Scope and assumptions
ROI % compares gain to the amount invested (cost). Illustrative.
Two investors both made $200. One risked $1,000, the other risked $10,000. The dollar profit is identical; the performance is not remotely comparable. ROI exists to make that distinction, and it does so with a single decision: dividing by what was put in, not by what came out.
Gain over cost
ROI % = (Final Value − Cost) ÷ Cost × 100
| Cost | Final | Gain | ROI |
|---|---|---|---|
| $1,000 | $1,200 | $200 | 20% |
| $500 | $650 | $150 | 30% |
| $2,000 | $1,800 | −$200 | −10% |
The third row matters as much as the first two. ROI carries a sign, and a loss is a legitimate result rather than a missing number.
Why cost is the denominator
Return is measured against the capital committed. Divide by the final value instead and you get a different metric — a margin — which answers "what share of the proceeds was profit?" rather than "how hard did my money work?"
Both are useful. They are not interchangeable, and swapping them changes the ranking of two investments.
The time blind spot
ROI as calculated says nothing about duration. A 20% return earned in one year and a 20% return earned in ten are the same ROI and very different achievements.
Annualising fixes it:
CAGR = (Final ÷ Cost)^(1 ÷ years) − 1
A 50% total return over five years is about 8.4% a year — noticeably less than the 10% that dividing 50 by 5 would suggest.
Nominal versus real
The percentage you compute is nominal. If inflation ran 3% over the holding period, a 20% nominal return is closer to 17% in purchasing power. Over long horizons the gap matters more than the difference between most competing investments.
ROI Figures That Overstate Returns
Dividing by the final value and calling it ROI; leaving fees and commissions out of the cost base; quoting a multi-year return without saying so; treating a negative result as an error rather than a loss; comparing ROIs across different holding periods; and forgetting that taxes reduce the final value.
Related growth & decay guides
For the rest of the growth-and-decay cluster, start at the hub: Population Growth Percentage. Adjacent: Inflation Percentage Calculation and Percentage Change Over Time.
FAQ
How do I annualise an ROI so different investments compare fairly? Use CAGR: (final ÷ cost)^(1 ÷ years) − 1. A 50% total return over five years is about 8.4% a year, not 10%.
Do fees belong in the cost base? Yes. Commissions, platform fees, and taxes paid to acquire the position are part of what you invested, so excluding them overstates the return.
What's the difference between ROI and ROE? ROI divides by total invested capital; ROE divides by shareholders' equity. ROE ignores debt, which is why a leveraged company can show a high ROE and a modest ROI.
How do I compare investments of very different sizes? Compare percentages, then look at absolute profit. A 40% return on $500 is $200; a 10% return on $50,000 is $5,000.
Sources: Investopedia, ROI; Corporate Finance Institute, ROI.
Disclaimer: Illustrative. Real returns involve risk, fees, and taxes. Informational only.
Sources and references
- Investopedia — Return on Investmenthttps://www.investopedia.com/terms/r/returnoninvestment.asp
- Corporate Finance Institute — ROIhttps://corporatefinanceinstitute.com/
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