How money grows: a simple guide to interest

Money that grows by itself

Imagine you put $100 into a special box. A year later, without doing anything at all, the box has $105 in it. The extra $5 came from nowhere — or at least, it feels that way. That extra money is called interest. It’s real, and it’s one of the most useful things to understand about how money works.

Why banks give you interest

Banks pay you interest because they want to use your money while you’re not. When you put money in a bank, the bank doesn’t lock it in a vault. It lends some of it to other people — people who want to buy a house or start a business. Those people pay the bank to borrow the money, and the bank shares a little of that with you for letting them use yours.

So when your money sits in a savings account, it’s actually busy. It’s helping other people, and the bank is paying you a small thank-you every year.

What the “rate” means

Interest is always written as a percentage, like “5% per year.” That just means: for every $100 you have, you get $5 back that year.

• $100 at 5% grows by $5 a year.
• $500 at 5% grows by $25 a year.
• $1,000 at 5% grows by $50 a year.

The bigger your savings, the bigger the interest you earn — which is why saving more makes your money grow faster.

Saving vs spending, seen differently

This is the magic bit. A dollar you spend is gone forever once you spend it. A dollar you save doesn’t just sit there — it actually makes more dollars, year after year, without you doing anything extra. That’s why people say saving is “paying your future self.” The money you save today gives you more money tomorrow, next year, and even further away than that.

It doesn’t mean you should never spend! Spending is fun and sometimes necessary. But knowing that savings work for you makes saving feel different — not like missing out, but more like planting something that will grow.

A quick example

Let’s say you have $200 and you earn 5% interest per year.

• After 1 year: you have $210.
• After 2 years: you have $220.
• After 5 years: you have $250.
• After 10 years: you have $300.

Your $200 grew by $100, just by sitting there. You didn’t do a thing. That’s interest in action.

Two kinds of growing: simple and compound

There are actually two ways money can grow, and the difference is small at first but enormous over time.

Simple growing is when interest only ever gets added to your starting amount. If you have $100 at 5% per year, you get $5 every year, no matter how big the pile gets. After ten years, you’d have $150 — your original $100 plus $50 of growth.

Compound growing is when the new growth gets to grow too. The bank doesn’t just give you $5 of growth on your original $100. Once the $5 is in your account, it also earns growth next year. So in year two, you earn growth on $105, not just on $100. In year three, you earn growth on $110.25. And so on.

After ten years with compound growing, your $100 at 5% becomes about $163 — that’s $13 more than simple growing produced. After thirty years, the difference becomes huge: $432 with compound growing versus $250 with simple growing. The pile that grows on its own grows almost twice as fast over a long time.

Most savings accounts use compound growing, which is great for you. Most loans also use compound growing, which is why owing money for a long time is bad — the amount you owe grows faster the longer you wait.

Why time matters more than the amount

Here’s a surprising thing: how long your money grows usually matters more than how much you start with.

Imagine two people. Sam puts $100 in a savings account at age 10 and never adds more. Alex puts $100 in a savings account at age 30 and never adds more. Both accounts earn 5% per year.

By the time both of them turn 50:

• Sam’s $100 has been growing for 40 years and is now worth about $704.
• Alex’s $100 has been growing for 20 years and is now worth about $265.

Same starting amount. Same growing rate. Sam ends up with almost three times as much money — just because Sam started earlier.

This is the most important secret in money. It’s not about how much you save. It’s about how long you let it grow. Small amounts saved when you’re young can become bigger than huge amounts saved when you’re older. Your time is the one thing that’s hard to get back, so using it well — even with tiny amounts — is worth a lot.

A common misunderstanding

Some people think they need a lot of money before saving is “worth it.” This is the opposite of true. The smaller and earlier you start, the more time the magic of growing has to work.

Saving $1 a week from age 8 turns into a surprising amount by the time you’re 25. Saving $10 a week from age 30 turns into less, even though you saved ten times as much per week, because there were fewer years for it to grow.

The lesson: don’t wait until you have “enough” to start saving. The “enough” gets a lot bigger when you start now.

Try it yourself

Use the simple interest explainer to type in a starting amount, a growth rate, and a number of years. The chart shows your money growing year by year — the blue part is what you started with, and the lighter part is all the interest you earned. Once you’ve got the idea, try the pocket money growth calculator to see how adding new money each week makes the growing even faster.