Compound Interest Explained: $300 a Month for 40 Years vs $1,000 for 20, What One Year of Delay Costs, and a Calculator (Money School, Lesson 11)

Compound interest is interest earned on interest: each year's growth is added to the pile, and next year's growth is calculated on the bigger pile. The formula is simple, and the effect is not: $300 a month invested for 40 years at 7% a year grows to about $787,000, from only $144,000 of your own money. The same $300 a month started ten years later reaches about $366,000; waiting a decade cuts the result by more than half, though you only skipped $36,000 of saving. The one thing you control most is time, so start with whatever you have this month, and run your own numbers in the calculator below. Here is the honest twist that most compound interest articles leave out: whether "a little for a long time" beats "a lot for a short time" depends on the rate. At 7%, $300 a month for 40 years beats $1,000 a month for 20 years by more than a quarter of a million dollars. At 4%, it loses. Time is the multiplier, and the rate decides how big a multiplier it is. This is Lesson 11 of Money School, the first lesson of Stage 3.

Jake turned 38 the month he finished Stage 2, and Ethan sent him a present: a single spreadsheet cell. It showed what $300 a month would have become if Jake had started investing it on his 25th birthday. Jake looked at it for a long time, then texted back one word, "Seriously?", and then a longer message an hour later: "OK. What do I do now that I'm 38?" That second message is the reason this lesson exists. The answer to "I should have started earlier" is never "too late"; it is "start now, and here is what now is still worth."

If you are reading this in your thirties, forties or fifties with nothing invested yet, you are in the company of most people your age, and nothing on this page is meant to make you feel behind. I did not start until I was 31, after years of thinking investing was for people who already had money. The table below would have changed my mind at 22 if anyone had shown it to me. Nobody did, so I am showing it to you, at whatever age you are, because the best time to start is always the earliest one you still have.

What compound interest is, in plain English

Simple interest is paid only on the money you put in. Put $1,000 in an account paying 5% simple interest and you earn $50 every year, forever, the same $50. Compound interest is paid on the money you put in and on the interest already earned. In year one you earn $50; in year two you earn 5% of $1,050, which is $52.50; in year three, 5% of $1,102.50. The extra is small at first and that is exactly why people underestimate it: the growth is invisible for years and then, quite suddenly, it is most of the pile.

The formula, for those who like it: a single amount grows to P × (1 + r)ⁿ, where P is the starting amount, r is the yearly rate and n is the number of years. For a monthly contribution it is a little longer, and the calculator below does it for you. You do not need the formula to use compound interest; you need three facts it implies.

1. Time matters more than amount. Because growth builds on growth, the years at the end of the period produce far more than the years at the start. Adding years adds the most powerful part.
2. The rate matters enormously over long periods. A few percentage points of difference, compounded for decades, can double or halve the result. That is why fees, Lesson 12's subject, are worth fighting over.
3. It works in both directions. A card balance at 22% compounds against you exactly the way an investment compounds for you, only faster. Lesson 3 and Lesson 9 were the defense; this lesson is the offense.

Jake: "So the first ten years are boring and the last ten years are where it happens."

Ethan: "Exactly. Which is why you can't skip the boring part. It's the part that makes the exciting part possible."

The table you will remember: $300 for 40 years vs $1,000 for 20

Here is the comparison the course page promised, with the honest twist included. One person invests $300 a month for 40 years, $144,000 of their own money in total. Another invests $1,000 a month for 20 years, $240,000. Both earn the same steady rate, compounded monthly. Which one ends up with more?

Read the 7% row twice. The person who put in $96,000 less ends up with $267,000 more, because their money had twenty extra years to compound. That is the whole case for starting small and early rather than waiting until you can afford to save a lot.

Now read the 4% row. At low rates, compounding is gentle, and the person who simply saved more wins. That is not a footnote; it is the most useful sentence on this page. Money in a savings account at 3.8%, as in Lesson 6, is the right home for your emergency fund, and it is not where long-term growth comes from. Over decades, the extra growth comes from owning productive assets, which is what Lesson 12 is about. Time is the multiplier; the rate decides how big the multiplier is.

What one year of delay costs, and what a decade costs

The cost of waiting is easy to see once you run it. Take $300 a month, a 7% yearly rate, and a finish line at age 65:

• Start at 25: about $787,000 at 65.
• Start at 26: about $731,000. One year of waiting cost about $56,500, from $3,600 of skipped saving.
• Start at 35: about $366,000. Ten years of waiting cut the result by more than half.

