The Sukanya Yojana interest rate is represented on a per-annum basis. But the interest is calculated monthly. The lowest SSY account balance in a month is considered for interest calculation; for instance, say your account balance on 1st May is Rs. 1 Lakh, and you deposit more on 5th May. In this case, the account balance on the 1st of the month (Rs. 1 Lakh) is considered for interest calculation.