Here’s the complete breakdown of your query on PNB’s 390-day FD at 6.60% and how long it’ll take for ₹5 lakh to become ₹10 lakh.
PNB’s New FD Rates (from September 1, 2025)
- General customers: 6.60%
- Senior citizens (60+): 6.90%
- Super senior citizens (80+): 7.20%
These rates apply to 390-day deposits (roughly 1 year + 1 month).
FD Doubling Time Formula
We use the Rule of 72 (approximation) and exact compound interest formula: T=ln(2)ln(1+r)T = \frac{\ln(2)}{\ln(1 + r)}T=ln(1+r)ln(2)
where:
- TTT = years to double
- rrr = annual interest rate (in decimal)
Case 1: General Customers (6.60%)
- Interest rate = 6.60% = 0.066
T=ln(2)ln(1.066)≈10.84 yearsT = \frac{\ln(2)}{\ln(1.066)} \approx 10.84 \text{ years}T=ln(1.066)ln(2)≈10.84 years
👉 ₹5,00,000 becomes ₹10,00,000 in 10 years 10 months (≈ 2036).
Case 2: Senior Citizens (6.90%)
- Interest rate = 6.90% = 0.069
T=ln(2)ln(1.069)≈10.44 yearsT = \frac{\ln(2)}{\ln(1.069)} \approx 10.44 \text{ years}T=ln(1.069)ln(2)≈10.44 years
👉 ₹5,00,000 doubles in 10 years 5 months (≈ 2036).
Case 3: Super Senior Citizens (7.20%)
- Interest rate = 7.20% = 0.072
T=ln(2)ln(1.072)≈9.97 yearsT = \frac{\ln(2)}{\ln(1.072)} \approx 9.97 \text{ years}T=ln(1.072)ln(2)≈9.97 years
👉 ₹5,00,000 doubles in just under 10 years (≈ 2035).
Interest Earned in One Cycle (390 Days)
Let’s calculate for General customer @ 6.60%:
- Principal = ₹5,00,000
- Rate = 6.60%
- Period = 390 days (~1.068 years)
Maturity Amount=5,00,000×(1+0.066×1.068)≈₹5,35,290\text{Maturity Amount} = 5,00,000 \times (1 + 0.066 \times 1.068) \approx ₹5,35,290Maturity Amount=5,00,000×(1+0.066×1.068)≈₹5,35,290
👉 In 390 days, you earn ₹35,290 interest.
- Senior Citizens (6.90%): Maturity ≈ ₹5,36,850 (interest ₹36,850)
- Super Seniors (7.20%): Maturity ≈ ₹5,38,400 (interest ₹38,400)
Why Investors Like FD
- Assured returns (no stock market risk).
- DICGC insurance up to ₹5 lakh.
- Liquidity (can break FD if needed, penalty applies).
- Suitable for all categories: individuals, HUFs, trusts, companies.
Final Takeaway
- For general investors, ₹5 lakh becomes ₹10 lakh in about 10 years 10 months.
- Senior citizens shave off ~5 months.
- Super senior citizens reach it under 10 years.
So, the higher your age bracket, the faster your FD doubles.