Compound interest means earning interest on your original amount and on interest previously added to that amount. So how does it work in practice? Interest earned in one period stays in the balance, so the next period’s interest is calculated on a larger amount.
It’s a powerful idea and this guide explains the formula, compares compound and simple interest, shows what can strengthen or weaken compounding.w
What is compound interest in simple terms
Compound interest is the interest you earn not only on your original amount but also on the interest already added to it. Each period, previously earned interest becomes part of the base for the next calculation. In most cases, financial institutions describe it as “interest on interest.”
Let’s take an example of €1,000 which earns a hypothetical 5% interest rate once a year:
- In year one, the interest is €50, so the balance becomes €1,050.
- In year two, the 5% rate applies to €1,050 rather than the original €1,000. The interest is €52.50, so the balance becomes €1,102.50.
- In year three, interest is calculated at €1,102.50.
The increase is small at first because the previously earned interest is still small. As the balance grows, the amount on which the next interest calculation is based also grows.
Three terms are important:
- Principal: the starting amount.
- Interest rate: the percentage applied during a stated period.
- Compounding frequency: how often earned interest is added to the balance and begins earning interest itself.
Compounding is a calculation method, not an investment guarantee. A formula can show what would happen at a constant rate, but it cannot make a variable investment return predictable.
The compound interest formula
The standard compound interest formula is: A = P(1 + r/n)^(nt)
For example, if we imagine a €5,000 earning on a hypothetical 10% annual rate, that compounds every year for approximately 30 years in total, then the calculation is as follows:
€5,000 × (1 + 0.10/1)^(1 × 30) = €87,247.01
The assumption is that there are no additional contributions, fees, or any other taxes in the process. What’s worth understanding is that at an unchanged 10% rate, the example is a mathematical illustration, not a forecast or guaranteed return.
If compound capital is added or withdrawn during the terms, each cash flow comes with different amounts of time to compounding and has to be included separately.
For a quick estimate, the Rule of 72 approximates how long an amount may take to double, and it’s done by dividing 72 by the expected annual rate. For example, at 9%, the estimate is eight years.
How compounding works at Yieldfund
Yieldfund offers optional partial reinvestment for each bond, and investors can decide for themselves if they want to reinvest or select a percentage of the weekly interest payment, up to a maximum of 50%.
When using Yieldfund, compounding means that interest initially paid out weekly is added to the investment amount and the interest is then calculated again on the total amount that is “invested”, all in accordance with the Declaration of Purchase, Bond Terms and Conditions and compounding addendum.
The portion not selected for reinvestment continues to be paid weekly on the first business day of the week, usually Monday.
If we take a practical example of a €100 weekly payment, with 50% reinvested, then €50 would be paid, and €50 would be deferred under the agreement.
At the end of the term, the amount invested with Yieldfund and deferred interest are paid out, including the interest that was created by allocating the difference in amount.
Compound interest vs. simple interest
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus interest already added to the balance.
Using a €5,000 starting amount and a hypothetical 5% annual rate for 20 years:
- With simple interest, €250 is added every year. The ending amount is €10,000.
- With annual compound interest, the ending amount is €13,266.49.
The rate and starting amount are identical, but the results differ because the compounded balance changes each year. With simple interest, the calculation base stays at €5,000. With compound interest, each year’s interest becomes part of the next year’s calculation base.
This does not mean that every product described as “compounding” is automatically better. Returns, risks, access to money, fees, tax treatment and contractual conditions must also be compared.
The compounding gap
In this example, both approaches start at €5,000 and use the same 5% rate. After 20 years, simple interest reaches €10,000 — but compound interest reaches €13,266.49. That extra €3,266 is interest earning its own interest. Over longer horizons and higher balances, the gap widens further.
Compound interest vs. compound returns
Compound interest and compound returns are related but not identical. Compound interest usually describes savings accounts and fixed-interest products. Compound returns are broader: they can include reinvested interest, dividends, and capital gains from assets whose value can rise or fall.
Stocks, funds and cryptoassets do not normally pay a fixed, guaranteed interest rate. Their future returns are variable. What’s important to know is that products don’t necessarily compound because they pay interest or dividends; what matters most is what happens to those earnings.
| Product or setting | What happens to earnings? | Does the balance compound automatically? |
| Fixed-interest account | Interest is credited to the balance | Yes, if credited interest remains in the account |
| Conventional coupon bond | Coupons are normally paid out | No; coupons must be reinvested separately |
| Distributing fund or ETF | Distributions are paid in cash | No, unless the investor reinvests them |
| Accumulating fund or ETF | Eligible income is retained within the fund | Returns can compound within the fund |
| Yieldfund compounding | The selected share of weekly interest can be added to the initial investment amount | Yes, interest accrues on that deferred amount under the applicable agreement |
What makes compounding faster or slower
The difference between quick or slow compounding varies based on variables that depend on investor behavior but also external factors such as taxes and available capital.
Time
Time gives previously earned interest or returns more opportunities to generate further growth. Starting earlier extends the number of compounding periods, but there is no universal year in which compounding suddenly becomes significant. The result always depends on the rate, cash flows, and costs.
Rate
At a higher positive rate, a balance grows faster. But higher expected returns commonly involve higher risk, and a high hypothetical rate should never be presented as certain.
Compounding frequency
When the same nominal annual rate is held constant, more frequent compounding produces a higher ending amount. The benefit has diminishing returns, however.
For example, €10,000 at a hypothetical 6% nominal annual rate for 10 years becomes:
- €17,908.48 with annual compounding.
- €18,214.89 with weekly compounding.
Contributions and withdrawals
Regular additions can increase the amount available to earn future returns. Withdrawals do the opposite: money taken out can no longer contribute to future compounding. A regular investing approach such as dollar-cost averaging can create discipline, but it does not protect against losses or guarantee a positive return.
Fees, taxes and inflation
Fees reduce the amount left to earn future returns, and even apparently small ongoing fees can have a major effect because they reduce the balance that remains invested.
Tax treatment varies by country. In the Netherlands (2026), savings and investments are taxed under Box 3 — but not on your actual interest. Instead, the tax authority applies a deemed return (6.00% for investments, a provisional 1.28% for savings) and taxes that at 36%, only on wealth above the tax-free allowance of €59,357 (€118,714 for fiscal partners). Because the tax is based on a deemed return rather than your real gains, its effect on compounding depends on your asset mix and balance.
The main point
Compound interest is not a shortcut. It is a mechanism: earnings remain in the balance and can generate further earnings. Time, rate, reinvestment, costs and withdrawals determine the mathematical result, while the product’s risks and contractual terms determine what that result means in practice.
At Yieldfund, partial reinvestment makes that choice explicit. Investors can balance weekly interest payments with deferred interest that accrues under their agreement, up to the permitted percentage. The right setting depends on the agreement and the investor’s own circumstances.
Frequently asked questions
What is compound interest in simple terms?
Compound interest means earning interest on the original amount and on interest previously added to that amount. It is commonly described as interest on interest.
Does compounding frequency matter?
Yes. At the same nominal annual rate, more frequent compounding produces a somewhat higher ending amount. The improvement becomes smaller as frequency increases, and rate, time, and costs often have a larger effect.
Does Yieldfund reinvest interest automatically?
No. The Yieldfund interest is not automatically reinvested, and investors can reinvest up to 50% of the interest paid out, while the not selected amount continues to be paid out.
