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Compound Interest Calculator

Estimate compound growth with an initial investment, regular contributions, an annual return assumption, and an investment period.

Your inputs

Everything runs in your browser. Nothing is sent anywhere.

$

Money you invest on day one. Minimum 0.

$

Amount added each contribution period. Minimum 0.

%

An assumption, not a prediction. 0–100%.

years

Whole years, 1–100.

Advanced options

How often you add money.

Contribution timing

Whether each deposit is invested at the start or the end of its period.

Year 20

Contributed $49,000 · Projected value $122,731 · Growth $73,731

Your projection

Estimated future value
$122,731
after 20 years at 8% assumed annual return
Total contributed
$49,000
$1,000 initial + $48,000 deposits
Estimated growth
$73,731
future value minus contributions

Growth over time

$0$30.7K$61.4K$92K$122.7KStartYear 10Year 20
Total contributed (hatched)Estimated growth (solid)

Estimate based on your assumptions. Real returns fluctuate; taxes, fees, and inflation are not included. Not financial advice.

Year-by-year breakdown
Year-by-year breakdown generated from the same calculation as the headline result and chart. Values rounded for display.
YearContributed to dateEst. growthProjected value
1$3,400$173$3,573
2$5,800$560$6,360
3$8,200$1,177$9,377
4$10,600$2,046$12,646
5$13,000$3,185$16,185
6$15,400$4,619$20,019
7$17,800$6,370$24,170
8$20,200$8,466$28,666
9$22,600$10,935$33,535
10$25,000$13,809$38,809
11$27,400$17,120$44,520
12$29,800$20,905$50,705
13$32,200$25,204$57,404
14$34,600$30,058$64,658
15$37,000$35,515$72,515
16$39,400$41,623$81,023
17$41,800$48,438$90,238
18$44,200$56,018$100,218
19$46,600$64,426$111,026
20$49,000$73,731$122,731

What-if scenarios

Explore how different choices could change the projection. These controls are independent — moving them does not change your main inputs above. Scenario figures are estimates from the same model, not guaranteed outcomes.

$200

20 years

8.0%

Scenario projected value: $122,731

Difference vs. your main calculation ($122,731): +$0

What is compound interest?

Compound interest is growth on top of growth. When your balance earns a return, that return is added to the balance — and the next period's growth is calculated on the new, larger balance. Repeat that hundreds of times and even modest contributions can snowball.

How compounding works

  1. You invest an initial amount, and optionally add contributions each period.
  2. Each period, the balance grows by the periodic rate derived from your assumed annual return.
  3. Growth stays invested, so the next period compounds a slightly bigger balance.
  4. Over time the growth slice overtakes the contributions slice — watch the two areas in the chart above cross.

The formula

With annual return r (as a decimal), compounding frequency n per year, and m contribution periods per year, the periodic rate is:

i = (1 + r/n)n/m − 1

Over N total contribution periods with contribution C per period and initial principal P:

  • End-of-period deposits: FVcontrib = C × ((1+i)N − 1) / i
  • Beginning-of-period deposits: multiply the above by (1+i)
  • Zero rate: FVcontrib = C × N
  • Initial principal: FVprincipal = P × (1+i)N

Variables: r = assumed effective annual return (decimal) · n = compounding periods per year · m = contribution periods per year · i = equivalent rate per contribution period · N = total contribution periods · C = contribution per period · P = initial principal. Full conventions are documented on the methodology page.

A worked example

Start with $5,000, add $300 every month, assume a 7% annual return compounded monthly for 30 years (end-of-period deposits):

  • Total contributed: $113,000 ($5,000 initial + $108,000 in deposits)
  • Projected value: $406,574
  • Estimated growth: $293,574
  • After just year 1: $9,079 from $8,600 contributed

Generated by the same calculation engine as the live calculator above — try entering these numbers yourself to reproduce it.

Contributions vs. growth

Your contributions are money you actually put in — they grow in a straight line. Growth is everything above that line: returns on your money, plus returns on previous returns. Early on, contributions dominate. Given enough time and return, growth usually takes over — which is why starting early matters more than starting big.

Assumptions and limits

  • The annual return is an assumption, not a prediction — real returns fluctuate, sometimes sharply.
  • Taxes, account fees, and inflation are excluded unless explicitly modeled.
  • Growth is modeled smoothly; real markets move in fits and starts.
  • Results are not guaranteed performance and not personalized financial advice.

Frequently asked questions

What is compound interest?

Compound interest means your returns start earning returns of their own. Instead of growth being calculated only on what you put in, it is calculated on your contributions plus all previous growth — so the balance can snowball over long periods.

How is this different from simple interest?

Simple interest is calculated only on the original amount. Compound growth is calculated on an ever-growing balance, which is why the projected curve bends upward over time while total contributions grow in a straight line.

Why does the annual return matter so much?

Because the return compounds every period, small differences in the assumed rate widen dramatically over decades. That is also why the rate is the most uncertain input — treat it as a what-if assumption, not a forecast, and try lower rates too.

What does contribution timing change?

Money deposited at the beginning of a period starts earning one period earlier than money deposited at the end. Over many periods this adds up to roughly one extra period of growth on the whole contribution stream.

Does this include taxes, fees, or inflation?

No. The model excludes taxes, fees, and inflation. Real investments also fluctuate year to year rather than growing smoothly. See the assumptions section and methodology page for the full picture.

Is this financial advice?

No. This calculator is for general education only. It cannot consider your personal situation. For decisions about real money, consider speaking with a qualified professional.