Compound Interest Calculator
Calculate compound interest on a deposit with optional ongoing contributions. Choose your compounding frequency — daily, monthly, quarterly, annual, or continuous — and see the effect on the final balance side-by-side. The same input math applies whether you're modeling a savings account, a CD, or a generic interest-bearing investment.
Year-by-year breakdown
| Year | Starting Balance | Contributions | Interest Earned | Ending Balance |
|---|---|---|---|---|
| Year 1 | $10,000 | $2,400 | +$520 | $12,920 |
| Year 2 | $12,920 | $2,400 | +$654 | $15,974 |
| Year 3 | $15,974 | $2,400 | +$795 | $19,168 |
| Year 4 | $19,168 | $2,400 | +$942 | $22,510 |
| Year 5 | $22,510 | $2,400 | +$1,095 | $26,005 |
| Year 6 | $26,005 | $2,400 | +$1,256 | $29,662 |
| Year 7 | $29,662 | $2,400 | +$1,425 | $33,486 |
| Year 8 | $33,486 | $2,400 | +$1,601 | $37,487 |
| Year 9 | $37,487 | $2,400 | +$1,785 | $41,672 |
| Year 10 | $41,672 | $2,400 | +$1,977 | $46,049 |
Compounding frequency comparison
For a given annual rate, more frequent compounding produces a slightly higher final balance. The differences are small at typical rates: the gap between daily and annual compounding on $10,000 at 4.5% for 10 years is roughly $80 — about 0.5% of the interest earned. Continuous compounding (the theoretical maximum) produces only a negligible improvement over daily.
The math
With principal P, annual rate r, and n compounding periods per year, the future value after t years is:
FV = P × (1 + r/n)^(n×t)
For continuous compounding, the formula is FV = P × e^(r×t). For recurring contributions, add the future value of an annuity for the contribution stream — that's what this calculator does internally, iterating month-by-month so the contribution-timing choice (beginning vs end of period) actually affects the result.
Why this matters more for long horizons than short ones
Compound growth is exponential. The interest earned in any given year is a function of the balance at the start of that year — and the balance at the start of year 20 is much larger than the balance at the start of year 2. Over a 30-year horizon, the majority of the final balance typically comes from interest, not from the original principal or the contributions.
Current US high-yield savings account rates
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