A recurring deposit calculator estimates the maturity value of a series of monthly deposits. Each instalment earns interest for a different length of time, so putting the total contributions into a fixed deposit formula for the full tenure gives the wrong model. HDFC Bank's official calculator explains that RD interest is compounded quarterly, while its interest page says interest starts from the date each instalment is paid.
| Illustration input | Assumption |
|---|---|
| Monthly deposit | Rs 1,000 |
| Number of deposits | 12 |
| Annual interest rate | 6%, assumed for teaching only |
| Total contributions | Rs 12,000 |
| Modelled maturity value | Rs 12,395.23 before deductions |
Recurring deposit calculator formula explained
A useful equal-month illustration is M = P × [(1 + i)N/3 minus 1] ÷ [1 minus (1 + i)−1/3]. Here P is the monthly instalment, N is the number of monthly instalments and i is the quarterly interest rate expressed as a decimal. For an annual decimal rate r, i = r ÷ 4. This model assumes the first instalment has N months to grow and the final instalment has one month.
The formula adds the maturity value of every deposit. Written another way, it is the sum of P × (1 + i)k/3 for k running from 1 to N. Dividing months by three converts the holding period into quarters. Fractional-quarter growth is an illustration assumption; it does not reproduce every bank's day-count rules or rounding procedure.
With an assumed annual rate of 6%, i = 0.06 ÷ 4 = 0.015. For 12 deposits of Rs 1,000, the calculation is 1,000 × [(1.015)4 minus 1] ÷ [1 minus (1.015)−1/3]. The result is Rs 12,395.23, comprising Rs 12,000 contributed and Rs 395.23 of modelled interest. The rate is an example, not a current bank offer or a promised return.
Why the instalment date changes RD interest
The first Rs 1,000 in this model grows for 12 months and becomes about Rs 1,061.36. The final Rs 1,000 grows for one month and becomes about Rs 1,004.98. Treating the full Rs 12,000 as though it arrived on the first day would give the later instalments interest for time they were never on deposit.
The official HDFC Bank RD interest page confirms that interest is calculated from the date each instalment is paid and compounded quarterly. Actual calendar dates therefore matter. Equal monthly contributions do not necessarily mean identical numbers of interest-bearing days between contributions.
This is the main difference from the lump-sum illustration in our FD quarterly compounding guide. In a lump-sum deposit, the principal enters together. In an RD, the calculator has to account for a sequence of cash flows. The compounding frequency alone cannot tell you the answer.
How to use a bank RD calculator
HDFC Bank's official RD calculator page asks for the monthly amount, tenure, senior citizen status and intended start date. It describes a tenure range from six months to ten years and says the calculator shows the estimated maturity amount and interest. Those are that bank's calculator inputs, not universal terms for all deposits.
Keep the total contributions separate from interest when reading the result. Also check whether the displayed figure is before deductions and whether all instalments are assumed to arrive on schedule. HDFC Bank labels the calculator amount indicative and directs customers to the FD or RD advice for the exact maturity amount. A teaching calculation should not replace that account-specific record.
For related background, see how deposit insurance counts bank balances and interest. A different monthly contribution product uses a different model, as explained in our SIP payment timing explainer. Matching the calculator to the product and its cash-flow dates is the first step to understanding its output.
