July 20, 2026
ROI Explained: The Formula, Common Mistakes, and When It Lies to You
Return on investment looks simple until you compare a 3-month project to a 3-year one. Here is the math that actually accounts for time.
Return on investment is the most-quoted metric in business and the most frequently misused. The formula itself is trivial: (gain minus cost) divided by cost. What trips people up is everything the raw number leaves out — time, risk, and what else you could have done with the money. Here is the formula, the mistakes that inflate it, and how to read an ROI figure without being misled by it.
The Basic ROI Formula
ROI = (Net Profit ÷ Cost of Investment) × 100. Net profit is total return minus the original cost. If you spend £5,000 on a marketing campaign and it generates £8,000 in attributable revenue, net profit is £3,000 and ROI is 60%.
Cost: £5,000
Return: £8,000
Net profit: £8,000 − £5,000 = £3,000
ROI: £3,000 ÷ £5,000 × 100 = 60%
Why Raw ROI Hides Time
A 60% ROI sounds identical whether it took three months or three years to achieve — it does not. Annualising the figure is what makes ROI comparable across different holding periods. The simplified annualised formula divides total ROI by the number of years (or uses the compound annual growth rate for a more precise figure over multi-year periods).
Same 60% ROI, two very different outcomes:
Campaign A: 60% ROI over 3 months → roughly 240% annualised
Campaign B: 60% ROI over 3 years → roughly 17% annualised (compound annual growth rate)
Comparing the two on raw ROI alone makes a mediocre 3-year investment look identical to an exceptional 3-month one.
ROI vs. IRR vs. Payback Period
ROI answers "how much did I gain relative to what I spent." It says nothing about when cash arrived, which matters if you are comparing projects with different timelines or multiple cash inflows spread over years. Two other metrics fill that gap:
- IRR (Internal Rate of Return): the annualised rate that makes the net present value of all cash flows equal zero. Standard in capital budgeting and used to compare projects with irregular, multi-year cash flows.
- Payback period: how long until cumulative returns equal the original cost. Useful when liquidity and risk exposure matter more than total return — a faster payback means less time your capital is at risk.
For a single lump-sum investment with a single lump-sum return, ROI is enough. Once cash flows happen in multiple stages, IRR gives a more honest comparison.
Common Mistakes That Inflate ROI
The most common error is an incomplete cost figure. A marketing campaign's "cost" is not just ad spend — it includes the hours staff spent producing creative, any software or agency fees, and the opportunity cost of running this campaign instead of another. Leaving these out overstates ROI substantially.
The second common error is cherry-picking the measurement window. Extending or shrinking the attribution period until the number looks good is a well-known way to manufacture an impressive ROI figure that does not hold up under scrutiny. Decide the measurement window before you look at results, not after.
A Worked Comparison Across Investment Types
Home renovation: £15,000 kitchen remodel adds £18,000 to resale value → ROI 20%, but realised only when the home sells, so it should be compared against holding-period alternatives, not annualised the same way as a liquid investment.
Stock investment: £2,000 invested, grows to £2,300 over 18 months → ROI 15%, annualised roughly 10% — directly comparable to a savings account APY or another fund's reported annual return.
Paid ad campaign: £1,000 spend generates £1,900 in sales over 30 days → ROI 90%, extremely high annualised, but only valid if the 30-day attribution window genuinely captures the campaign's full effect and not future organic sales it would have generated anyway.
Using the Calculator
Enter your investment cost and total return into the ROI calculator to get both the raw ROI percentage and the underlying profit figure. When comparing two options with different timeframes, calculate both and mentally annualise before deciding which is genuinely the better use of capital.
Try the calculator