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Compound Interest Calculator

See how savings grow when interest earns interest - with monthly contributions and a year-by-year breakdown of the balance.

Compound Interest Calculator

Balance after 30 years

$691,150

$190,000 contributed · $501,150 of growth

Total contributed

$190,000

Interest earned

$501,150

Growth multiple

3.64x

Balance ÷ contributed

Money doubles in

10.3 yrs

Rule of 72

With simple interest on the starting amount alone you would have $31,000. Compounding adds $660,150.

What made up the balance

  • Money you put in$190,000
  • Interest it earned$501,150

Growth over time

The gap between the two bands is compounding at work - it widens every year.

ContributedInterest

Year 30: $691,150

Balance over time split between contributions and compound interest
Balance over time split between contributions and compound interest
PeriodContributedInterestTotal
Year 0$10,000$0$10,000
Year 1$16,000$919$16,919
Year 2$22,000$2,339$24,339
Year 3$28,000$4,294$32,294
Year 4$34,000$6,825$40,825
Year 5$40,000$9,973$49,973
Year 6$46,000$13,782$59,782
Year 7$52,000$18,299$70,299
Year 8$58,000$23,578$81,578
Year 9$64,000$29,671$93,671
Year 10$70,000$36,639$106,639
Year 11$76,000$44,544$120,544
Year 12$82,000$53,455$135,455
Year 13$88,000$63,443$151,443
Year 14$94,000$74,587$168,587
Year 15$100,000$86,971$186,971
Year 16$106,000$100,683$206,683
Year 17$112,000$115,820$227,820
Year 18$118,000$132,486$250,486
Year 19$124,000$150,790$274,790
Year 20$130,000$170,851$300,851
Year 21$136,000$192,796$328,796
Year 22$142,000$216,760$358,760
Year 23$148,000$242,892$390,892
Year 24$154,000$271,345$425,345
Year 25$160,000$302,290$462,290
Year 26$166,000$335,905$501,905
Year 27$172,000$372,384$544,384
Year 28$178,000$411,934$589,934
Year 29$184,000$454,777$638,777
Year 30$190,000$501,150$691,150
Maximum value $691,150

Year by year

YearContributedInterestBalance
0$10,000$0$10,000
1$16,000$919$16,919
2$22,000$2,339$24,339
3$28,000$4,294$32,294
4$34,000$6,825$40,825
5$40,000$9,973$49,973
6$46,000$13,782$59,782
7$52,000$18,299$70,299
8$58,000$23,578$81,578
9$64,000$29,671$93,671
10$70,000$36,639$106,639
11$76,000$44,544$120,544
12$82,000$53,455$135,455
13$88,000$63,443$151,443
14$94,000$74,587$168,587
15$100,000$86,971$186,971
16$106,000$100,683$206,683
17$112,000$115,820$227,820
18$118,000$132,486$250,486
19$124,000$150,790$274,790
20$130,000$170,851$300,851
21$136,000$192,796$328,796
22$142,000$216,760$358,760
23$148,000$242,892$390,892
24$154,000$271,345$425,345
25$160,000$302,290$462,290
26$166,000$335,905$501,905
27$172,000$372,384$544,384
28$178,000$411,934$589,934
29$184,000$454,777$638,777
30$190,000$501,150$691,150

Interest overtakes contributions once the balance is large enough that a year of growth exceeds a year of saving - the point where compounding starts doing the heavy lifting. Here, growth has already passed contributions by 164%.

How the compound interest calculation works

Compound interest is what happens when the interest you earn starts earning interest of its own. Over a year or two the effect is barely visible. Over decades it dominates everything else about saving, which is why it is worth understanding precisely rather than approximately.

The formula for a lump sum is:

A = P × (1 + r/n)^(n×t)

P is what you start with, r is the annual rate as a decimal, n is how many times a year interest compounds, and t is the number of years.

Take $10,000 at 7% for 30 years, compounded annually. That is 10,000 × 1.07³⁰ = $76,123. You contributed $10,000; compounding produced the other $66,123. Simple interest over the same period would have paid just $21,000 in total — the difference is entirely interest earning interest.

Compounding frequency matters, though less than people expect. That same $10,000 at 7% for 30 years produces $76,123 compounded annually, $81,165 monthly, and $81,646 daily. Moving from annual to monthly gains you about 6.6%; moving from monthly to daily gains another 0.6%. Frequency is worth understanding, but rate and time matter far more.

Regular contributions are where compounding gets genuinely powerful. Add $500 a month to that $10,000 at 7% for 30 years and the balance reaches about $690,000, of which $190,000 is contributions and roughly $500,000 is growth.

Two rules worth carrying around:

  • The Rule of 72. Divide 72 by your annual return to find the doubling time. At 7%, money doubles roughly every 10.3 years — so a 30-year horizon gives you almost three doublings, and the last doubling adds more than the first two combined.
  • Time beats amount. Someone investing $200 a month from age 25 to 35 and then stopping ends up ahead of someone investing $200 a month from 35 to 65, despite contributing a third as much. The first ten years get the most compounding periods.

Use the calculator above to test contributions, rates, and compounding frequency, then read the yearly table to see when growth overtakes contributions.

Disclaimer: This calculator provides estimates for informational purposes only and is not financial advice. Your actual rate, taxes, fees, and payment will vary - confirm figures with a licensed lender or financial professional.

Frequently asked questions

What is the difference between simple and compound interest?

Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all interest already earned, so the balance grows faster over time. On $10,000 at 7% for 30 years, simple interest pays $21,000 while annual compounding pays over $66,000.

How much does compounding frequency actually matter?

Less than most people assume. $10,000 at 7% for 30 years grows to about $76,100 compounded annually, $81,200 monthly, and $81,600 daily. The jump from annual to monthly is meaningful; beyond monthly the gains are marginal. Your rate of return and time horizon matter far more.

What is the Rule of 72?

Divide 72 by your annual percentage return to estimate how many years it takes for money to double. At 7% that is about 10.3 years, at 9% about 8 years. It is a mental shortcut rather than an exact formula, but it is accurate enough for typical rates between 4% and 12%.

What return rate should I assume?

For long-term stock market investing, 6% to 8% after inflation is a common planning assumption based on historical averages. Savings accounts and CDs pay far less. Higher assumed returns make projections look better but do not make them more likely - it is safer to plan conservatively.

Does this account for inflation and taxes?

No. The result is a nominal figure before inflation and before any tax on interest, dividends, or capital gains. To think in today's purchasing power, subtract your inflation assumption from the return rate - a 7% return with 3% inflation is roughly 4% in real terms.

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