Formulas
General (monthly deposit, compounding n times/year):
Let r = R/100 (annual rate, decimal), total months m, and monthly growth factor
g = (1 + r/n)^(n/12).
Then for end-of-month deposits: FV = P × (g^m − 1) / (g − 1)
For beginning-of-month deposits: FV = P × (g^m − 1) / (g − 1) × g

Popular India (Quarterly compounding, monthly deposit):
With q = 1 + r/4, m = 12t:
End-of-month: FV = P × [ q^(4t) − 1 ] / [ q^(1/3) − 1 ]
Beginning-of-month: multiply by q^(1/3).
These are algebraically equivalent to many bank/fintech calculators.

Disclaimer: This tool is for illustration only. Actual RD maturity depends on bank/NBFC rules, compounding/crediting practices, deposit date cut-offs, penalties, taxes, and rounding.