Invest

Compound Interest Calculator

Watch a starting balance and a monthly habit turn into a number with a comma you didn't expect. Growth stacked on contributions, year by year.

Try it A steady rate, not a market

Starting amount $

Monthly contribution $

Annual return

7%

Years

30 yr

Final balance$724,501 You contributed$194,109 Growth$530,392

Growth is 2.7× what you put in.

Monthly compounding at a steady rate — real markets don't move in straight lines. No taxes, fees, or inflation modeled. Arithmetic, not advice.

How it works

Compound interest is growth on growth. In year one, only the money you put in earns anything. In year two, last year's earnings start earning too — and every year after that, the pile working for you is bigger than the pile you built yourself. That's why the curve above doesn't rise in a straight line: it bends, and the bend is the whole story.

The defaults tell it plainly. Start with $14,109, add $500 a month at 7% a year, and after 30 years you're holding about $724,501 — of which only $194,109 ever came out of your pocket. Growth crosses over your contributions around year 16, and the final decade alone adds about $407,000, more than the entire balance at year 20. The lazy-looking early years aren't wasted; they're the fuse.

Two honest footnotes. Monthly compounding beats annual slightly, and this page contributes at month-end like most paychecks do. And the same exponent works in reverse for costs — see what a 1.86% fund fee really costs for the unhappy mirror image. In ClariFi, this math runs on your real balances: every savings goal carries a forecast that says whether you're on track, in dollars per month, not vibes.

The formula

r  = rate / 12                        monthly rate
g  = (1 + r)^(12 × years)             growth factor
FV = P₀ × g + C × (g − 1) / r         final balance
contributed = P₀ + C × 12 × years
growth      = FV − contributed         ← the part time built
(r = 0 is just FV = P₀ + C × 12 × years — no growth, no bend)
Example: $14,109 + $500/mo at 7% for 30 years
         → $724,501 final · $194,109 contributed · $530,392 growth

Honest assumptions

One steady rate, compounded monthly, forever. Real markets lurch — the average matters, but so does the order it arrives in.
Contributions land at month-end and never change for the whole run. Real habits pause and grow.
No taxes, no fund fees, no inflation. Each one takes a real bite; the fee bite has its own calculator.
The result is in future dollars. To think in today's dollars, run a lower rate (return minus expected inflation).
This page does arithmetic. It doesn't know your life — it's a starting point, not a plan, and not advice.

Questions people ask

Does daily compounding beat monthly?

Barely. At this page's defaults, the gap between daily and monthly compounding is a fraction of a percent of the final balance — noise next to the rate and the years. Savings rates quoted as APY have already folded the compounding frequency in. Monthly is the honest middle ground for investment math, since most people contribute monthly anyway.

What return should I assume?

Nobody knows the next 30 years, which is why this is a slider and not a promise. The 7% default is in the range often cited for long-run broad stock-index returns before inflation; bonds and savings accounts sit lower. The useful move is to run the same plan at 5% and at 9% and treat the two answers as a range, not a prediction.

Does this include inflation?

No — the final balance is in future dollars, which will buy less than the same number today. The standard workaround: subtract expected inflation from your return and re-run. At 7% growth with roughly 3% inflation, slide the rate to 4% and read the result as today's purchasing power.

Why is the growth bigger than my contributions?

Because growth compounds and contributions don't — each deposit is a one-time event, but every dollar of growth keeps earning for the rest of the run. At the defaults you put in $194,109 and growth adds $530,392, about 2.7× as much, with the crossover around year 16. That's not magic; it's the exponent doing what exponents do.

Related calculators

ClariFi makes tools, not advice. Nothing on this page is a recommendation to buy, sell, or sign anything.

In the app

ClariFi runs this math on your real accounts.

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iPhone only for now, iOS 17+. Tools, not advice.