How the compound sum is worked out
The starting amount grows by (1 + r/n) raised to n times the years. r is the yearly rate as a decimal. n is how many times a year the interest is compounded: 1, 2, 4, 12, or 365 for daily.
A monthly addition is paid at the end of the month. Its monthly rate is the one that matches that compounding, so a yearly 10% is not simply 10% divided by 12. At monthly compounding, the monthly rate is exactly the yearly rate divided by 12. Simple interest uses the nominal rate and does not add interest back. Each addition earns simple interest only for the months still left.
A worked example
₹1,00,000 at 8%, compounded once a year, for 5 years, with nothing added. Year 1 ends at ₹1,08,000, the same as simple interest. Year 2 ends at ₹1,16,640. After five years the compound value is ₹1,46,933. Simple interest ends at ₹1,40,000. Compounding adds ₹6,933.
₹1,00,000 at 10% compounded every quarter for one year becomes ₹1,10,381. That is 1,00,000 times (1.025) to the power 4, rounded to the rupee. Simple interest on the same sum is ₹1,10,000.
₹1,000 at the end of each month, nothing at the start, 12% compounded monthly, for one year, becomes ₹12,683. The same deposits at simple interest become ₹12,660. The last ₹1,000 earns nothing, because it is paid at the end.
8% is not a rate from a bank or from the Ministry of Finance. It is a round number so the example is easy to check. Replace it with the rate you want.
What this is not
A bank fixed deposit often compounds for whole quarters and then pays simple interest on a leftover. That sits on the FD calculator. A recurring deposit uses a different quarterly series, on the RD calculator. A SIP is a market return, not a promised rate, on the SIP calculator.
Interest on a bank deposit is usually taxable. Interest on PPF and Sukanya Samriddhi is not. This page does not take tax off, because the product and your slab are unknown.