Simple interest
Simple interest is the most basic way to calculate a return: interest is always calculated on the initial capital, not on the interest you have already earned. In other words, there is no "snowball": each period you earn exactly the same amount.
The formula
Interest = C × r × t
Where C is the initial capital, r is the interest rate per period (0.05 for 5 %) and t is the number of periods (usually years). The final value is simply the capital plus that interest:
Final value = C × (1 + r × t)
Example: €2,000 at 4 % a year for 3 years. Interest = 2,000 × 0.04 × 3 = €240. Final value = €2,240.
Notice that you earn €80 every year (2,000 × 0.04), year after year, with that figure never changing.
Where simple interest shows up in real life
Although compound interest dominates long-term investing, simple interest is still very common:
- Bond coupons. A bond that pays a fixed coupon (say 3 % of face value each year) pays you the same amount every time. If you don't reinvest that coupon, you are earning simple interest.
- Some short-term deposits and loans. If interest is paid at maturity or settled periodically and you withdraw it, it is not capitalized.
- Late-payment interest. Many surcharges and statutory interest amounts are calculated on a simple basis over the outstanding amount.
The difference from compound interest
The key is whether interest is reinvested or not:
| Year | Capital | Simple interest (4 %) | Compound interest (4 %) |
|---|---|---|---|
| 1 | €2,000 | €80 | €80.00 |
| 2 | — | €80 | €83.20 |
| 3 | — | €80 | €86.53 |
| Total | €240 | €249.73 |
Over 3 years the difference is small, but because the compound exponent grows over time, at 20 or 30 years the gap is enormous. That is why, for your long-term investments, compounding is what matters.
To fully understand why compounding takes off over time, see the dedicated guide on compound interest.
Why this matters when investing in bonds
A bond's coupon is simple interest: you're paid the same fixed amount each year on the face value. The difference between an ordinary bond and a powerful long-term one is what you do with that coupon: spend it and it stays pure simple interest; reinvest it into more bonds or the market and you turn it into compound interest. That decision, repeated over 20 years, separates two very different outcomes. It's exactly the logic behind a bond ladder.
For long-term investing, what really moves the needle is compound interest: reinvest, don't withdraw.