Compound interest is the reason a small amount saved early can end up bigger than a large amount saved late. It is interest that earns interest, and it works quietly in the background while you do other things. It also works against you when you owe money.
This article is the reading companion to the free Mondosol lesson Money Matters: Compound Interest, part of the Money course. Read it, watch the videos, or press play and listen first, then practise in the lesson.
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In this article
What compound interest is
Interest is the price of using money. When you save, the bank pays you interest. When you borrow, you pay the bank interest.
With simple interest, you earn interest only on the money you first put in. With compound interest, the interest is added to your balance, and next period you earn interest on the bigger total.
- Principal: the money you start with.
- Rate: the percentage paid per year.
- Time: how long the money stays invested or saved.
- Compounding: how often interest is added (yearly, monthly, daily).
Simple vs compound: a side-by-side
Say you save 10 000 kr at 5% a year and never add anything. The numbers below are an invented example, not a forecast: real rates change and are often lower or higher.
| After | Simple interest | Compound interest | Extra from compounding |
|---|---|---|---|
| 10 years | 15 000 kr | 16 289 kr | 1 289 kr |
| 20 years | 20 000 kr | 26 533 kr | 6 533 kr |
| 30 years | 25 000 kr | 43 219 kr | 18 219 kr |
The gap is small in year one and large in year thirty. That growing gap is the whole story of compounding.
Why time matters more than the amount
Because interest builds on interest, the last years of a long saving period add far more than the first years. Starting ten years earlier can matter more than saving much more later.
This is why people say the best time to start was yesterday, and the second best time is today. Even small amounts count.
Saving a little every month
Most people do not have a lump sum. They save a bit each month. The table shows 1 000 kr saved at the end of every month at an invented 5% a year, interest added monthly.
| Time | You paid in | Balance at the end | Interest earned |
|---|---|---|---|
| 10 years | 120 000 kr | 155 282 kr | 35 282 kr |
| 20 years | 240 000 kr | 411 034 kr | 171 034 kr |
| 30 years | 360 000 kr | 832 259 kr | 472 259 kr |
Doubling the time does more than double the result. Your own deposits grow in a straight line, while the interest part keeps curving upward.
The rule of 72
A quick way to estimate how long money takes to double: divide 72 by the yearly rate. At 5% it is about 14 years (72 ÷ 5 ≈ 14.4). At 8% it is about 9 years. It is a rough rule of thumb, not an exact answer.
The same maths works against debt
When you owe money and do not pay it off, interest is added to what you owe, and next time you pay interest on that bigger amount. This is why credit card and consumer loan balances can grow fast.
- Pay more than the minimum when you can.
- Pay the most expensive debt first.
- Check the yearly rate, not only the monthly payment.
For the debt side in more detail, read Make Money: Avoiding Debt Traps, How Interest Makes Debt Grow.
Watch: compound interest explained
Two short videos show the idea in different ways. Rates and products differ by country, so use them for the concept.
Video: “Compound Interest Explained in One Minute | One Minute Economics” by One Minute Economics, on YouTube.
Video: “What Is Compound Interest? | Investopedia” by Investopedia, on YouTube.
Take it further
Reading is a good start. These options help you go from knowing to doing:
- Mondosol+: all courses, live coaching sessions and members-only events, with a 30-day money-back guarantee. See the plans.
- Investment Checklist (PDF): a simple step-by-step checklist from Mondosol. Educational, not advice.
- 1:1 coaching with Elio: a live session on Google Meet to turn this topic into your own plan.
Frequently asked questions
Is compound interest the same as interest on interest?
Yes. Each time interest is added, it becomes part of the balance, so the next interest payment is calculated on a larger amount.
Does more frequent compounding make a big difference?
It helps a little: monthly compounding grows slightly faster than yearly at the same rate. The rate and the time matter much more.
Do savings accounts pay compound interest?
Many do, but terms differ. Check how often interest is added and what the yearly rate is. Rates change over time.
Is the 5% in your examples realistic?
It is only an illustration. Savings rates and investment returns change, and investments can lose value. Never treat an example as a promise.
What should I do first: pay debt or save?
It depends on the interest rates. Expensive debt usually costs more than savings earn, but a small emergency buffer is also important. Compare your own numbers.
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Read our independent eToro review: what you can trade, what it costs and the risks to know first. It is a look at one platform, not a recommendation to invest.
Affiliate disclosure: the review page contains affiliate links, so Mondosol may earn a commission if you sign up through them, at no extra cost to you. Investing involves risk and you can lose some or all of your money. Educational, not financial advice.
Keep learning
- Course: Money
- Lesson (Practise it): Money Matters: Compound Interest
- Read next: Make Money: The Power of Saving and Compound Interest Basics and Make Money: Make Money Work for You, How Saving and Investing Grow
- Related: Make Money: Emergency Funds
Educational, not financial advice. All numbers are invented examples for learning, not predictions. This post links to our eToro review page, which contains affiliate links (see the disclosure above).
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