CalcOak

Compound Interest Calculator: see how your savings grow year by year

Final balance
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Total contributed
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Interest earned
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Effective rate (APY)
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Balance by yearContributionsInterest
Stacked bar chart of contributions and interest per year
YearContributions to dateInterest that yearBalance

How to use the compound interest calculator

Enter what you have now, what you plan to add and how often, the annual rate your account or fund pays, and how many years you will leave it. Results update as you type. The default scenario shows the idea clearly: $10,000 up front plus $200 a month at 7% compounded monthly for 20 years ends at about $144,600. You put in $58,000 of that yourself; the remaining $86,600 is interest, and more than half of it arrives in the final seven years, which is why the orange share of each bar keeps widening.

The compounding menu changes how often interest is credited, and the APY card shows the effect. A nominal 7% is 7.00% APY with yearly compounding, 7.23% with monthly and 7.25% with daily, because each credit starts earning on the previous one. Banks usually advertise APY for savings and APR for lending, so if you are copying a figure from a savings account, choose yearly compounding to avoid counting the effect twice. Contribution timing decides whether each deposit earns interest in the period it is made: paying in on the first of the month rather than the last adds roughly one month of growth to every deposit, which is a small but free improvement.

Why use this instead of a rough rule of thumb

The "rule of 72" tells you that money doubles every 10 years or so at 7%, but it says nothing about regular deposits, which is how most people actually save. Deposits made at a different rhythm from the compounding, say weekly pay-ins into a daily-compounding account, are awkward to work out by hand. This calculator converts the nominal rate to an exact per-deposit rate and simulates every period, so the yearly table matches what a spreadsheet would produce to the cent.

It is also useful for comparing decisions rather than just predicting a number. Try the same deposit at 5% and 8% to see how much a fee-heavy fund costs you, or halve the years to see what starting late does. The copy button gives you a plain-text summary to paste into notes or a message, and nothing you enter leaves your browser.

Further reading

Frequently asked questions

How does this calculator handle monthly contributions with daily or yearly compounding?+

It first converts the nominal annual rate into an effective annual rate: APY = (1 + r/m)^m − 1, where m is the number of compounding periods per year. It then derives an equivalent rate for each contribution period, i = (1 + APY)^(1/n) − 1, with n = 12 for monthly, 52 for weekly or 1 for yearly deposits, and simulates the balance period by period. That keeps the yearly growth exactly right regardless of how the two frequencies line up.

What is the difference between the nominal rate and APY?+

The nominal rate (often labelled APR) is the headline figure before compounding is taken into account. APY, or effective annual rate, is what you actually earn over a year once interest starts earning interest. At 7% nominal, monthly compounding gives an APY of about 7.23%, and daily compounding gives 7.25%. The gap is small at low rates but widens as the rate rises.

Does it matter whether I contribute at the start or the end of each period?+

Yes, slightly. A deposit made at the start of the month earns interest for that month; one made at the end does not. Over ten years of $100 monthly deposits at 6%, the start-of-month option ends about $82 ahead. Most savings accounts credit interest on the balance you held during the month, so start-of-period is usually the closer match if you pay in on payday.

Is the total interest figure taxed or inflation-adjusted?+

No. The results are gross of tax and in nominal terms. If your account is taxable, or you want to see purchasing power rather than the raw balance, reduce the rate you enter accordingly: for example, a 7% return with 3% inflation is roughly a 4% real return.

Can I use it for a loan or credit card balance instead of savings?+

You can, because compound interest works the same way in reverse. Enter the balance as the initial deposit, set contributions to zero and use the card's APR with daily compounding to see how a balance grows if left unpaid. For a repayment plan with fixed monthly payments, the loan amortisation schedule linked below is the better fit.

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