Why does one skipped year cost so much more than the $3,600 you did not save? Because the year you lose is not the first year; it is effectively the last one, the year when the pile is largest and a year of growth adds the most. Every year you start earlier adds a year at the end of the curve, where compounding is strongest.

If you are 38 like Jake, this is not a reason for despair; it is a reason to start this month. At 7%, $300 a month from 38 to 65 still grows to about $287,000 from $97,200 of your own money, and anything you add from future raises, using the split from Lesson 4, raises it further. The math punishes waiting; it rewards starting at any age.

Jake: "So I can't get the 25-year-old version. But I can stop losing another fifty grand by waiting until 39."

Ethan: "That's the right way to read it. The most expensive year is the one you're about to skip."

The rule of 72: how fast money doubles, both ways

A shortcut that needs no calculator: divide 72 by the yearly rate, and you get roughly how many years it takes money to double. It is approximate and surprisingly accurate for everyday rates.

That last row is the uncomfortable mirror of everything else in this lesson. A $3,000 card balance left to compound at 22.36% for five years, with nothing paid, grows to about $9,100. It is the same math that turns $300 a month into $787,000, running in the other direction and three times faster. This is why the floor in Stage 2 comes before the reservoir in Stage 3: there is no investment that reliably beats a 22% debt.

Inflation: compound interest's quiet opposite

Inflation compounds too, and it works against money that sits still. Consumer prices were about 3.7% higher in September 2026 than a year earlier. At that pace, prices double in about 19 years by the rule of 72, which means $10,000 kept in cash would buy roughly half as much in two decades. Your money grows in real terms only when its rate beats inflation.

That gives you three honest categories for any money you hold:

• Losing ground: cash and accounts paying less than inflation. Fine for small amounts you need this week; costly for anything held for years.
• Holding ground: high-yield savings and short-term Treasury bills, which roughly keep pace in a year like this. The right home for an emergency fund, where safety and access matter more than growth.
• Gaining ground over long periods: a diversified mix of investments held for many years, which has historically grown faster than inflation with ups and downs along the way. That is the reservoir Stage 3 builds.

When you compare any investment, subtract inflation in your head. A 7% return in a year of 3.7% inflation is about 3.3% of real growth in buying power; the rule of 72 then says buying power doubles in about 22 years rather than 10. The table above uses plain rates because that is what statements show; the real version is a little slower, and still far ahead of cash.

Compounding frequency, APY and why it matters less than you think

Banks and investment pages talk about interest that compounds daily, monthly or yearly, and the APY you saw in Lesson 6 includes that effect. More frequent compounding does earn a little more: $10,000 at 5% compounded yearly is $10,500 after a year, and compounded daily it is about $10,513. The difference is real and small. The difference between a 4% and a 5% rate, or between 10 years and 20, is enormous by comparison.

So when you compare accounts, compare APY to APY, as Lesson 6 said, and then stop worrying about compounding frequency. When you compare investments, the frequency is meaningless; what matters is the long-run rate after fees, and the number of years you leave the money alone. Spend your attention where the effect is large: start date, contribution, rate and fees.

Fees: the compound interest you pay someone else

A fee charged as a percentage of your balance every year is compound interest in reverse, paid to someone else. Run $300 a month for 40 years at 7%, and you get about $787,000. Run it at 6%, which is what a 1% yearly fee does to a 7% return, and you get about $597,000. That one percentage point, a number that sounds tiny on a brochure, costs about $190,000 over a working life, a quarter of the result.

This is why Lesson 12 spends so long on what an index fund is and why its fees are so low, and why the course keeps saying that fees are the only part of investing you fully control. You cannot choose the market's return; you can choose not to hand a quarter of it away. Before you invest in anything, find its yearly cost, written as a percentage, and run it through the calculator below at your rate minus that percentage.

How to start compounding this month, with any amount

None of this is useful until money is actually compounding, so here is the order, using everything earlier in the course. If you have finished Stage 2, most of it is already in place.

1. Floor first. An emergency fund in a high-yield account (Lessons 2 and 6), no card balances at 22% (Lessons 3 and 9). Compounding against you at 22% wipes out compounding for you at 7%.
2. Take any employer match. If your employer matches retirement contributions, contribute at least enough to get all of it. Lesson 13 covers the accounts; the match alone is a 50% or 100% return on the matched dollars before any compounding begins.
3. Choose an amount you can keep paying. $50 a month that you never stop beats $500 a month that you stop in March. Consistency is what compounding feeds on.
4. Automate it on payday. The same habit from Lesson 1: money that moves before you see it is money you never miss. Lesson 16 turns this into a full payroll system.
5. Raise it with every raise. The split from Lesson 4 sends part of each raise to the reservoir automatically, so your contribution grows without a new decision each time.
6. Leave it alone. The hardest step. Compounding needs years, and the years include falls. Lesson 12 explains what "the market went down" means for someone with decades to go.

