📈 Compound Interest Calculator

Calculate how your money grows over time with compound interest, optional regular contributions, multiple compounding frequencies, and a full year-by-year breakdown.

Free No Account Year-by-Year
Calculator Inputs

The initial amount you are investing.

% / year

⚠ Rate looks very high — double check it's a percentage, not a decimal.

How often interest is added to your balance.

Amount added at each contribution interval.

Final Balance

after — compounded

Total Interest
Total Invested
Eff. Annual Yield
APY / EAR

Balance Breakdown

Principal ()
Contributions ()
Interest ()
⏱

Doubling time

years (exact)

💰

Rule of 72

years (approx.)

Growth Over Time

Balance Principal

Year-by-Year Breakdown
Yr Opening Balance Contributions Interest Earned Closing Balance
Export:

💰

Final Balance

See how much your investment grows over time

📈

Compound Growth

Interest earns interest — the snowball effect

🔢

APY / EAR

Effective annual yield accounting for compounding

📅

Year-by-Year

Full annual breakdown of growth

➕

Contributions

Add regular top-ups monthly, quarterly, or annually

⏱

Doubling Time

How long until your money doubles

💡 Compound Interest Formula

A = P × (1 + r/n)^(n×t)

P = principal · r = annual rate · n = compounds/year · t = years

The compound interest formula, with contributions added

The lump sum uses the standard compound interest formula A = P x (1 + r / n) raised to the power of n x t, where P is the principal, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years. Choosing monthly compounding sets n to 12, quarterly to 4, daily to 365 and weekly to 52. Selecting continuous compounding switches to the limiting form A = P x e raised to the power of r x t.

Regular contributions are handled by the future value of an annuity. Each deposit compounds for the time remaining until the end of the term, and the series sums to PMT x (((1 + i) to the power of N) minus 1) divided by i, where i is the periodic rate and N the number of payments. Setting contribution timing to the beginning of the period multiplies that total by a further (1 + i), because every payment earns one extra period of interest. Total interest is simply the final balance minus the principal and all contributions.

Worked example: 10,000 at 8 percent for ten years

Take a principal of 10,000, an annual rate of 8 percent, a term of ten years, monthly compounding and a contribution of 200 per month paid at the end of each month.

The periodic rate is 0.08 / 12, or 0.0066667, and there are 120 periods. The lump sum grows to 10,000 x 1.0066667 to the power of 120, which is 10,000 x 2.21963, or 22,196. The monthly deposits contribute 200 x ((2.21963 minus 1) / 0.0066667), which is 200 x 182.945, or 36,589.

Adding the two gives a closing balance of about 58,785. Of that, 10,000 was the opening principal and 24,000 came from 120 monthly deposits, a total of 34,000 paid in. The remaining 24,785 is compound interest. Notice that interest exceeds the contributions themselves over this term. Switching contribution timing to the beginning of each month lifts the annuity portion by a factor of 1.0066667, adding roughly 244 to the final figure.

Interpreting the projection and its limits

Treat the output as a smooth illustration of one scenario, not a forecast. It assumes the rate you entered holds unchanged for the entire term and that every contribution is made on schedule. Real savings rates move, market returns vary year to year, and a single average figure hides the sequence of returns that matters greatly near the end of a long term.

The result is also a nominal figure. It ignores inflation, so 58,785 in ten years buys less than 58,785 today; entering a real rate, meaning your expected return minus expected inflation, gives a rough purchasing-power view instead. Tax on interest or dividends, platform and fund charges, and any account fees are not deducted, and each of those reduces the effective compounding rate.

Compare accounts using the same compounding frequency, or use the advertised annual equivalent rate, because 8 percent compounded daily is not the same as 8 percent compounded annually. This tool provides general information only and is not financial advice.

Frequently Asked Questions

Simple interest is calculated only on the original principal, so it grows in a straight line. Compound interest is calculated on the principal plus all interest already credited, so growth accelerates. On 10,000 at 8 percent for ten years, simple interest pays 8,000 while annual compounding pays about 11,589.
Less than most people expect. On 10,000 at 8 percent for ten years with no contributions, annual compounding reaches about 21,589, monthly about 22,196 and daily about 22,253. Moving from annual to monthly gains roughly three percent of the balance; the step from monthly to continuous adds very little more.
Choose whichever matches your actual payment date. Beginning-of-period means each deposit earns interest for one extra compounding period, so it produces a slightly higher balance. A standing order leaving your account on the first of the month is beginning-of-period; money swept in after payday at month end is end-of-period.
No. It reports a nominal balance before tax, charges and inflation. To approximate purchasing power, enter a real rate instead, meaning your expected return minus expected inflation. For after-tax growth, reduce the rate by your marginal tax rate on savings income or model the account as tax-sheltered if it is.
Continuous compounding is the mathematical limit as compounding periods become infinitely frequent, calculated as P times e to the power of rate times years. No retail savings account works this way; it appears in finance theory and options pricing. In practice it produces only a fraction of a percent more than daily compounding.