How this calculator works
This model invests each monthly deposit at the beginning of its month. A quarterly nominal rate is converted into an equivalent monthly growth factor, so a deposit held for one month receives one third of a quarter's exponential growth. Actual bank and Post Office products can use different stub-period, maturity-rounding and missed-deposit rules. This is a general RD projection, not an exact Post Office passbook calculation. The term solver chooses the first whole month that reaches the goal.
Formula
Monthly i = (1 + nominal annual rate/4)^(1/3) - 1. Maturity = monthly deposit x [(1 + i)^n - 1]/i x (1 + i). At zero interest, maturity = monthly deposit x n.
Worked example
Twelve INR 100 deposits at 0% mature at INR 1,200. With a 12% quarterly-compounded nominal rate, one INR 100 beginning deposit held three months becomes INR 103.
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Frequently asked questions
Why do beginning deposits earn extra interest?
The first deposit is held for the full term and the last deposit still earns one month of interest.
Does this match every bank?
No. Product-specific rounding, calendar dates and partial quarters can change an actual maturity quote.
Can I solve for a negative rate?
The reverse rate search is limited to 0% through 100% nominal annual interest.