ROI Calculation Explained: The Formula, a Worked Example, and Where People Get It Wrong
ROI answers a simple question, did this pay off, and by how much, but the formula's simplicity hides a real trap: a return figure on its own says nothing about how long it took to earn. A 45% return over eight months and a 45% return over eight years are wildly different outcomes described by an identical number.
Here's the formula, two worked examples from different kinds of spending, and the specific ways this calculation gets misused when comparing two options against each other.
What ROI actually means
Return on investment measures how much value you gained relative to what you put in, expressed as a percentage of the original outlay. It's deliberately general. The same formula works whether "investment" means a stock purchase, a piece of equipment, an ad campaign, or the time you spent building a side project, which is exactly why it needs to be paired with judgment about what counts as the investment and what counts as the return in each specific case.
The formula
ROI% = (Final value − Investment) ÷ Investment × 100
For anything held over more than a year, it's worth also computing the annualised return, since ROI alone doesn't account for time at all:
CAGR% = ((Final value ÷ Investment)^(1 ÷ Years) − 1) × 100
CAGR (compound annual growth rate) answers "what steady yearly rate would have produced this same total result," which is the number that actually allows a fair comparison between investments of different lengths.
Example: a marketing campaign
A small business spends ₹80,000 on a two-month ad campaign. The campaign drives sales that generate ₹1,50,000 in gross profit (revenue minus the direct cost of goods sold on those specific sales, not total revenue).
ROI% = (1,50,000 − 80,000) ÷ 80,000 × 100 ≈ 87.5%
That's a strong return for two months. Because the holding period is short, the annualised figure would be dramatically higher than the headline ROI, but annualising a two-month campaign result and treating it as a repeatable yearly rate is its own mistake, covered below.
Example: an equipment purchase
A workshop buys a ₹5,00,000 machine that increases production capacity. Three years later, the additional profit it's generated, net of maintenance costs, totals ₹7,25,000.
ROI% = (7,25,000 − 5,00,000) ÷ 5,00,000 × 100 = 45%
CAGR% = ((7,25,000 ÷ 5,00,000)^(1 ÷ 3) − 1) × 100 ≈ 13.2%
A 45% total return over three years and a 13.2% annualised return are the same result described two ways, and the annualised figure is the one that's actually comparable to, say, a loan's interest rate or a different investment's yearly performance.
Profit vs revenue: pick the right one
The single most common ROI error is using revenue where profit belongs. A marketing campaign that generates ₹5,00,000 in sales sounds impressive against an ₹80,000 spend, but if delivering those sales cost ₹3,50,000 in materials and labour, the actual gain is ₹1,50,000 in gross profit, not ₹5,00,000. Using revenue as the "final value" inflates ROI into a number that has nothing to do with what the business actually kept.
The rule: "final value" in the formula should always represent value you actually retained, after the direct costs of generating it, never a top-line figure that still has real expenses buried inside it.
Why ROI on its own can mislead you
Plain ROI has no concept of time built in, which makes it dangerous for comparing two different opportunities unless you separately check how long each one took.
| Investment | ROI | Holding period | Annualised (CAGR) |
|---|---|---|---|
| A | 45% | 8 months | ≈ 75% |
| B | 45% | 8 years | ≈ 4.8% |
Anyone comparing these two using ROI alone would (correctly) call them identical, and would be badly wrong about which one is the better use of capital. Always annualise before comparing investments of different lengths, and be cautious about extrapolating a short campaign's annualised figure forward as if it would repeat all year. A two-month result rarely scales linearly to twelve.
Common mistakes
- Using revenue instead of profit. Covered above, the single most frequent source of an inflated, meaningless ROI figure.
- Ignoring your own time as a cost. A freelance project that "returned" a profitable amount looks different once you value the hours it took at a reasonable rate. See how to set your freelance rate for how to put a real number on that time in the first place.
- Comparing investments of different lengths without annualising. Directly addressed above. A short-term ROI figure is not comparable to a long-term one without converting both to the same annual basis.
- Excluding hidden or ongoing costs. Maintenance, financing interest, storage, or opportunity cost of tying up cash all reduce the real return and are easy to leave out of a quick calculation.
- Not comparing against a real alternative. A positive ROI in isolation tells you the investment beat doing nothing. It doesn't tell you it beat the next-best thing you could have done with the same money, which is usually the more useful comparison.