Compound interest, explained simply

Resources · Home loans and saving · 4 min read Starting out

Key points

• Compound interest is interest earned on interest, as well as on the original amount.
• It starts slowly and speeds up, so time matters as much as the rate.
• Small regular additions often do more than the starting amount.
• The same maths works against you on debt, where unpaid interest gets charged interest too.

Simple versus compound interest

Compound interest sounds technical. It isn’t. But to see why it matters, start with its plainer cousin.

With simple interest, you only earn interest on the amount you started with. Put $5,000 away at 5% simple interest and you earn $250 every year, forever. Same amount, year after year.

With compound interest, each interest payment gets added to the balance. Next time, the interest is worked out on that bigger balance. So you earn interest on your interest.

At first the gap is tiny. Give it years and it gets big.

A worked example

Here’s what we’re assuming:

• Starting amount: $5,000
• Interest: 5% a year, paid monthly at one-twelfth of that rate (about 0.42% a month)
• No withdrawals, no fees, no tax

The same maths works in any currency.

After 10 years, compounding gives you $735.12 more than simple interest. After 30 years, it’s $9,838.94 more. Look at the third decade on its own. The extra growth there is larger than in the first two decades put together.

That’s the whole idea in one line. Compounding rewards time more than anything else.

Adding a little regularly

Hardly anyone drops in one lump sum and walks away. You add money as you go. So here’s the same $5,000 at 5% a year, compounded monthly, with $100 added at the end of each month:

By 30 years, interest makes up more than 60% of the balance. And the regular $100 does far more of the work than the starting $5,000 ever did. You don’t need a big sum to begin. You need to keep going.

What makes compound interest stronger or weaker

Four things shape the result.

Time comes first. The longer money sits untouched, the more growth builds on growth, which is why starting earlier usually beats starting bigger.

Rate matters too. A higher rate compounds faster. But in real life, higher potential returns tend to come with more risk of losing money, so a bigger number isn’t free.

Frequency plays a smaller part. Interest compounded monthly grows a little faster than interest compounded yearly at the same stated rate. Usually not by much.

Then there’s what leaks out. Anything you withdraw stops compounding. Fees and tax on the interest shave down the rate you actually get.

The rule of 72

Want a quick feel for how long it takes money to double? Divide 72 by the yearly rate. At 6%, that’s about 12 years. At 4%, about 18. At 9%, about 8.

It’s a rough guide, not an exact answer. Still handy for seeing how rate and time pull on each other.

When compounding works against you

Debt compounds too. On a credit card, unpaid interest is added to the balance, and next month you’re charged interest on it. How credit cards charge interest walks through this step by step.

The assumptions this time:

• Balance: $5,000
• Interest: 22% a year, charged monthly at one-twelfth of that rate
• No payments made and no new spending (for illustration only, as cards require minimum payments)

Three years, not one new purchase, and the balance has nearly doubled. That’s why high-interest debt can feel like it’s growing faster than you can pay it. It’s the same maths as savings. Just pointed the other way.

The upside? Paying down high-interest debt has a clear, predictable effect. Every dollar you repay stops compounding against you.

Putting it together

Compound interest isn’t magic. It’s steady arithmetic that pays off for patience and consistency. On savings, the levers are simple: start when you can, add regularly, then leave it alone. On debt there’s really just one lever. Shrink the balance the interest is charged on.

Common questions

What is compound interest in simple terms?

It’s interest earned on interest, as well as on the original amount. Each interest payment joins the balance, so the next one is worked out on a bigger number.

How often is compound interest calculated?

It varies. Monthly and yearly are common. Monthly compounding grows a little faster than yearly at the same stated rate, but the gap is usually small.

Does compound interest apply to debt?

Yes. On a credit card, unpaid interest is added to the balance and then charged interest itself. That’s why high-interest balances can grow so quickly.

Try it

The compound interest calculator shows how a starting amount and regular additions could grow using your own numbers. To see how compounding is affecting your debts, find your Freedom Date.

General information only, not financial advice. Your situation is your own.