Compound interest calculator
Growth with regular contributions, and a chart of contributions versus interest.
131.4% on top of what you put in
$10,000.00 to start + $120,000.00 added
7% nominal, compounded 12× a year
Rule of 72 estimate: 10.3 years
What you put in Total including growth
Year by year
| Year | Contributed | Interest | Balance |
|---|---|---|---|
| 1 | $16,000.00 | $919.19 | $16,919.19 |
| 2 | $22,000.00 | $2,338.58 | $24,338.58 |
| 3 | $28,000.00 | $4,294.31 | $32,294.31 |
| 4 | $34,000.00 | $6,825.16 | $40,825.16 |
| 5 | $40,000.00 | $9,972.70 | $49,972.70 |
| 6 | $46,000.00 | $13,781.53 | $59,781.53 |
| 7 | $52,000.00 | $18,299.43 | $70,299.43 |
| 8 | $58,000.00 | $23,577.68 | $81,577.68 |
| 9 | $64,000.00 | $29,671.22 | $93,671.22 |
| 10 | $70,000.00 | $36,639.02 | $106,639.02 |
| 11 | $76,000.00 | $44,544.25 | $120,544.25 |
| 12 | $82,000.00 | $53,454.70 | $135,454.70 |
| 13 | $88,000.00 | $63,443.02 | $151,443.02 |
| 14 | $94,000.00 | $74,587.14 | $168,587.14 |
| 15 | $100,000.00 | $86,970.62 | $186,970.62 |
| 16 | $106,000.00 | $100,683.03 | $206,683.03 |
| 17 | $112,000.00 | $115,820.45 | $227,820.45 |
| 18 | $118,000.00 | $132,485.91 | $250,485.91 |
| 19 | $124,000.00 | $150,789.85 | $274,789.85 |
| 20 | $130,000.00 | $170,850.72 | $300,850.72 |
How it works
Compound interest is interest earning interest. The balance grows exponentially rather than linearly, which is why the shape of the chart above is a curve that steepens — and why the last decade of a long investment contributes more than the first two combined.
FV = P(1 + i)ⁿ + C · [((1 + i)ⁿ − 1) / i]
P = starting amount C = contribution per period
i = rate ÷ periods n = periods × years Nominal rate versus APY
A “7% APR compounded monthly” does not earn 7% a year — it earns 7.23%, because each month's interest starts earning too. APY (or AER) folds that in, which is why it is the honest number for comparing savings accounts. The gap widens with both the rate and the frequency.
Time beats amount
Someone investing $200 a month from age 25 to 35 and then stopping usually ends up ahead of someone starting at 35 and paying in for thirty years. Ten years of head start compounds for the whole remaining period. There is no way to buy that back later.
The rule of 72
Divide 72 by the percentage rate to estimate the doubling time. At 7%, money doubles in about ten years; at 3%, twenty-four. It is a mental shortcut accurate to within a few percent for rates between about 4% and 15%.
This ignores inflation, tax and fees
A 7% return with 3% inflation is roughly 4% in real purchasing power. A 1% annual fund fee removes far more than 1% of the final balance, because it also removes everything that fee would have compounded into. Both effects are large over decades.