Compound Interest
Returns earning returns change the shape of long-term saving. A worked example shows why the final years do most of the work.
Compound Interest
Most people meet compound interest as a formula and never feel it. The formula is the least interesting part. What matters is that compounding is heavily back-loaded: the last five years of a thirty-year run add more money than the first fifteen, and almost nobody who gives up at year eight ever finds that out.
Compound interest is the eighth wonder of the world. He who understands it, earns it; he who doesn't, pays it.
One year, then the next
Put $10,000 somewhere that earns 7% a year. After twelve months it has paid $700. What happens to that $700 decides everything else. Spend it, and next year you earn $700 again on the same $10,000. Leave it in, and next year's 7% is paid on $10,700, which is $749. The extra $49 is interest on interest. It is small, and it never stops growing.
- 01Start with a balance$10,000
- 02It earns a return7% is $700 in year one
- 03The return stays investedThe balance is now $10,700
- 04Next year's return is bigger7% of $10,700 is $749
The loop repeats every year. Each turn starts from a larger balance, so each year's gain is larger than the one before.
Illustrative. 7% a year, no withdrawals, fees or taxes.
Simple interest is the version where the $700 is taken out every year. After thirty years you have your $10,000 plus $21,000 of interest, $31,000 in all. Compounded, the same $10,000 at the same 7% ends at about $76,000.
- Simple interest
- Compound interest
The two lines are almost indistinguishable for the first several years. That is the whole problem.
Illustrative. Assumes a constant 7% annual return with no fees or taxes.
Look at the shape rather than the endpoint. At year ten the gap is $2,672: real, but easy to shrug off. By year twenty it is $14,697. By year thirty it is $45,123. The rate never changed. The compounding just had more money to work on.
- Your $10,000
- Interest on the $10,000
- Interest on interest
The largest part of the final balance is money nobody deposited and the original $10,000 never earned directly.
Illustrative. $10,000 at 7% a year for 30 years. Simple interest on the deposit is $700 a year.
Why the early years matter most
This part changes behavior, so it is worth saying plainly. In a thirty-year run, money you put in during the last decade ends up worth far less than money you put in during the first, because it has less time to compound. Early contributions are worth more not because they are larger, but because they are older.
Here is what $100 a month, saved for one decade only, is worth at year thirty depending on which decade it was:
The same $12,000 of deposits. The first decade ends up worth four times the last.
Illustrative. $100 a month at 7% a year, compounded monthly, no fees or taxes.
Starting age is the one thing you cannot buy back
The most expensive mistake available to a young person is waiting. Not because they miss returns, but because they miss the cheapest compounding years.
Two people save $200 a month at 7%. The first starts at 25, stops at 35 and never adds another cent. The second starts at 35 and pays in every month for thirty years.
- Saves from 25 to 35, then stops
- Saves from 35 to 65
The early saver paid in $24,000. The late saver paid in $72,000, three times as much, and still finishes $36,974 behind.
Illustrative. $200 a month at 7% a year, compounded monthly, no fees or taxes.
The late saver did nothing wrong. Thirty years of steady saving is exactly the behavior this site recommends. The early saver simply had a ten-year head start, and a head start is the one input that cannot be made up with effort later.
The same force runs backwards
Compounding is not a wealth machine. It is a machine, and it works on whatever balance you carry.
Take a $5,000 credit card balance at 22% APR. If you pay only the minimum, here assumed to be the month's interest plus 1% of the balance, it takes about eighteen and a half years to clear and costs $7,726 in interest. Pay a fixed $250 a month instead and it is gone in 26 months for $1,286 of interest. Same card, same rate. The difference is how long the compounding is allowed to run.
- First payment
- $142
- $250
- Time to clear
- About 18.5 years
- Better: 26 months
- Interest paid
- $7,726
- Better: $1,286
- Total paid
- $12,726
- Better: $6,286
VerdictThe minimum payment shrinks as the balance shrinks, which is what keeps the debt alive for nearly two decades.
Illustrative. 22% APR compounded monthly, no new purchases. Minimum payment formulas vary by issuer.
This is why order matters. Paying off high-interest debt before investing is not a cautious preference. A guaranteed 22% from clearing a card beats a hoped-for 7% from a fund every time.
Fees compound too
An expense ratio looks trivial because it is quoted as a small number. It is not small, because it is charged every year against a balance that compounding is trying to grow.
Two funds, $10,000 each, the same 7% return before costs, held for thirty years. One charges 0.05% a year, the other 1.20%. The cheap one ends around $75,063. The expensive one ends around $54,271. That is a gap of about $20,800, paid for a difference of 1.15 percentage points that most people never look at.
- 0.05% fee
- 1.20% fee
The gap widens with time in the same way returns do, because a fee is compounding with a minus sign.
Illustrative. $10,000 at 7% before costs, fee subtracted from the return each year, no taxes.
Making it work when the numbers feel small
The honest obstacle is that $100 a month does not feel like it becomes anything. So here is the sequence the arithmetic actually rewards:
Clear the expensive debt first
Any balance above roughly 8% APR returns more from repayment than a diversified portfolio is likely to earn.
Automate a fixed amount
Move it on payday so the decision is made once, not every month. The amount matters less than the consistency.
Reinvest everything
Dividends and distributions have to buy more of the fund. If they land in your checking account, compounding stops.
Leave it alone
Every switch restarts the flat part of the curve and costs fees on the way in and out.
Check it once a year, not weekly
The curve only looks impressive across years. Watching it monthly is how people talk themselves out of it.
Compounding is not a strategy you adopt. It is a property of time that you either let work or interrupt. The people it works for are rarely the ones earning the most. They are the ones who started, automated something small, reinvested it and left it alone long enough for the curve to turn. You will not see it working for years, and that is not a sign it is failing. It is what the curve looks like from the inside.
56 more deep dives are in the Navigator library.
