Compound Interest Calculator
Enter your starting principal, interest rate, and how often it compounds to see how your money grows over time. Add an optional recurring contribution to see how regular deposits accelerate growth, choose whether you deposit at the start or end of each period, and enter an inflation rate to see what your balance is worth in today's dollars.
Results
| Future value (nominal) | - |
|---|---|
| Total contributed | - |
| Total interest earned | - |
| Real value (today's dollars) | - |
Growth over time
Year-by-year balance
| Year | Total contributed | Balance |
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Understanding compound interest
Compound interest is the engine behind almost every long-term savings and investing goal. The idea is simple: the interest you earn is added back to your balance, and the next round of interest is then calculated on that larger balance rather than on your original deposit alone. In other words, your interest starts earning interest. This "interest on interest" effect is what separates compounding from simple interest, where you earn the same flat amount every period on the principal and nothing more. Over a year or two the gap is modest, but stretched across decades it becomes the dominant force in your account, often contributing far more to the final balance than your own deposits. Understanding how it behaves, and which levers change the outcome, is the whole point of this calculator.
Why interest on interest matters so much
With simple interest, $5,000 at 7% earns a flat $350 every year, forever. With compound interest, that first $350 is added to your balance, so in year two you earn 7% on $5,350, not $5,000. The extra is small at first (about $24.50), but it never stops growing. Each year the base gets bigger, so each year's interest gets bigger, and the curve bends upward. This is why compounding is sometimes described as exponential growth: the balance does not rise in a straight line, it accelerates. The practical lesson is that the early years feel disappointingly slow while the later years do the heavy lifting, which rewards patience and punishes anyone who gives up early or dips into the balance.
How compounding frequency changes the result
At the same stated (nominal) annual rate, the balance also depends on how often interest is credited. Compounding frequency is the number of times per year interest is calculated and added back: annually credits it once, monthly twelve times, and daily 365 times. The more often interest is credited, the sooner each piece of it begins earning its own interest, so a higher frequency always produces a slightly higher balance at the same nominal rate. Consider $5,000 at 7% for 10 years. Compounded annually it grows to about $9,836. Compounded monthly it reaches about $10,048. Compounded daily it edges up to roughly $10,068. The jumps shrink as frequency rises because there is a mathematical ceiling (continuous compounding), but the direction is always the same: more frequent compounding beats less frequent at an identical nominal rate. Switch the frequency dropdown above and watch the future value tick upward while every other input stays fixed.
The rule of 72
A quick way to gauge compounding without any tool is the rule of 72: divide the number 72 by your annual percentage rate, and the result is roughly how many years it takes your money to double. At 7%, that is 72 / 7, or about 10.3 years, which is why the $5,000 default nearly doubles over the 10-year horizon. At 3% it would take about 24 years; at 9%, only 8. The rule is an approximation that works best for rates between about 4% and 12%, but it is excellent for sanity-checking a result or comparing two rates in your head. It also makes the cost of a lower rate vivid: shaving two points off your return can add a decade to your doubling time.
Recurring contributions and deposit timing
Most people do not just park a lump sum, they add to it regularly, and those deposits compound as well. A recurring contribution turns your account into an annuity, and the order in which deposits and interest are applied changes the total. This calculator exposes that choice through the contribution timing option. With end-of-period timing (an ordinary annuity), each deposit arrives at the close of the period and earns interest only from the next period onward. With start-of-period timing (an annuity due), every deposit sits in the account one extra period, so the contributed portion of your balance is multiplied by one additional factor of the per-period rate. The effect is one-directional: start-of-period timing always yields at least as much as end-of-period timing whenever you are contributing and earning a positive rate, and the gap widens with larger contributions, higher rates, and longer horizons. Your starting principal grows exactly the same either way; only the deposits are affected. If you are paid on the first and invest immediately, start-of-period is the more accurate model.
Nominal versus inflation-adjusted (real) value
The future value this tool reports is a nominal figure: the actual number of dollars you would see in the account. But a dollar decades from now buys less than a dollar today, so the nominal number overstates how much better off you really are. Enter a figure in the inflation rate input and the calculator also reports the real value, which discounts the nominal balance back into today's purchasing power using real value = nominal / (1 + inflation) raised to the number of years. If your $10,048 arrives after 10 years of 3% inflation, it is worth only about $7,476 in today's dollars. The real figure is the more honest measure of progress, because it answers the question that actually matters: not how many dollars you will have, but what those dollars will buy.
A worked example
Use the page defaults: a $5,000 initial principal, a 7% annual rate, monthly compounding, 10 years, no recurring contribution, end-of-period timing. The monthly rate is 7% divided by 12, or about 0.5833% per month, applied across 120 months. The future value comes to about $10,048. Because you contributed nothing beyond the opening $5,000, the entire gain of roughly $5,048 is interest, slightly more than the principal itself. Notice this is a touch above the annual-compounding figure of $9,836, the extra coming purely from crediting interest twelve times a year instead of once. Now try adding a $200 monthly contribution: total deposits rise to $29,000 and the balance climbs well past $40,000, with interest still doing a large share of the work. Switch timing to start-of-period and the figure nudges higher again, since each deposit now earns one extra month of growth.
Common mistakes to avoid
- Confusing APR and APY. The nominal rate (APR) ignores compounding; the annual percentage yield (APY) folds it in. A 7% nominal rate compounded monthly is an APY of about 7.23%. When comparing products, always compare the same measure, and prefer APY for savings.
- Ignoring inflation. A big nominal number can hide flat or shrinking purchasing power. Always enter an inflation rate and read the real value before deciding a goal is funded.
- Underestimating the time horizon. Because growth accelerates, the last decade usually dwarfs the first. Cutting your horizon short, or starting late, removes exactly the years that compound hardest.
- Not reinvesting. Compounding only works if interest and dividends stay in the account. Spending the earnings each period converts your investment back into simple interest and flattens the curve.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest pays a flat amount each period on your original principal only. Compound interest adds each period's interest back to the balance, so the next period's interest is calculated on a larger sum. Over long horizons compounding produces dramatically more because the interest itself keeps earning interest.
Does compounding frequency really change how much I earn?
Yes, though modestly. At the same nominal rate, crediting interest more often lets each piece start earning sooner. At 7% over 10 years, $5,000 grows to about $9,836 compounded annually, $10,048 monthly, and $10,068 daily. The gains shrink as frequency rises toward the continuous-compounding ceiling.
How does the rule of 72 work?
Divide 72 by your annual percentage rate to estimate the years needed to double your money. At 7% that is 72 / 7, about 10.3 years. It is an approximation that is most accurate for rates between roughly 4% and 12%, and it is handy for quick comparisons without a calculator.
Should I choose start-of-period or end-of-period contributions?
Pick whichever matches reality. If you deposit at the start of each period (start-of-period, an annuity due), each contribution earns one extra period of interest, so the future value is slightly higher. End-of-period (an ordinary annuity) assumes deposits land at the close of the period. Use the timing dropdown to compare them.
Why is the real value lower than the future value?
The future value is nominal: the actual dollars in the account. The real value discounts that figure by the inflation rate you enter, showing what the balance would buy in today's dollars. Since prices tend to rise over time, the real value is lower and is the more meaningful measure of how much wealthier you have become.