Complete guide
Reviewed July 2026Compound interest is interest earned on interest. Where simple interest pays only on your original principal, compounding adds each period's interest to the balance so the next period earns on a larger base — a feedback loop that turns steady rates into exponential growth given enough time.
This calculator computes the future value of any principal at any rate, for any duration, with your choice of compounding frequency — annually, semi-annually, quarterly, monthly or daily — and shows exactly how much of the final amount is your money versus earned interest.
Einstein probably never called compounding the eighth wonder of the world, but the math earns the reputation honestly: ₹1 lakh at 8% becomes ₹2.16 lakh in 10 years, ₹4.66 lakh in 20, and ₹10.06 lakh in 30. The last decade earns more than the first two combined.
The compound interest formula
A = P × (1 + r/n)^(n×t) A = final amount P = principal (initial investment) r = annual interest rate (decimal) n = compounding periods per year t = time in years Compound interest earned = A − P
Every variable matters, but they don't matter equally. Time sits in the exponent — it's the most powerful input. Rate multiplies inside the bracket — second most powerful. Frequency (n) fine-tunes: more frequent compounding helps, but with rapidly diminishing returns.
Step-by-step example
- P = ₹1,00,000, r = 8% (0.08), monthly compounding (n = 12), t = 5 years.
- Periodic rate: 0.08 ÷ 12 = 0.006667.
- Number of periods: 12 × 5 = 60.
- (1.006667)^60 = 1.4898.
- A = 1,00,000 × 1.4898 = ₹1,48,985. Interest earned: ₹48,985.
- Simple interest at the same rate would earn ₹40,000 — compounding added ₹8,985 (22% more) in just five years.
Does compounding frequency matter?
Moving from annual to daily compounding adds about 0.33% of effective yield at 8% — real but modest. The lesson: when comparing products, convert everything to the effective annual rate (EAR) and compare those; a 7.9% daily-compounded account loses to an 8.3% annually-compounded one.
| Frequency | Final amount | Effective annual rate |
|---|---|---|
| Annually (n=1) | ₹2,15,892 | 8.000% |
| Semi-annually (n=2) | ₹2,19,112 | 8.160% |
| Quarterly (n=4) | ₹2,20,804 | 8.243% |
| Monthly (n=12) | ₹2,21,964 | 8.300% |
| Daily (n=365) | ₹2,22,535 | 8.328% |
The Rule of 72
Divide 72 by your annual rate to estimate the years needed to double your money: at 8%, about 9 years; at 12%, about 6; at 6%, about 12. It's accurate within a few months for rates between 4% and 15% — a fast sanity check on any projection this calculator produces.
Time beats rate: the case for starting early
The early starter invests less than half as much and stops entirely at 35 — yet finishes 75% richer, because her contributions compound for 25–35 years each. Nothing else in personal finance produces asymmetries like this. Compounding rewards the calendar more than the contribution.
| Investor | Invests during | Total invested | Value at 60 |
|---|---|---|---|
| Early starter | Age 25–35 only (10 yrs) | ₹1,00,000 | ₹18.9 lakh |
| Late starter | Age 35–60 (25 yrs) | ₹2,50,000 | ₹10.8 lakh |
How to use this calculator
- Enter your principal — the lump sum you're investing today.
- Set the annual interest rate. Use the quoted nominal rate; the frequency setting handles the rest.
- Choose the number of years.
- Select the compounding frequency your product actually uses: Indian FDs typically compound quarterly, savings accounts daily or quarterly, bonds semi-annually, most loan interest monthly.
- Read the final amount and the interest portion, and sanity-check with the Rule of 72.
Common mistakes
- Comparing nominal rates across different compounding frequencies — always compare effective annual rates.
- Ignoring taxes: interest on FDs and savings is taxed yearly at slab rates in India, which meaningfully slows real compounding versus tax-deferred instruments like PPF.
- Ignoring inflation: 8% nominal growth during 6% inflation is ~2% real growth. Run projections in real terms for long-horizon goals.
- Interrupting the compounding: withdrawing 'just the interest' converts compound growth back into simple interest.
- Assuming a constant rate for market investments — for equities, use long-run averages and treat outputs as estimates.
Frequently asked questions
Glossary
- Principal
- The original sum invested or borrowed, before any interest.
- Compounding frequency
- How often interest is added to the balance (annually, quarterly, monthly, daily).
- Effective annual rate (EAR)
- The true one-year growth rate after intra-year compounding: (1 + r/n)^n − 1.
- Nominal rate
- The quoted annual rate before accounting for compounding frequency.
- Rule of 72
- Doubling-time estimate: 72 ÷ rate ≈ years to double.
- Real return
- Growth after subtracting inflation — the change in actual purchasing power.
- Future value
- What a sum today becomes after compounding at a given rate for a given time.
- Continuous compounding
- The theoretical limit of infinitely frequent compounding: A = Pe^(rt).
Key takeaways
Compound interest grows money exponentially because each period's interest joins the base for the next. The formula A = P(1+r/n)^(nt) makes three things clear: time (in the exponent) dominates, rate comes second, frequency is a rounding matter. Start early, compare products by effective annual rate, defend the compounding from taxes and interruptions — and keep the same force off your liabilities.
Enter your principal, rate and term above — then double the years and watch what the exponent does.