Compound Interest Calculator

Watch money grow on money — with regular contributions, and with inflation honestly accounted for.

Reviewed Free, no sign-upFormulas shown and tested

$
$
%
Compounding
%
Value after 20 years
$300,850.72
≈ $166,573.75 in today's money
You invested: $130,000 (43.2%)Growth earned: $170,851 (56.8%)Final$300,851
  • You invested$130,00043.2%
  • Growth earned$170,85156.8%
0: $10,0001: $16,9192: $24,3393: $32,2944: $40,8255: $49,9736: $59,7827: $70,2998: $81,5789: $93,67110: $106,63911: $120,54412: $135,45513: $151,44314: $168,58715: $186,97116: $206,68317: $227,82018: $250,48619: $274,79020: $300,851
$0 → $300,851
Final balance$300,850.72
Buying power after 3% inflation$166,573.75
Compounding overtook you. Growth ($170,850.72) is now larger than everything you put in ($130,000.00). That crossover is the whole point of starting early.
$300,850.72 in 20 years will buy roughly what $166,573.75 buys today at 3% inflation. Real returns are what matter — this is the number most calculators leave out.

Your money vs. the money it earns

The blue block is what you put in; the green is growth on top. Watch the year the green overtakes the blue — after that, compounding is doing more work than you are.

1 — You invested: $16,0001 — Growth: $9192 — You invested: $22,0002 — Growth: $2,3393 — You invested: $28,0003 — Growth: $4,2944 — You invested: $34,0004 — Growth: $6,8255 — You invested: $40,0005 — Growth: $9,9736 — You invested: $46,0006 — Growth: $13,7827 — You invested: $52,0007 — Growth: $18,2998 — You invested: $58,0008 — Growth: $23,5789 — You invested: $64,0009 — Growth: $29,67110 — You invested: $70,00010 — Growth: $36,63911 — You invested: $76,00011 — Growth: $44,54412 — You invested: $82,00012 — Growth: $53,45513 — You invested: $88,00013 — Growth: $63,44314 — You invested: $94,00014 — Growth: $74,58715 — You invested: $100,00015 — Growth: $86,97116 — You invested: $106,00016 — Growth: $100,68317 — You invested: $112,00017 — Growth: $115,82018 — You invested: $118,00018 — Growth: $132,48619 — You invested: $124,00019 — Growth: $150,79020 — You invested: $130,00020 — Growth: $170,851
  • You invested
  • Growth
peak $300,851
Full year-by-year table
YearInvestedGrowthBalance
1$16,000.00$919.19$16,919.19
2$22,000.00$2,338.58$24,338.58
3$28,000.00$4,294.31$32,294.31
4$34,000.00$6,825.16$40,825.16
5$40,000.00$9,972.70$49,972.70
6$46,000.00$13,781.53$59,781.53
7$52,000.00$18,299.43$70,299.43
8$58,000.00$23,577.68$81,577.68
9$64,000.00$29,671.22$93,671.22
10$70,000.00$36,639.02$106,639.02
11$76,000.00$44,544.25$120,544.25
12$82,000.00$53,454.70$135,454.70
13$88,000.00$63,443.02$151,443.02
14$94,000.00$74,587.14$168,587.14
15$100,000.00$86,970.62$186,970.62
16$106,000.00$100,683.03$206,683.03
17$112,000.00$115,820.45$227,820.45
18$118,000.00$132,485.91$250,485.91
19$124,000.00$150,789.85$274,789.85
20$130,000.00$170,850.72$300,850.72

The idea that makes long-term investing work

Simple interest pays you on what you deposited. Compound interest pays you on what you deposited plus everything it has already earned. That small difference is why time matters more than almost anything else in investing.

Put $10,000 in at 7% and add $500 a month. After 20 years you'd have contributed $130,000 — and the balance would be roughly $300,000. More than half of it is money you never earned at a job. Leave it another ten years and the growth portion dwarfs the contributions entirely.

The formula, in the open

A = P(1 + r/n)nt  +  C × [ ((1 + r/n)nt − 1) ÷ (r/n) ]
A = final amount · P = starting amount · C = contribution each period · r = annual rate (decimal) · n = periods per year · t = years

The first term grows your starting balance. The second is the future value of a stream of contributions. This calculator computes both and shows you the split, so you can see the exact year growth overtakes your own deposits.

Three things that decide the outcome

  • Time — by far the biggest lever. Starting at 25 instead of 35 can roughly double the final balance for the same monthly amount, because the earliest dollars compound the longest.
  • Rate. The gap between 5% and 8% looks small annually and enormous after thirty years. It's also the number you control least — which is why fees matter: a 1% annual fee is a 1% cut of your compounding.
  • Consistency. Regular contributions through good and bad markets beat trying to time entries. The calculator assumes you keep going; real returns depend on you actually doing that.

The honest caveat: real markets don't return a smooth 7% every year — they lurch up and down and occasionally fall hard. This calculator shows a steady average, which is the right way to plan, but not what any individual decade will look like. Treat the output as a projection, not a prediction.

Why we show inflation

Most growth calculators end at the big number. That number is misleading on its own. At 3% inflation, prices roughly double every 24 years — so a million dollars three decades out has the buying power of about $400,000 today.

It's not a reason to give up; it's a reason to aim higher and to prefer returns that beat inflation. That's why the inflation-adjusted figure sits right under the headline here.

Want a specific target instead?

This tool answers "what will I have?". If you know the number you need and want to work backwards, the savings goal calculator solves for the monthly amount required. And if your money is going into a workplace retirement plan, the 401(k) calculator includes employer matching, which is the highest guaranteed return available to most people.

Frequently asked questions

What is compound interest?

Compound interest is interest earned on your interest. In year one you earn a return on your deposit; in year two you earn a return on the deposit plus the first year's gains, and so on. Over long periods the growth on past growth becomes larger than your original contributions.

What is the compound interest formula?

For a lump sum: A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate as a decimal, n is compounding periods per year and t is years. With regular contributions you add the future value of an annuity: C × [((1 + r/n)^(nt) − 1) ÷ (r/n)].

How often should interest compound?

More frequent compounding earns slightly more. At 12% a year, annual compounding gives 12.00% effective while monthly gives 12.68%. The difference is real but small — the rate itself and the time invested matter far more than the compounding frequency.

What return rate should I assume?

Be conservative and label it an assumption. Historically, a broad US stock index has averaged roughly 7% a year after inflation over very long periods, but individual years swing wildly and past performance guarantees nothing. Running the numbers at a few different rates is more useful than trusting a single one.

Why does the calculator show an inflation-adjusted value?

Because $1,000,000 in thirty years will not buy what $1,000,000 buys today. At 3% inflation it buys roughly what $412,000 buys now. Any projection that ignores inflation makes the future look better than it is, so we always show both numbers.

Educational tool, not financial advice. FinCalc performs mathematical calculations to help you plan and understand your options. It does not know your full financial situation and is not a substitute for advice from a licensed financial adviser, lender or tax professional. Rates, fees, taxes and terms vary by provider and location — always confirm the numbers with the institution before making a decision.
Written and maintained by Sastihari S — Software engineer & builder. Formulas are published on the page and covered by automated tests. Published , last reviewed . Questions or a correction? Get in touch.