How RD interest is calculated
In a recurring deposit you pay the same amount every month for a fixed term. Each instalment earns interest from the month it's paid, and banks and the post office compound that interest every quarter. So the first instalment earns interest for the whole term and the last one for just a month.
The calculator adds up every instalment's value at maturity: P × (1 + r/4)^(4 × months left ÷ 12), where P is the monthly deposit and r the yearly rate ÷ 100. For example, ₹5,000 a month for 12 months at 7% gives about ₹62,311: ₹60,000 of deposits and about ₹2,311 of interest. Over 5 years, the same RD comes to about ₹3,59,664 from ₹3 lakh deposited.
Banks round interest at each quarterly credit, so their figure can differ from this one by a few rupees.