Compound Interest Calculator
Project the future value of a balance with regular contributions, any compounding frequency, and a year-by-year breakdown.
Results are provided as is, for general information. Check anything important before relying on it. See the Terms of Use.
About this tool
Projects how a balance grows when interest is added back and left to earn more interest. Enter a starting amount, an annual rate, a term in years and how often interest compounds — annually, semi-annually, quarterly, monthly or daily — and optionally a recurring contribution paid monthly or yearly, at the start or end of each period. The result is the future value, the total you paid in (starting amount plus every contribution), the interest earned, and a year-by-year table of starting balance, contributions, interest and ending balance. The calculation runs a monthly simulation: each month the balance grows by a factor derived from the nominal rate at your chosen compounding frequency, and contributions are added before that growth (start) or after it (end). A 0% rate simply returns what you paid in.
How it works
A = P × (1 + r ÷ n)^(n × t) P = starting amount, r = annual rate, n = compounding periods per year, t = years
Interest is added to the balance and then earns interest itself. More frequent compounding gives a slightly higher result at the same rate. Regular contributions are added month by month and compound from the month they go in.
| Question | Working | Answer |
|---|---|---|
| $10,000 at 5% for 10 years, compounded yearly | 10,000 × 1.05^10 | $16,288.95 ($6,288.95 interest) |
| The same, compounded monthly | 10,000 × (1 + 0.05 ÷ 12)^120 | $16,470.09 ($6,470.09 interest) |
| $200 a month at 7% for 30 years, starting from zero | 360 deposits, compounded monthly | $243,994.20 ($72,000 deposited) |
When to use it
Questions
Does more frequent compounding really matter?
A little. At 6% nominal, one year on $10,000 is $600 compounded annually, about $616.78 compounded monthly, and about $618.31 compounded daily. The gap widens with higher rates and longer terms but is usually small next to the effect of the rate itself and of regular contributions.
What is the difference between contributions at the start and end of the period?
A start-of-period contribution is in the account for that whole period, so it earns that period's interest; an end-of-period contribution is added after interest is applied and starts earning next period. Over decades the start-of-period option comes out noticeably ahead. End of period is the default because it matches how most people actually save (money goes in after payday).
Is this before or after tax and inflation?
Before both. The future value is in nominal dollars at the rate you enter — it does not deduct tax on interest, account fees, or the effect of inflation on what those dollars will buy. For a rough 'real' picture, enter a rate reduced by your expected inflation.
How is the monthly growth worked out for, say, quarterly compounding?
The nominal annual rate is divided by the number of compounding periods per year to get the periodic rate, then converted to an equivalent monthly factor: (1 + rate/n) raised to the power n/12. Simulating in monthly steps this way keeps one code path for every compounding frequency and lines up cleanly with monthly contributions.
What is the difference between the nominal rate and APY?
The tool takes the nominal annual rate plus a compounding frequency — how loan documents and savings ads state it. APY (effective annual yield) is what the nominal rate becomes once compounding is applied: 6% nominal compounds monthly to about 6.17% effective. At 10 years on 10,000, that is 17,908.48 compounded annually versus 18,193.97 monthly — figures the schedule shows directly. If your bank quotes APY, compounding is already baked in; entering it as the nominal rate with annual compounding is the safe interpretation.
How much do regular contributions change the outcome?
More than intuition suggests, because each contribution starts earning immediately. 10,000 at 6% monthly-compounded over 10 years is 18,193.97; adding a 100-at-end-of-month contribution brings the total to 34,581.90 — of which 12,000 is deposits and the remaining 4,387.93 of the gain is interest earned on everything so far. The year-by-year table shows the contribution and interest lines evolving separately, which is where the 'how much did I actually earn' answer lives.
Where does start-versus-end contribution timing matter?
Everywhere in principle, visibly at high rates and long horizons. A start-of-period contribution earns that period's growth; an end-of-period one misses it — one period's head start, compounding for the whole term. The default is end-of-period because money usually arrives after payday, not as a mathematical endorsement. The schedule makes the difference explicit: run both timings on the same inputs and compare the future values.
Why does the schedule show one row per year?
Because the simulation runs monthly but people read annually: a 30-year monthly table would be 360 rows of near-identical lines. Each annual row folds twelve monthly growth steps and any contributions, so the rows reconcile exactly with the headline future value, contributions and interest. The monthly mechanics remain inside the math even where the display is annual — the row numbers are compound results, not approximations.
Is this exactly what my bank would compute?
The model is the standard one — full-precision simulation at your chosen compounding frequency, rounded only for display — and projections from any source with the same rate and frequency will match it. Actual account statements can differ through average-daily-balance methods, fee timing and institution-specific rounding; treat the statement as ground truth for what happened, and this tool as the clean-model projection of what should happen.
What does a 0% rate mean here?
It is accepted and meaningful: the future value is exactly the money you put in, with the schedule's interest column zero throughout — a useful baseline that shows every dollar of projected growth attributable to the rate. Negative rates are rejected by design: a savings balance shrinking by policy is a fee model, not a compound-interest one, and fees are better subtracted as a separate step.