By Tim King · Updated 16 September 2026

Compound Interest Calculator

Savings Growth With Weekly, Monthly or Yearly Contributions and Effective Rate

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Starting amount

Contribution per month (optional)

Contribution frequency

Interest rate (% per year)

Compounding frequency

Number of years

Result

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How the Compound Interest Calculator works

This compound interest calculator shows the balance year by year. Put in a starting balance, what you add and how often, the rate and the number of years, and the final balance appears as you type — alongside how much of it you put in yourself, how much is interest, and the effective annual rate the compounding frequency actually produces.

Compound interest calculator: how the balance grows

Compound interest is interest earning interest. In year one a balance earns a return; in year two the return is calculated on the original balance plus last year's return, and so on. That is the whole mechanism, and it is why the growth curve bends upward instead of running in a straight line. Over five years the bend is barely visible. Over thirty it dominates everything else on the page.

Compound interest with monthly contributions: a worked example

Start with $10,000, add $200 a month, earn 6% compounded monthly, and leave it for ten years. The starting balance alone grows to $18,194. The 120 contributions total $24,000 and grow to $32,776, because the early ones compound for nearly the full decade. The final balance is $50,970, of which $34,000 is money you put in and $16,970 is interest. The formula behind it is A = P(1 + r/n)^(nt) + c × ((1 + r/n)^(nt) − 1) ÷ (r/n), where P is the starting amount, c the contribution per period, r the annual rate, n the periods per year and t the years.

Effective annual interest rate calculator: the nominal rate is not the rate you get

This is the point most calculators quietly skip. A rate quoted as "6% per year, compounded monthly" does not return 6% over the year. Each month you earn 0.5%, and the following month earns 0.5% on a slightly larger balance, so the year finishes at 6.17%. That figure — the effective annual rate — is what you actually receive, and it is the only honest way to compare two products quoted on different compounding cycles. This calculator shows it explicitly, because a 6.0% account compounding daily beats a 6.1% account compounding annually, and no amount of staring at the headline numbers will tell you that.

Compounding frequency matters far less than people think

Having established that daily beats annual, here is the deflating part: across the whole realistic range, the difference is small. On a $10,000 balance at 6% over ten years, annual compounding returns $17,908 and daily compounding returns $18,220 — about $312, or 1.7%, for the most extreme change in frequency available. Adding just $50 a month to that same balance adds roughly $8,200. Contribution size and time in the market are the levers that move the outcome; compounding frequency is a rounding detail worth checking and then forgetting. If you are choosing between accounts, compare effective rates and fees, then stop thinking about it.

Where contributions land

Contributions here are added at the end of each period, which is the conservative convention and matches how a salary-day transfer behaves in practice. Paying at the start of each period instead gives every contribution one extra period of growth, worth roughly one period's interest across the whole run — a few hundred dollars on a decade-long plan, not a different answer.

How much will my savings grow? A reference table

The table shows what a $10,000 lump sum becomes with monthly compounding and no further deposits, and what $200 a month becomes from nothing. Scale the rows: $50,000 grows to five times the lump sum figure, and $400 a month to twice the contribution figure.

Rate	$10,000 after 10 yrs	$10,000 after 20 yrs	$10,000 after 30 yrs	$200/month for 20 yrs	$200/month for 30 yrs
1%	$11,051	$12,213	$13,497	$53,112	$83,926
1.5%	$11,617	$13,496	$15,679	$55,937	$90,859
2%	$12,212	$14,913	$18,212	$58,959	$98,545
2.5%	$12,837	$16,479	$21,153	$62,195	$107,074
3%	$13,494	$18,208	$24,568	$65,660	$116,547
3.5%	$14,183	$20,117	$28,533	$69,374	$127,083
4%	$14,908	$22,226	$33,135	$73,355	$138,810
4.5%	$15,670	$24,555	$38,477	$77,625	$151,877
5%	$16,470	$27,126	$44,677	$82,207	$166,452
5.5%	$17,311	$29,966	$51,874	$87,125	$182,722
6%	$18,194	$33,102	$60,226	$92,408	$200,903
6.5%	$19,122	$36,564	$69,918	$98,084	$221,236
7%	$20,097	$40,387	$81,165	$104,185	$243,994
7.5%	$21,121	$44,608	$94,215	$110,746	$269,489
8%	$22,196	$49,268	$109,357	$117,804	$298,072
8.5%	$23,326	$54,412	$126,925	$125,400	$330,141
9%	$24,514	$60,092	$147,306	$133,577	$366,149
9.5%	$25,761	$66,361	$170,949	$142,385	$406,607
10%	$27,070	$73,281	$198,374	$151,874	$452,098
10.5%	$28,446	$80,919	$230,185	$162,101	$503,280
11%	$29,891	$89,350	$267,081	$173,128	$560,904
11.5%	$31,409	$98,656	$309,872	$185,020	$625,819
12%	$33,004	$108,926	$359,496	$197,851	$698,993

