Compound Interest Calculator
Watch your savings multiply through the power of compounding. Calculate future balance with regular monthly additions and custom compounding intervals.
Annual Growth Schedule
Year-by-year compounding trajectory, annual contributions, and balance growth.
| Year | Annual Deposits | Total Deposited | Interest Earned (Yr) | Total Accrued Interest | Ending Balance |
|---|---|---|---|---|---|
| Year 1 | $6,000 | $16,000 | +$1,055 | $1,055 | $17,055 |
| Year 2 | $6,000 | $22,000 | +$1,641 | $2,695 | $24,695 |
| Year 3 | $6,000 | $28,000 | +$2,275 | $4,970 | $32,970 |
| Year 4 | $6,000 | $34,000 | +$2,961 | $7,932 | $41,932 |
| Year 5 | $6,000 | $40,000 | +$3,705 | $11,637 | $51,637 |
| Year 6 | $6,000 | $46,000 | +$4,511 | $16,148 | $62,148 |
| Year 7 | $6,000 | $52,000 | +$5,383 | $21,531 | $73,531 |
| Year 8 | $6,000 | $58,000 | +$6,328 | $27,859 | $85,859 |
| Year 9 | $6,000 | $64,000 | +$7,351 | $35,210 | $99,210 |
| Year 10 | $6,000 | $70,000 | +$8,459 | $43,669 | $113,669 |
| Year 11 | $6,000 | $76,000 | +$9,659 | $53,329 | $129,329 |
| Year 12 | $6,000 | $82,000 | +$10,959 | $64,288 | $146,288 |
| Year 13 | $6,000 | $88,000 | +$12,367 | $76,655 | $164,655 |
| Year 14 | $6,000 | $94,000 | +$13,891 | $90,546 | $184,546 |
| Year 15 | $6,000 | $100,000 | +$15,542 | $106,088 | $206,088 |
| Year 16 | $6,000 | $106,000 | +$17,330 | $123,419 | $229,419 |
| Year 17 | $6,000 | $112,000 | +$19,267 | $142,685 | $254,685 |
| Year 18 | $6,000 | $118,000 | +$21,364 | $164,049 | $282,049 |
| Year 19 | $6,000 | $124,000 | +$23,635 | $187,684 | $311,684 |
| Year 20 | $6,000 | $130,000 | +$26,095 | $213,778 | $343,778 |
The Mathematics of Exponential Financial Compounding
The Compound Interest Formula
For a single lump-sum investment, future balance follows the exponential equation:
Where A is final balance, P is principal, r is annual interest, n is compounding frequency per year, and t is total years.
The Rule of 72
A classic mental math shortcut used by Wall Street investors to quickly approximate how many years it takes an asset to double:
At a steady 8% annualized stock market return, your principal doubles roughly every 9 years (72 / 8 = 9).
Time: The Ultimate Leverage
Because interest generates its own interest, growth is non-linear. In a 30-year horizon, more than half of your total lifetime portfolio gains occur in just the final 6 to 7 years.
Starting to invest 10 years earlier with half the monthly contribution often beats starting later with double the contribution.