Compound Interest Calculator
See how savings grow with regular contributions.
Added at the end of every month. Leave at 0 for a lump sum only.
Balance after 20 years
$343,778
At 8% your money doubles roughly every 9.0 years (Rule of 72).
Growth year by year
Each column is your balance at the end of that year, split into what you put in and what the compounding earned.
- Your contributions
- Interest earned
View as table
| Year | Contributed | Interest | Balance |
|---|---|---|---|
| 1 | $16,000 | $1,055 | $17,055 |
| 2 | $22,000 | $2,695 | $24,695 |
| 3 | $28,000 | $4,970 | $32,970 |
| 4 | $34,000 | $7,932 | $41,932 |
| 5 | $40,000 | $11,637 | $51,637 |
| 6 | $46,000 | $16,148 | $62,148 |
| 7 | $52,000 | $21,531 | $73,531 |
| 8 | $58,000 | $27,859 | $85,859 |
| 9 | $64,000 | $35,210 | $99,210 |
| 10 | $70,000 | $43,669 | $113,669 |
| 11 | $76,000 | $53,329 | $129,329 |
| 12 | $82,000 | $64,288 | $146,288 |
| 13 | $88,000 | $76,655 | $164,655 |
| 14 | $94,000 | $90,546 | $184,546 |
| 15 | $100,000 | $106,088 | $206,088 |
| 16 | $106,000 | $123,419 | $229,419 |
| 17 | $112,000 | $142,685 | $254,685 |
| 18 | $118,000 | $164,049 | $282,049 |
| 19 | $124,000 | $187,684 | $311,684 |
| 20 | $130,000 | $213,778 | $343,778 |
How the Compound Interest Calculator works
Compound interest pays you interest on your interest. Over short periods the effect is small; over decades it dominates. This calculator shows the full curve, separating what you contributed from what the compounding actually earned.
Not financial advice. This calculator is for planning and illustration, not financial advice. Real products carry fees, taxes, and terms it does not model. Confirm figures with your lender or a qualified adviser before committing.
Frequently asked questions
What is the compound interest formula?
A = P(1 + r/n)^(nt), where P is the starting principal, r is the annual rate as a decimal, n is how many times per year interest compounds, and t is years. Regular contributions are added with a separate future-value-of-an-annuity term.
How does compounding frequency affect returns?
More frequent compounding earns slightly more. At 8% over 30 years, monthly compounding beats annual by roughly 3-4% of the final balance. It matters, but far less than the rate itself or how long you stay invested.
What is the Rule of 72?
Divide 72 by the annual return to estimate the years needed to double your money. At 8% that is about 9 years; at 12%, about 6. It is a rough mental shortcut, accurate to within a few percent for rates between 6% and 15%.
Does this account for inflation or tax?
No, the result is a nominal figure. To estimate purchasing power, subtract expected inflation (historically 2-3%) from your return rate before entering it, which gives you a real, inflation-adjusted balance.