Buy a new backpack every August from 9th grade through senior year of college and you will spend about $600. Skip the yearly replacement and invest that money instead and you would have roughly $7,400 by the time you are 50. Same eight backpacks. Very different ending.
One purchase per school year, invested instead and left alone until you turn 50.
Starting at age 14. Growth is compounded yearly and is an estimate, not a promise.
This started as a dumb argument about a backpack. Somebody said a $75 backpack was not a big deal, which is true, and somebody else said it is a big deal eight times in a row, which is also true. So we did the math properly instead of guessing, and the answer was a lot larger than either of us expected.
Here are the assumptions, all of them stated up front so you can disagree with any of them. You buy one backpack every school year for four years of high school and four years of college, starting the August you turn 14. Each one costs $75, which is a normal price for the kind of brand-name pack that most people actually carry, not the cheapest one on the shelf and not a designer one. Instead of buying them, you put that $75 into a broad stock market index fund each year and never touch it again until you are 50. The money grows at an average of 8% a year, which is roughly what the US stock market has averaged over long stretches before inflation.
The reason the number is so much bigger than $600 has nothing to do with backpacks and everything to do with how long the money gets to sit. The $75 you would have spent at 14 has 36 years to grow, and 36 years at 8% multiplies money by about sixteen. That single first backpack is worth $1,198 on its own. Every year you wait, the multiplier shrinks, which is why the same $75 spent at 21 only becomes $699. Time is doing almost all of the work here, and time is the one resource you currently have more of than every adult you know.
Every row is one backpack you did not buy, and what that $75 is worth on your 50th birthday at 8% a year.
| School year | Your age | Grows for | How big it gets | Worth at 50 |
|---|---|---|---|---|
| 🎒 9th grade | 14 | 36 yrs | $1,198 | |
| 🎒 10th grade | 15 | 35 yrs | $1,109 | |
| 🎒 11th grade | 16 | 34 yrs | $1,027 | |
| 🎒 12th grade | 17 | 33 yrs | $951 | |
| 🎒 College year 1 | 18 | 32 yrs | $880 | |
| 🎒 College year 2 | 19 | 31 yrs | $815 | |
| 🎒 College year 3 | 20 | 30 yrs | $755 | |
| 🎒 College year 4 | 21 | 29 yrs | $699 | |
| $600 spent on backpacks becomes | $7,433 | |||
Read that top row one more time. A single $75 backpack bought in 9th grade is a $1,198 decision. Not because backpacks are expensive, but because you are 14, and being 14 is worth about sixteen times its face value if you point it at the right thing. Nobody is going to tell you that at the store.
And notice the shape of the column. The numbers shrink every single year, and they keep shrinking for the rest of your life. This is the actual reason adults keep telling teenagers to start early and then fail to explain why. It is not discipline. It is that the exponent on the front row is the biggest one you will ever have.
Eight backpacks, all of them long gone, most of them replaced while the old one still worked fine. You spent $600 and own nothing.
One backpack that made it through, and eight small deposits you never noticed making. At 50 it is a used car, an emergency fund, or a chunk of a house deposit.
"Same eight years. Same eight backpacks. One version ends with a landfill and one ends with $7,433."
and you pick which one at 14To be completely fair to Option A: it is not really $0, because you did get eight backpacks and you did need something to carry books in. That is the honest objection to this whole article and it deserves a real answer, which is the next section. But hold on to the size of the gap, because it survives the objection almost entirely intact.
You do. So here is the version of this that survives contact with reality.
Nobody is suggesting you carry your textbooks in your arms for eight years to make a spreadsheet look good. The real comparison was never eight backpacks against zero backpacks. It is eight backpacks against one, because a well made pack genuinely lasts that long and most people replace theirs for reasons that have nothing to do with it being broken.
So run it that way. Buy one solid $75 backpack in 9th grade, one with a real warranty, and keep it. You skip the other seven purchases, which is $525 you never spend. Invested the same way, that comes to about $6,235 by 50. You lose roughly $1,200 off the headline number and you get to have a backpack the entire time, which seems like a reasonable trade.
And if you want the full $7,433 anyway, it is genuinely available: a hand-me-down from an older sibling, a $12 thrift store find, or the pack that is already sitting in your closet from last year working perfectly well. The math does not care where the bag came from. It only cares that the $75 went somewhere that grows instead of somewhere that ends up in a landfill in 2032.
The backpack is just the clearest example of a habit that shows up everywhere: replacing something on a schedule instead of when it stops working. Same math, different object.
Keeping a phone one extra year, once, at 16
The phone in your pocket does the same things in year three that it did in year one. The upgrade is a habit somebody sold you, not a need.
$15 a month through all four years of high school
Small, automatic, and invisible, which is exactly what makes it the most expensive category on this page. Go read your subscriptions right now.
One $250 pair skipped at age 16
Not saying never buy nice shoes. Saying know that the sticker price is not the price, and decide anyway.
$5 a week from 14 through 22
Everyone jokes about this one because it feels preachy. The number is the number, and it is the biggest on this page by a wide margin.
