268

Calculation: Cites to recent items 881/ Number of recent items 268 = 3.287

I would like to tell you that some journals use different methods to calculate it,

Please go through the following web site.

https://clarivate.com/essays/impact-factor/

The right calculation is 150/100=1.5. See also this example:

For 2012, ISI has these numbers for the Journal of Geophysical Research:

Number of items published in 2011:              2707

Number of items published in 2010:              2999

Cites in 2012 to items published in 2011:      7422

Cites in 2012 to items published in 2010:      10688

2012 Impact Factor   =   18110 / 5706   =   3.174

Suppose one journal has 100 papers published in year 2011-12.

These 100 paper together have got the total citations of 150 in 2013 (even some got 0 citation).

IF (2013)= 150/100=1.5