Compound Interest Calculator
Project growth from a starting balance, regular contributions and a rate of return.
Compound growth
| Year | Contributed | Interest | Balance |
|---|---|---|---|
| 1 | $3,600.00 | $840.68 | $14,440.68 |
| 2 | $3,600.00 | $1,161.69 | $19,202.37 |
| 3 | $3,600.00 | $1,505.92 | $24,308.29 |
| 4 | $3,600.00 | $1,875.02 | $29,783.31 |
| 5 | $3,600.00 | $2,270.81 | $35,654.12 |
| 6 | $3,600.00 | $2,695.22 | $41,949.34 |
| 7 | $3,600.00 | $3,150.30 | $48,699.63 |
| 8 | $3,600.00 | $3,638.28 | $55,937.91 |
| 9 | $3,600.00 | $4,161.53 | $63,699.44 |
| 10 | $3,600.00 | $4,722.61 | $72,022.06 |
| 11 | $3,600.00 | $5,324.26 | $80,946.31 |
| 12 | $3,600.00 | $5,969.39 | $90,515.70 |
| 13 | $3,600.00 | $6,661.16 | $100,776.87 |
| 14 | $3,600.00 | $7,402.94 | $111,779.81 |
| 15 | $3,600.00 | $8,198.35 | $123,578.16 |
| 16 | $3,600.00 | $9,051.25 | $136,229.41 |
| 17 | $3,600.00 | $9,965.81 | $149,795.22 |
| 18 | $3,600.00 | $10,946.48 | $164,341.70 |
| 19 | $3,600.00 | $11,998.05 | $179,939.75 |
| 20 | $3,600.00 | $13,125.63 | $196,665.39 |
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Not financial advice. Results are estimates for general information only and are not financial, investment or tax advice. Figures ignore fees, taxes and inflation unless stated. Consult a qualified adviser before making a decision.
Frequently asked questions
What is compound interest?
Interest calculated on the principal plus all previously accumulated interest, so the balance grows exponentially rather than in a straight line.
How much does compounding frequency change the result?
Less than most people expect. At 7% over 30 years, monthly rather than annual compounding adds roughly 3% to the final balance. Rate and time matter far more.
Does this account for inflation and tax?
No. Figures are nominal and before tax. To think in today's money, enter a real rate — your expected return minus expected inflation.
When are contributions applied?
At the end of each compounding period, which is the standard ordinary-annuity convention and the conservative assumption.
About the Compound Interest Calculator
Compound interest earns returns on previous returns. The future value of a lump sum is:
FV = P × (1 + r/n) ^ (n × t)
where P is the principal, r the annual rate, n the compounding periods per year and t the number of years. Regular contributions add an annuity term, calculated here for a deposit made at the end of each period.
Two things dominate the outcome. Time matters more than rate, because the exponent grows faster than the base: 7% over 30 years multiplies your money by 7.6, while 9% over 20 years multiplies it by 5.6. Contributions dominate early and returns dominate late — the year-by-year table shows the crossover point where annual interest first exceeds annual contributions.
Results are nominal. At 3% inflation, money halves in purchasing power roughly every 24 years, so subtract your inflation assumption from the rate to reason in today's money.