Please do not read this as "never buy anything." That is a miserable way to be a teenager and it is not how any of this works. The point is much narrower and much more useful: every purchase has a second price tag that nobody prints, and for someone your age that second tag is roughly ten to sixteen times the first. Knowing that does not mean you always say no. It means you say no to the things you did not really want, which turns out to be most of them, and yes to the things you did.
If you want the framework for telling those two apart, that is exactly what needs vs. wants is for. And the mechanism that makes all these numbers work is explained properly in how compound interest works.
Every article like this one leaves these out. Here they are, because a number you understand is worth more than a number that impresses you.
The US stock market has averaged roughly that over long periods, but it does not deliver 8% a year. It delivers 22% one year, negative 18% the next, and a long flat stretch in the middle. Over 36 years the average tends to show up. Over any given three years it absolutely does not, and past performance genuinely does not guarantee future results. Drag the return slider down to 6% and you get about $4,022, which is a lot less exciting and still enormous next to $600.
Prices rise over time, so a future dollar is worth less than a current one. Adjusting for inflation, that pile might have the buying power of something closer to $3,000 in today's money. That is the honest version. It is still five times what you put in, and the comparison against buying eight backpacks, which leaves you with exactly nothing, does not change at all.
Growth in a normal brokerage account gets taxed when you sell. Growth inside a Roth IRA does not, which is why that account is such a big deal for teenagers specifically. If you have earned income from a job, that is where this money should go. See Roth IRA for teens.
Under 18 you need a parent or guardian to open a custodial brokerage account or a custodial Roth IRA with you. This is a ten minute task that people put off for years. It is also the only actual obstacle in this entire article, which should tell you something.
The math is the easy part. This is the part that decides whether any of it happens.
The whole plan, four boxes
screenshot this part ✂️
Backpack, phone, headphones, cleats, a subscription. One item, chosen on purpose, that still works and that you were going to swap out anyway. One is enough. Choosing five means keeping zero, which is how every version of this fails.
A custodial Roth IRA if you have a job and earned income, since the growth comes out tax free later. A plain custodial brokerage account if you do not. Most major brokerages let you do this online for free with no minimum. Budget one evening, not one weekend. This guide is written for your parent if they want the details.
Not "eventually," not "when I get around to it." The day you decide not to buy the thing, transfer that exact dollar amount. Money that stays in checking gets spent on something else within about two weeks, and then this whole article was just a fun fact.
One low-cost fund that holds the whole market. Not a stock somebody recommended, not crypto, not whatever is trending. Then do not check it. The single hardest part of a 36 year plan is leaving it alone during the years it goes down, and it will go down. Start with what index funds are.
Because each $75 sits for a different length of time and grows the whole way. The $75 from 9th grade has 36 years to compound at 8%, which multiplies it by about sixteen and turns it into $1,198 on its own. The one from senior year of college only has 29 years, so it becomes $699. Add up all eight and you get $7,433. The money you put in never changes, which is what makes this feel like a trick. It is not a trick, it is just what an exponent looks like when the time period is longer than you have been alive.
It is a reasonable long-run assumption for a broad US stock index before inflation, and it is the figure most calculators use, but it is an average of a very bumpy ride rather than a rate you receive each year. Some years are up 25% and some are down 20%, and there have been decade-long stretches that went almost nowhere. If you want a more conservative view, slide the calculator to 6%, which gives about $4,022 on the same $600. The lesson does not depend on the exact rate. It depends on the number of years, which is the part you actually control.
No. The realistic version is buying one good backpack in 9th grade and keeping it for all eight years instead of replacing it every August. That skips seven purchases rather than eight, which comes to about $6,235 by 50 instead of $7,433. You give up $1,200 of the headline number and you have a bag the whole time. If you can use a hand-me-down or something already in your closet, you get the full amount, but nobody should be carrying loose textbooks around to win an argument with a calculator.
In a custodial account opened with a parent, since you cannot open a brokerage account alone before 18. If you have earned income from a job, a custodial Roth IRA is the best option available to you, because the growth comes out completely tax free in retirement and a teenager has more decades of growth ahead than anyone else who will ever open one. Without earned income, a plain custodial brokerage account works, holding a broad low-cost index fund. Both are free to open at most major brokerages and take about ten minutes.
It reduces the number without changing the conclusion. Rising prices mean $7,433 in the 2060s buys noticeably less than $7,433 buys today, perhaps in the neighborhood of $3,000 in current buying power depending on what inflation actually does. But inflation hits the alternative too, and the alternative is that you spent the $600 and own eight worn out backpacks. Comparing an inflation-adjusted several thousand dollars against zero is still not a close comparison.
Then it stays roughly $600, and after inflation it is worth less than $600. A regular savings account is the correct place for money you will need in the next few years, like saving for a car or an emergency fund, because it does not go down. It is the wrong place for money you will not touch for three decades, because the growth that produced $7,433 in this article is entirely from investing, not from saving. Both matter, and they are for different jobs.
Not remotely, and the calculator will show you why. A $75 backpack skipped at 17 is still $951 by 50, which is more than twelve times what you would have spent. What you have lost is the very front of the curve, and there is nothing to do about that except not lose any more of it. The genuinely bad move is deciding that because you missed 14, the whole idea is pointless. Someone who starts at 17 finishes far ahead of someone who starts at 25, and someone who starts at 25 finishes far ahead of someone who never starts.