Chartered Accountant
                
                   213 Points
                   Joined December 2011
                
               
			  
			  
             
            
             The concept of effective rate of interest is used when the rate of interest is stated as certain percent per annum, but is compounded at a shorter interval, like semi-annually or quarterly or monthly etc.
for example, Rs. 100 is invested for 2 years at 10% p.a. compunded semi-annually.
Thus, the intrerest comes to Rs. 21.55 rather than Rs.21 if it was compunded annually,
Thus, effective rate of interest becomes 10.25% when interest is compounded semi-annually.
effective rate= (1+r/k) the whole raised to power k minus 1
where, r= stated rate of interest 
k is the no. of times interest is compounded in a year.
here, in the above example, effective rate woulod become [1+(0.1 divided by 2)]raised to two -1
that is, 1.05 raised to two is 1.1025, from which when one is subtracted, we get 0.1025, i.e. 10.25%