Compound interest calculator: your own numbers

Put in what you have now, what you can add each month, how many years until you need it, and a yearly rate. Try the same numbers at 4%, 6% and 8% to see the range, and try your rate minus a 1% fee to see what fees cost you.

The line that tells you the most is the third one: what starting a year later would cost. For most people it is larger than they expect, and it is the best argument for doing the exercise below this week rather than next month.

The United Kingdom and Australia: the same math, different accounts

Compound interest is arithmetic, and arithmetic does not change at the border. What changes is where the compounding happens most efficiently, because tax wrappers let it run without being trimmed each year. In the United Kingdom, the Individual Savings Account lets interest and investment growth compound free of tax, and workplace pensions add employer contributions on top of yours; Lesson 13 covers both. In Australia, superannuation is the compounding machine most workers already own, with compulsory employer contributions and decades to run; checking its fees, as in the fees section above, is one of the highest-value fifteen minutes an Australian worker can spend.

Wherever your paycheck lands, the rule is the same: start early, add regularly, keep fees low, and leave it alone for a long time.

This week's exercise: run your own table, do this with me

Fifteen minutes and the calculator above. I keep the results in the dated note from Lesson 1, and I rerun them every birthday.

1. Write your age and the age you would like to stop needing a paycheck. The difference is your number of years.
2. In the calculator, put in what you have invested now (zero is fine) and an amount you could add monthly without strain.
3. Run it at 4%, 6% and 8%. Write all three results down; the truth will be somewhere in that range, and it will not arrive in a straight line.
4. Read the "one year later" line. Write that number at the top of the page.
5. If you have no retirement contribution yet, find out this week whether your employer offers a match (the Stage 0 question from the course page) and how to start.
6. Set a reminder for your next birthday to rerun the table with whatever you have added.

If the numbers are smaller than you hoped, that is normal and honest. What matters is that they are now pointing in the right direction, with time on their side.

Honest aside: compound interest is not a promise

Every table on this page assumes a steady rate, year after year. Real investments do not behave like that. Some years they rise sharply, some years they fall, and the long-run average is only visible after the fact. A 7% average might arrive as +20%, -15%, +10%, +3%, and so on. The math of compounding still holds over long periods, but the path is bumpy, and the bumps are where people make the expensive mistake of selling at the bottom.

So treat the tables as a picture of how time works, not as a forecast of what you will have. The honest promise of compounding is narrower and still powerful: money left to grow for decades at a reasonable rate, with low fees and steady additions, ends up far larger than the same money left in cash. Lesson 12 is about how to get a reasonable rate with low fees without having to pick winners.

Where this fits in Money School, and what comes next

Lesson 11 opens Stage 3, "Build the reservoir." Stage 2 built the floor: a fund you can reach, a score that keeps borrowing cheap, insurance for the losses you cannot survive, a ranked debt list, and the habit that stops scams. With the floor in place, money can finally be left alone long enough to compound.

Lesson 12 explains what an index fund actually is: why owning the whole market beats picking, what a 0.03% fee versus a 1% fee does over 30 years, and what "the market went down" means for someone with decades to go. Then Lesson 13 covers the retirement accounts where compounding runs most efficiently: 401(k), IRA, pension and super. The course page shows every lesson and which ones are live.

Compound interest: your questions, answered straight

Jake ran his own table that night. At 38, with nothing invested yet, $300 a month to 65 showed about $287,000 at 7%, and sending half of his next two raises to it took the figure past $500,000. He set up the transfer for the first of the month, the day after the shop's payroll, and put the "one year later" number, about $22,800, on a note above his desk to stop himself pushing it to next year. Then he sent Ethan a spreadsheet cell of his own: the same calculation for Ethan's daughter, who had just turned 16. Ethan wrote the line below on the back of the birthday card.

Revision note. Written October 24, 2026, as Lesson 11 of Money School. The growth figures are our own arithmetic at the stated rates, compounded monthly; the inflation figure is the change in consumer prices from September 2025 to September 2026, and the card rate is the August 2026 average on balances paying interest. "Great oaks from little acorns grow," says the old proverb; it forgot to mention that the acorn needs forty years.