Compound Interest: The Year Growth Takes Over
Compound interest is interest earned on interest already earned, so the balance grows faster the longer it runs. The effect is back-loaded: on a monthly deposit at a realistic rate, growth does not exceed total contributions until nearly two decades in.
How it works
Simple interest pays on what you deposited. Compound interest pays on the interest too. That one difference is the whole subject, and it is small at first and enormous later.
The mechanism is multiplication rather than addition. Each period’s balance becomes the base for the next period, so the amount earned grows even when nothing new is added.
Which makes it back-loaded, and the shape of that is the part almost nobody sees before they experience it.
What the curve actually looks like
Take $500 a month for 40 years at 7% a year, compounded monthly. The balance reaches $1,312,407. Total deposited: $240,000. Everything else — $1,072,407 — is growth.
Those two lines on the chart are the whole story. The straight one is what you paid in. The curved one is the balance. For a long time they are almost the same line.
Growth first exceeds total contributions in month 217 — year 18.1. For eighteen years, more than half the balance is money you deposited. The compounding everyone talks about is doing less work than your paycheck is.
| Annual rate | Growth overtakes contributions | Balance after 40 years |
|---|---|---|
| 5% | year 25.3 | $763,010 |
| 7% | year 18.1 | $1,312,407 |
| 10% | year 12.8 | $3,162,040 |
Read the table by column. The same $240,000 of deposits produces $763,010 or $3,162,040 depending only on the rate. And the rate’s main effect is on the date the curve turns — it pulls the crossover from year 25 to year 13.
In practice: the part that decides outcomes
Year eight looks like a savings account. The balance is roughly what you put in, the growth is a modest fraction, and nothing about it feels like the charts in the articles.
That is the point at which most people conclude it is not working. It is working exactly as the arithmetic requires; the visible part simply arrives later.
Time is doing more than the rate is. The last ten years of the 7% run add more to the balance than the first twenty-five combined, because they compound on the largest base the account has ever had.
Which is why an interruption costs more than it looks. Missing a year of deposits early does not remove that year’s $6,000 — it removes what that $6,000 would have become by year 40.
A fee is a rate reduction and compounds identically. A 1% annual charge on a 7% return is a 6% return, and the table above shows what a point of rate is worth across four decades.
And the same mechanism runs against you. At 3% a year, a dollar buys 41 cents after 30 years. The $1,312,407 is a nominal figure, and inflation is the page on what it is really worth.
What compounding is not
It is not fast. Every chart that makes it look fast is drawn on a 40-year axis, where the first two decades are compressed into the left edge.
It is not a property of any particular investment. Compounding is arithmetic; the rate is the uncertain part, and the tables here assume a rate rather than promising one.
It is not a reason to accept any rate on offer. The gap between 5% and 7% across 40 years is larger than most people’s total contributions, which makes costs and fees a first-order decision.
And it does not repair an interruption. Restarting after a gap resumes the curve at a lower base, and the missing compounding is not recoverable later.
When it fails
Withdrawals break it completely. The curve depends on the base never shrinking, and money taken out removes both the amount and everything it would have earned.
The second failure is expecting the shape too early. An account behaving exactly correctly looks disappointing for its first decade, and the response to that disappointment is usually to reach for a higher rate at a much higher risk.
A third is treating the assumed rate as a fact. 7% is a modelling input on this page, chosen because it is a common assumption, and the real sequence of returns is neither smooth nor known in advance.
And a fourth is compounding a nominal number and spending a real one. The balance grows in dollars and is spent in groceries, and only one of those is on the chart.
There is a fifth that catches people who understand all of the above. Compounding at a high rate requires surviving every year at that rate, and a single large loss resets the base the way a withdrawal does. A 50% fall needs a 100% gain to return to where it started, so an approach that averages a high return with occasional deep losses compounds far worse than its average suggests.
The practical version of everything on this page is unglamorous. Start earlier than feels necessary, automate the deposit so it does not require a decision each month, keep the fee as close to zero as the options allow, and do not touch it. Those four things account for almost all of the difference between the tables above and what most accounts actually do.
The original data
5 of the 24,971 videos measured for this site cover compound interest, at a median of 35,773 views — a small supply on a topic with broad interest, and none of it publishes the crossover.
The crossover figures were computed for this page. $500 monthly, compounded monthly, at 5%, 7% and 10%, over 40 years. Change the deposit and the crossover year does not move at all — it depends only on the rate, which is why it is the figure worth carrying rather than the balance.
Related
Inflation and savings is this arithmetic pointed the other way. Retirement accounts is the wrapper that decides how much of the growth you keep. And financial independence is the target this curve is usually aimed at.
I understood compounding as an idea for years before I plotted the crossover, and seeing that it sits at year eighteen changed what I expected from my own accounts. Nothing was wrong with them. They were just early.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.