How to use the compound interest calculator
Enter your starting principal, interest rate, and duration. Choose years or months for the duration. Add regular contributions if you plan to top up. Everything updates as you type.
Optionally add inflation and tax rates to see your real, post-tax return. Use the Goal seek panel to work backwards from a target.
The compound interest formula
A = P × (1 + r/n)^(n×t)
Where P is the principal, r is the annual rate (decimal), n is compounding periods per year, and t is time in years.
Why inflation-adjusted matters
A rupee today buys more than a rupee in 20 years. If you earn 10% and inflation is 6%, your real return is roughly 4% — not 10%. This calculator shows both the nominal and the inflation-adjusted value so you can see what your money will actually buy.
Rule of thumb: Real return ≈ Nominal return − Inflation rate.
Why post-tax matters
Interest is usually taxed as ordinary income. If you're in the 30% tax bracket and earn 10%, you keep roughly 7% after tax. FDs and many debt investments fall in this category. Equity investments have different tax treatment — check the specific rules that apply.
Compounding frequency matters
| Frequency | Effective annual rate at 10% nominal |
|---|---|
| Yearly | 10.00% |
| Half-yearly | 10.25% |
| Quarterly | 10.38% |
| Monthly | 10.47% |
| Daily | 10.52% |
The rule of 72
Divide 72 by your interest rate to estimate how many years it takes to double. At 8%, money doubles in about 9 years. At 12%, in about 6 years.
Common questions
What is compound interest?
Interest earned on principal plus previously accumulated interest. It grows exponentially over time.
Why does inflation matter in compound interest?
A rupee today buys more than a rupee in 20 years. The real return is your nominal return minus inflation. A 10% return with 6% inflation gives a real return of roughly 4%.
How does tax affect compound returns?
Interest is usually taxed as ordinary income. Post-tax returns show what you actually keep after tax.
Can I calculate in months instead of years?
Yes. Switch the duration mode to Months and enter the number of months directly.
What is the goal seek feature?
You give it a target amount and a number of years, and it works out the monthly contribution you need to reach it.
Is my data saved?
Your inputs are stored only in your browser's local storage. Nothing is sent to any server.