Compound Interest Calculator
This calculator shows what a lump sum plus steady monthly deposits can become when returns compound. Compounding — earning interest on interest — is the engine behind long-term wealth building, and small deposits made early matter more than most people expect.
Educational estimate only — not financial advice. Results depend entirely on the numbers and assumptions you enter; real-world returns, rates, taxes, and fees will differ.
What is compound interest?
Compound interest is interest calculated on both the original amount you put in and on all the interest that has already been added to it. In the first period you earn interest only on your principal. In the second period you earn interest on the principal plus the first period's interest. From there the effect snowballs: every period, the base on which interest is calculated gets a little bigger, so the balance grows faster and faster even though the interest rate itself never changes.
This is the opposite of simple interest, where interest is always calculated on the original principal alone and growth is a straight line. With compounding, growth is a curve that bends upward. The difference looks small over a year or two, but over ten, twenty, or thirty years it becomes the single biggest factor in how a savings account, a retirement fund, or an investment portfolio performs. Investors sometimes call this the snowball effect, because a small early advantage keeps picking up more mass as it rolls.
Two things control how powerful compounding becomes: the rate of return and time. A higher rate makes the curve steeper, but time is the ingredient people underestimate. Because growth accelerates, the last years of a long savings plan often add more dollars than the first years did, even with identical deposits. That is why starting early — even with small amounts — usually beats starting later with larger amounts.
Regular deposits supercharge the process. Each deposit starts compounding from the moment it lands, and deposits made in the early years have the longest time to grow. A household that deposits $200 a month for ten years at a 7% average return contributes $24,000 of deposits, but if it also started with $10,000 invested, the interest earned along the way adds thousands more on top — money that was never taken out of a paycheck.
How compound interest is calculated
The calculator applies the standard compound interest formula once for your initial amount and once for your stream of deposits, then adds the two together. Your initial amount grows by the compounding factor for the full term. Your deposits are treated as a series of equal payments, one per compounding period, each of which compounds for the time it remains invested; deposits you enter as a monthly amount are grouped into the compounding period you select.
The result depends on four inputs you control: how much you start with, how much you add each month, the annual rate, and how long the money stays invested. It also depends on compounding frequency — how often interest is credited. Monthly compounding is the default because most savings accounts, and the way most people model investment returns, credit growth monthly. Daily compounding produces a slightly higher result and annual compounding a slightly lower one, because interest starts earning its own interest sooner.
One honest warning about this formula: it assumes a constant rate of return, which real investments never deliver. A stock portfolio might average 7% over a decade while swinging up 20% one year and down 12% the next. The compound interest formula smooths that ride into a single steady curve. For savings accounts and CDs the rate really is close to constant, so the result is a reliable projection. For market investments, treat the output as the center of a range of possible outcomes, not a promise.
The formula also assumes deposits arrive on schedule and are never interrupted, and that you reinvest everything with no withdrawals, taxes, or fees. Each of those real-world frictions reduces the final balance. Even so, the calculator is the right planning tool: it shows the mechanics clearly, lets you test how sensitive the outcome is to the rate, the deposit, and the time horizon, and gives you a target to aim at.
Worked example
Suppose you start with $10,000, deposit $200 a month, earn a 7% annual rate compounded monthly, and leave the money alone for 10 years. The calculator is pre-loaded with exactly these numbers, so the result at the top of this page matches the table below.
| Initial amount | $10,000.00 |
| Total deposits (120 × $200) | $24,000.00 |
| Balance after 5 years | $28,494.83 |
| Interest earned over 10 years | $20,713.58 |
| Future value after 10 years | $54,713.58 |
Read the table carefully and one line jumps out: you put in $34,000 of your own money ($10,000 at the start plus $24,000 of deposits), yet the account ends at about $54,713.58. The difference — roughly $20,713.58 — is interest the account earned by itself. That is compounding doing work that your paycheck never had to do. Extend the same plan to 20 years and the interest share grows far larger, because the balance that interest is calculated on keeps expanding.
The five-year figure is equally instructive. Halfway through the plan the balance is about $28,494.83, well past the halfway mark of the final value in time but not in dollars — the second five years add more than the first five. This back-loaded pattern is normal for compounding and is the main reason financial planners urge people to start retirement saving in their twenties rather than waiting for a bigger salary in their thirties.
How compounding frequency changes the result
Compounding frequency answers a simple question: how often is interest added to your balance so it can start earning interest itself? With annual compounding, a year's interest is credited once, at year end. With monthly compounding, one twelfth of the annual rate is credited each month. With daily compounding, interest lands every day. The underlying annual rate is the same in all three cases, but the more frequently interest is credited, the slightly larger the final balance, because credited interest joins the principal sooner.
How much does it matter? On $10,000 at 7% for 10 years with no deposits, annual compounding gives about $19,672, monthly about $20,097, and daily about $20,138. The gap between monthly and daily is small — around $41 in this case — but the gap between annual and monthly is real money. When you compare savings accounts, this is exactly why banks quote an APY (annual percentage yield) alongside the nominal rate: APY already bakes in the compounding frequency, so it is the honest number to compare. Our APY calculator shows the conversion directly.
For long-horizon investing the frequency question mostly fades into the background, because the return itself is an average of volatile market results rather than a credited rate. Frequency matters most for deposit products — savings accounts, money market accounts, and CDs — where the rate is contractual and the crediting schedule is written into the account terms.
Compounding when you borrow instead of save
Compounding is neutral machinery: it multiplies whatever it is applied to, including debt. Credit card balances compound monthly (and interest is often calculated daily), which is why a balance carried at a high APR grows so aggressively when only minimum payments are made. The same formula that builds a saver's balance builds a borrower's balance — the only difference is who receives the interest.
This symmetry explains a common piece of financial advice: paying off a card charging 22% APR earns you a guaranteed 22% 'return' in avoided interest, which no savings account can match. If you carry revolving debt, run the numbers both ways — this calculator for what your savings could earn, and the loan payoff calculator for what extra payments save you — and let the arithmetic decide where the next dollar goes.
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Frequently asked questions
What is compound interest in simple words?
It is interest earned on interest. You deposit money, it earns interest, and that interest is added to your balance — so next period you earn interest on a bigger amount. Over time the growth speeds up, which is why long-term savers benefit so much from starting early.
How often should interest compound for the best result?
More frequent compounding always produces a slightly higher balance at the same stated rate: daily beats monthly, which beats annual. The differences are modest at typical savings rates, so choose accounts by their APY — the yield that already accounts for compounding — rather than by the compounding schedule alone.
Does this calculator work for investments like index funds?
It gives a reasonable projection if you enter the average annual return you expect, but real markets do not grow in a smooth line. Treat the result as an estimate of the middle outcome. Actual results will vary year by year and may be higher or lower than the projection.
What happens if I stop making deposits halfway through?
The money already invested keeps compounding; only the future deposits stop. You can model it here by running the calculator twice: once up to the year you stop, then use that balance as the initial amount with a $0 deposit for the remaining years.
Why does starting early matter so much?
Because compounding is back-loaded. In the example above, the second five years add more growth than the first five. Money invested at 25 has roughly twice as many doubling cycles ahead of it as money invested at 45, so each early dollar does the work of several later dollars.