Compound Interest Calculator

How to use the compound interest calculator

The compound interest calculator shows how savings or an investment grow when interest is added to the balance and then earns interest itself. Enter a starting amount, a regular deposit if you make one, an interest rate and a number of years, and the result updates as you type.

1. Starting amount: what you put in today (it can be zero if you’re only making regular deposits).
2. Regular deposit: how much you add, and how often: every month, quarter or year. Tick the box if you deposit at the start of each period rather than the end.
3. Interest rate per year and years: rates from −50% to 100% and terms up to 100 years work.
4. Compounding: how often interest is added, from yearly to daily.
5. Read the results: the future value, how much of it you paid in and how much is interest, the effective yearly rate, how long the money takes to double, a chart of the balance and a year-by-year table.

The figures are before tax, fees and inflation, and assume a fixed rate. Real investments go up and down; this is a planning tool, not financial advice.

The compound interest formula

For a single amount, the future value after t years is:

FV = P × (1 + r ÷ n)^{n × t}

where P is the starting amount, r is the yearly rate as a decimal and n is the number of times interest is added each year (see compound interest).

Worked example: 10,000 at 7% a year, compounded yearly, for 20 years: 10,000 × 1.07²⁰ = 38,696.84. Of that, 28,696.84 is interest. With monthly compounding instead, the same deposit grows to 40,387.39, because interest is added (and starts earning) twelve times a year.

Adding regular deposits

Regular saving is where compounding becomes powerful. The calculator adds each deposit on its schedule and lets it earn interest from then on. When the deposit schedule and the compounding schedule differ (say, monthly deposits with yearly compounding), it converts the rate exactly to the deposit period, so the result is precise instead of approximate.

Worked example: 200 a month for 30 years at 6% a year, compounded monthly, deposits at the end of each month. You pay in 72,000 and end with about 200,903, so about 128,903 is interest. That’s the effect of time: in the last years, interest earned each year is larger than the deposits.

Depositing at the start of each period instead of the end gives each deposit one extra period of interest, so the result is a little higher.

How often interest is compounded

More frequent compounding means slightly more interest at the same headline rate. The calculator shows the effective yearly rate, which puts every option on the same footing. In banking this is usually called the annual percentage yield (APY) or annual equivalent rate (AER).

The difference between monthly and daily is small; the rate and the time you leave the money invested matter far more.

How long until your money doubles?

The calculator shows how many years it takes for a sum to double at your rate, using the exact formula: years = ln 2 ÷ ln(1 + effective rate). At 7% a year compounded yearly, that’s 10.24 years.

The quick mental version is the rule of 72: divide 72 by the rate. 72 ÷ 7 = 10.3 years, very close to the exact answer. At 3% it’s about 24 years, at 10% about 7. If the rate is zero or negative, the money never doubles, and the calculator says so.

Compound vs simple interest

With simple interest, interest is paid only on the original amount: 10,000 at 7% earns 700 every year, so after 20 years you’d have 24,000. Switch the calculator to Simple interest to compare the two directly, or see the simple interest calculator. With compound interest, interest is added to the balance and earns interest too, so the same deposit reaches 38,696.84. The gap grows every year, which is why time is the biggest factor in long-term saving.

Loans work the same way in reverse: interest on a loan balance is what makes long loans expensive. To see monthly payments and total interest on a loan or mortgage, use the loan calculator or the mortgage calculator.

Reading the chart and the table

The chart shows the balance at the end of each year, split into the money you paid in (dark) and the interest (blue), so you can see when interest starts to take over. The year-by-year table lists, for each year, the total paid in, the interest earned and the balance, which is handy for planning milestones or checking a statement from your bank. The US Securities and Exchange Commission’s Investor.gov also explains how regular contributions add up.

Why starting early matters

Because interest earns interest, the years at the end of a long term add the most. Compare two savers who each put in 300 a month at 5% a year, compounded monthly. One saves for 15 years: they pay in 54,000 and end with about 80,187. Keeping the same habit for 30 years gives about 249,678, roughly three times as much from twice the time, because the later years build on a much larger balance. Try it in the calculator by changing only the number of years; the chart shows the blue interest part taking over.

Raising the deposit helps too, but in a straight line: twice the deposit gives twice the result. Extra years work like a curve, which is why small, early, regular savings often beat larger savings started later.

Currency and rounding

Amounts are shown in the site’s currency and rounded to cents for display; the calculation itself keeps full precision until the end, so rounding doesn’t build up over long terms. Rates can include decimals, such as 4.35%.

Ask in plain words

• “What will 10,000 grow to at 7% for 20 years?”
• “If I save 300 a month at 5%, how much will I have in 15 years?”
• “How long does it take to double my money at 4%?”

Type it under the calculator or in the AI workspace. To compare rates as percentages, the percentage calculator helps.

Privacy

Amounts you enter stay in your browser; nothing is uploaded, stored or used to train AI. More in Trust and privacy.

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