Everybody’s heard that compound interest is “the eighth wonder of the world.” Most of us heard it from someone trying to sell us something, and it never came with an explanation we could use.

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So here’s the plain version.

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The basic idea

Compound interest is interest you earn on your money, plus interest on the interest you already earned.

Say you put $1,000 somewhere that pays 5% a year.

• After year one, you’ve earned $50. You now have $1,050.
• In year two, that 5% applies to $1,050, not $1,000. You earn $52.50.
• In year three, it applies to $1,102.50. You earn $55.13.

The raise is small each year, but it keeps growing because the base keeps growing.

Compare that to simple interest, where you only ever earn the original $1,000. That’s $50 every year, no matter what. After 10 years, you’d have $1,500. With compounding, you’d have $1,629.

That $129 gap is the whole concept. It’s not dramatic at 10 years, but it keeps widening.

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‍Time does most of the work

Same $1,000, same 5%, left alone:

• 10 years: about $1,629
• 20 years: about $2,653
• 30 years: about $4,322

The second ten years add more than the first ten, and the third ten add more than both. We’ll be honest, though: nobody feels that at year three. For a long stretch, it looks like nothing is happening.

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‍Adding money matters more than the rate

Most of us aren’t sitting on a lump sum. We’re finding $20 or $50 when the month allows.

At 5%, putting in $50 a month looks like this:

• 10 years: about $7,760 (you put in $6,000)
• 20 years: about $20,550 (you put in $12,000)

The interest is the smaller part of that early on. In the first few years, what you contribute matters far more than what it earns. Compounding is the thing that gradually changes that.

It also doesn’t need a perfect streak. Missed months happen: the car needs a repair, a bill comes in higher than expected. A skipped month slows things down. It doesn’t undo them.

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‍A quick shortcut: the Rule of 72‍

Want a rough sense of how long it takes money to double? Divide 72 by the interest rate.

• At 5%, about 14 years (72 ÷ 5 = 14.4) 
• At 2%, about 36 years
• At 8%, about 9 years

It’s an estimate, not an exact answer, but it’s handy for sizing up a rate quickly.

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‍How often it compounds

Interest can compound yearly, monthly, or daily. More frequent is better for you, but on everyday amounts the difference is small. On $1,000 at 5%, yearly compounding gives you $1,050 after a year, and daily gives you about $1,051.27. Worth knowing, not worth losing sleep over. The rate and how long you leave the money alone matter much more.

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‍The part nobody puts on the poster

Compounding works in both directions. The same math that grows savings grows debt.

Take $1,000 on a card at 24% APR. That’s roughly 2% a month. If nothing gets paid for a year, the balance is about $1,268. And next month’s interest is charged on this month’s interest, which is why a balance can grow even when you’re not spending anything new.

So when a rate is on your side, time helps. When it’s against you, time costs you. That’s the main reason many of us put paying down high-rate debt ahead of nearly everything else.‍

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Where to start

You don’t need a lot or a perfect plan. Here’s what tends to work:

• Start with whatever amount doesn’t squeeze the light bill or gas money
• Leave it alone as long as you can, since every withdrawal resets some of the growth
• Add to it when a good month comes along, and don’t sweat the bad ones

A $500 cushion that sits and earns something is a better foundation than a big plan that falls apart in March.