The 30-year column at 7% is more than double the 20-year column: the last ten years of a thirty-year run produce more growth than the first twenty combined, which is the whole argument for starting early.

Investment growth calculator: what is not modelled

No tax, no inflation, no fees, and no variation in the rate. Real returns are taxed, real accounts charge, and a rate held flat for thirty years is a modelling assumption rather than a forecast. Inflation is the big one: at 2.5% a year, money loses roughly half its purchasing power over thirty years, so a balance that looks life-changing in nominal terms is considerably less so in today's money. Treat the output as a comparison between scenarios, not a prediction of your future bank balance.

Compound interest worked examples

First example. $5,000 at 6% compounded monthly for 15 years: 5,000 × (1 + 0.06 ÷ 12)^180 = $12,270.47.

Second example. $300 a month at 8% for 25 years, with nothing to start: 300 × ((1 + 0.08 ÷ 12)^300 − 1) ÷ (0.08 ÷ 12) = $285,307.92, of which $90,000 is your own money.

How many years to double? Divide 72 by the rate: at 6% about 12 years, at 8% about 9.

FAQ

What is the difference between the nominal rate and the effective rate?

The nominal rate is the headline figure before compounding is applied, while the effective annual rate is what you genuinely earn once interest starts earning interest within the year. At 6% compounded monthly the effective rate is 6.17%, and that is the number to compare between products.

Does daily compounding actually beat monthly compounding?

Slightly, but far less than the marketing suggests. On $10,000 at 6% over ten years, moving from annual to daily compounding is worth around $312 in total, while adding $50 a month to the same account is worth thousands. Compare effective rates, then focus on contributions.

Are contributions added at the start or the end of each period?

At the end of each period, which is the conservative assumption and matches a transfer made on pay day. Contributing at the start of each period gives every payment one extra period of growth, which lifts the final balance by roughly one period's interest over the whole term.

Does this account for tax, fees or inflation?

No. The result is a nominal, pre-tax, pre-fee figure at a rate held constant for the whole term. Inflation matters most over long runs — at 2.5% a year, purchasing power roughly halves across thirty years — so discount long-term results before treating them as real spending money.

Why does the same total contribution produce so much more when it starts earlier?

Because every dollar compounds for the time it is invested, and that time is what the exponent in the formula acts on. Ten years of contributions left alone for twenty more years typically beats twenty years of the same contributions started ten years later, despite the smaller total in.

What is the formula for compound interest?

For a lump sum, A = P(1 + r/n)^(nt): principal times one plus the rate per period, raised to the number of periods. At 6% compounded monthly for ten years that is 10,000 × (1.005)^120 = $18,194. Regular contributions add c × ((1 + r/n)^(nt) − 1) ÷ (r/n), the future value of an ordinary annuity, which is what the calculator uses when the contribution field is filled in.

How long does it take to double my money?

Divide 72 by the annual rate for a quick estimate: at 6% money doubles in about 12 years, at 9% in eight. The rule of 72 is accurate to within a few months for rates between 4% and 12%; the exact figure is ln 2 ÷ ln(1 + r), which at 6% is 11.9 years. Enter the rate and adjust the years field until the interest earned matches the starting amount to check.