Selurio · Blog
ROI for Beginners — How to Measure Whether an Investment Pays Off
You put in €10,000 and a few years later you have €15,000. Is it a good deal? "I made €5,000" is a correct answer, but an incomplete one — it says nothing about how fast or whether another opportunity might have been better. This is exactly where ROI comes in.
ROI (return on investment) is the simplest way to reduce "did this pay off" to a single number in percent. This article explains how it is calculated, why the annualized return is more honest than the total, and where the number lies.
What is ROI
ROI measures profit relative to what you put in, in percent. The formula is elementary:
Total return = (final value − initial investment) / initial investment × 100%
For the example above: (15,000 − 10,000) / 10,000 = 50%. The investment grew by half of what was put in.
The power of ROI is that it makes deals of different sizes comparable. A €500 profit on €1,000 invested (50%) is far better than a €500 profit on €10,000 invested (5%), even though the absolute profit is the same.
Total return hides time
Here is the trap. A 50% total return sounds wonderful — but over what period? Over 2 years it is excellent. Over 20 years it is poor. The total return collapses the whole period into a single number and says nothing about its length.
That is why the annualized return exists (CAGR — compound annual growth rate). It answers the question: "by what percentage per year, on average, did the investment grow, if the growth were even?"
Annualized return = ((final / initial)^(1 / years) − 1) × 100%
For our example over 5 years: a 50% total return translates to about 8.45% per year. That you can now compare with everything else — a deposit, an ETF, another investment.
| Period | Total return | Annualized (CAGR) |
|---|---|---|
| 2 years | 50% | ≈ 22.5% |
| 5 years | 50% | ≈ 8.45% |
| 10 years | 50% | ≈ 4.14% |
| 20 years | 50% | ≈ 2.05% |
The same 50% total return means something entirely different depending on the period. That is why, when someone promises you "a 50% return," your first question should be: "over what period?"
Calculate your own case in the return (ROI) calculator — change the period from 5 to 10 years and see how the annualized return collapses, even though the profit in euros stays the same.
An example with a loss
ROI works in both directions. If you put in €10,000 and after 3 years you have €8,000, the total return is −20% and the annualized about −7.2%. Negative numbers are just as informative: they show how much and how fast you were losing.
One treacherous property of percentages: a 50% loss is NOT offset by a 50% gain. If €10,000 falls by 50% to €5,000, to get back you need a 100% gain. That is why avoiding large drawdowns weighs more than chasing large gains — a theme we explore in risk and return.
Where ROI misleads
ROI is convenient but simplistic. Beware of three things:
- It does not account for time by itself. Always look at the annualized return, not just the total.
- It does not account for risk. 10% from a stable ETF and 10% from a speculative stock look identical in ROI, but carry entirely different risk.
- It does not account for taxes, fees, and inflation. Your real return is lower than the nominal one. 8% at 4% inflation is about 4% real growth — see real return.
| Question | What ROI misses |
|---|---|
| Over what period? | The total return hides the period — calculate CAGR |
| At what risk? | A single number does not distinguish safe from risky |
| Real or nominal? | Inflation, taxes, and fees are not included |
How to use it wisely
ROI is a tool for comparison and stocktaking, not a buy signal. Use it to assess a past investment or to compare alternatives under the same assumptions — but never on its own, detached from the period and the risk.
For long-term planning, combine ROI with compound interest: the annualized return is exactly the rate r that compounding uses to project growth forward.
What this does NOT mean
- The past is not the future. A return calculated for a past period does not guarantee the same going forward.
- It is not a recommendation. Selurio is an educational platform — the goal is to understand the number, not to tell you what to buy.
- The numbers are a guide. The actual result depends on taxes, fees, timing, and luck.
Continue reading
- Risk and return — why a higher return means higher risk.
- Compound interest — the eighth wonder — how the annualized return grows over time.
- Real return — how much is left after inflation.
- Investing for beginners — the whole picture step by step.
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