How FD interest is calculated
In a cumulative fixed deposit, interest is added to the deposit at regular intervals and then earns interest itself. Most Indian banks compound FD interest every quarter. The maturity amount is A = P × (1 + r/n)^(n × t), where P is the deposit, r the yearly rate ÷ 100, n the number of times interest is compounded a year (4 for quarterly) and t the tenure in years.
For example, ₹1 lakh at 7% for 5 years, compounded quarterly, matures at about ₹1,41,478, so you earn about ₹41,478 in interest. Compounded yearly, it would be about ₹1,40,255: the more often interest is added, the more you earn, though the difference is small.
With a monthly or quarterly payout FD, interest is paid into your account instead of being reinvested, so the deposit doesn't grow. A ₹5 lakh FD at 7% pays about ₹2,917 a month or ₹8,750 a quarter. Some banks pay slightly less for monthly payouts, because they discount the quarterly interest for paying it early. Payout FDs suit people who want a regular income, such as retirees.