Banks compute RD maturity with quarterly compounding on monthly deposits:
M = R × ((1 + i)n − 1) ÷ (1 − (1 + i)−1/3)
- R — monthly deposit
- i — quarterly interest rate (annual rate ÷ 4 ÷ 100)
- n — number of quarters (years × 4)
Total invested = R × months. Interest earned = M − total invested. The −1/3 exponent adjusts for deposits made monthly within each quarter.