ROI is one of those terms that gets used constantly in business but is often calculated incorrectly. The formula itself is simple. The difficulty is in correctly identifying what counts as a cost, what counts as a return, and how to apply the result to real decisions.
This guide explains the ROI formula, covers the common ways the calculation goes wrong, looks at annualised ROI for comparing investments with different time horizons, and gives practical examples across hiring, advertising, equipment, and software.
Use the ROI Calculator to run your own numbers alongside this guide.
The ROI Formula
Return on investment is expressed as a percentage and calculated as:
ROI = (Net gain / Cost) x 100
Where net gain is the return minus the cost. Or equivalently:
ROI = ((Return - Cost) / Cost) x 100
A simple example: you spend £5,000 on a marketing campaign and it generates £12,000 in revenue from sales you attribute to that campaign. Your net gain is £7,000. Your ROI is (£7,000 / £5,000) x 100 = 140%.
That looks impressive. But whether 140% is a good result depends on your industry, your timeframe, and whether you have calculated the cost and return correctly.
What Counts as a Cost
This is where most ROI calculations go wrong. People tend to include the direct financial outlay but miss the indirect costs.
For a marketing campaign, the cost includes:
- Ad spend
- Agency or freelancer fees for creative and management
- The cost of landing pages, tools, or software used specifically for the campaign
- Your own time spent managing the campaign (valued at your hourly rate or opportunity cost)
For hiring an employee, the cost includes:
- Salary
- Employer National Insurance (currently 13.8% on earnings above £9,100)
- Employer pension contributions
- Any equipment, software licences, or workspace costs
- Recruitment costs (agency fees, time spent interviewing)
- Training and onboarding time from existing staff
For equipment, the cost includes:
- Purchase price or lease cost
- Installation and setup
- Maintenance and servicing
- Training for staff who will use it
- Financing costs if purchased on credit
If you leave out the indirect costs, your ROI figure will look better than it is. This leads to poor decisions.
What Counts as a Return
Returns also need careful definition. The most common mistake is using revenue instead of profit.
If you spend £5,000 on ads and generate £12,000 in revenue, but those sales have a 40% gross margin, your actual return is £4,800 in gross profit. The ROI becomes (£4,800 - £5,000) / £5,000 x 100 = -4%. The campaign lost money, even though the revenue looked good.
Always use the appropriate profit metric for your return, not gross revenue.
For some investments, the return is a cost saving rather than revenue generated. If a piece of software costs £1,200 per year and saves you 3 hours per week at a time value of £50 per hour, that is 156 hours saved, worth £7,800 per year. The ROI on the software is 550%. Reductions in cost are just as valid as increases in revenue when calculating return.
Annualised ROI: Comparing Investments With Different Timeframes
A basic ROI calculation does not account for time. A 100% ROI over 3 years is much less impressive than a 100% ROI over 3 months.
When you want to compare two investments with different payback periods, use annualised ROI:
Annualised ROI = ((1 + ROI/100)^(1/years) - 1) x 100
Investment A: 80% ROI over 2 years = approximately 36% annualised Investment B: 50% ROI over 6 months = approximately 125% annualised
Investment B is significantly better on an annualised basis, even though its headline ROI is lower. This matters when you are deciding how to allocate a limited budget.
For shorter projects, you can simplify by multiplying the return to a yearly figure rather than using the compounding formula. The important point is to normalise for time before comparing.
When ROI Is a Poor Metric
ROI is useful for decisions with clear, measurable returns and defined costs. It is a poor tool for several common situations.
Brand building. Awareness campaigns, PR, content marketing, and community building often produce returns that are diffuse, long-term, and hard to attribute. You could measure brand search volume, share of voice, or customer lifetime value over time, but attributing these changes to a single investment is difficult. ROI calculations in this context often produce misleading results.
Long payback periods. If a piece of equipment has a 7-year productive life and pays back in year 3, the ROI looks poor in year 1. You need to model the full expected return over the asset's life and discount future cash flows appropriately. Simple ROI is not the right tool here; net present value (NPV) is.
Strategic necessity. Some investments are not optional. If your competitors all use a particular software platform and clients expect you to use it too, the ROI calculation is less relevant than the cost of not making the investment. Not all spending decisions are purely financial optimisations.
Hiring for growth. The return on a new hire is highly dependent on what they do and how quickly they become productive. ROI models for headcount are often more useful for scenario planning than for precise calculation.
ROI Benchmarks by Investment Type
While every business and situation is different, these rough benchmarks help calibrate your expectations.
Paid advertising (PPC, social ads). A minimum viable ROI is often quoted at 2:1, meaning £2 returned for every £1 spent (ROI of 100%). This covers platform costs and leaves some margin. High-performing campaigns in competitive markets often target 4:1 or better. Below 2:1 is typically not worth the effort.
Email marketing. Email consistently produces high ROI, often cited in industry surveys at 30:1 or more for well-run programmes. This reflects the low cost of sending email compared to the value of conversions to an engaged list.
Software and tooling. Any software investment that saves more than its annual cost in staff time is worth considering. Tools that generate revenue (CRM, sales automation) should aim for at least 5:1 to justify the disruption of implementation.
Equipment. Productive life matters. Equipment with a 5-year life needs to generate enough value over that period to justify the cost, accounting for maintenance and capital tied up. There is no single benchmark; it depends on utilisation and alternatives.
Hiring. A new hire in a revenue-generating role (sales, account management) can often be benchmarked against a revenue-per-head target. A sales hire costing £50,000 per year in total employer costs should contribute significantly more than that in gross profit if they are effective.
Common Mistakes in ROI Calculations
Forgetting opportunity cost. Money spent on one thing cannot be spent on another. If you are choosing between two investments, the real cost of choosing one includes the foregone return from the other. This is especially relevant for small businesses with limited capital.
Confusing revenue with profit. Always calculate returns on a profit basis, not revenue. Using revenue overstates the return for any business with significant costs of goods sold or service delivery.
Ignoring time to payback. A positive ROI that takes 4 years to realise may not be worth as much as a smaller but faster return, particularly in uncertain environments.
Attributing too much to one campaign. Marketing attribution is difficult. Be honest about what you can and cannot credit to a specific investment. Over-attribution makes ROI calculations unreliable.
Not tracking actuals vs estimates. Many ROI calculations are done before an investment and then never revisited. Track actual results against projections and use the data to improve future estimates.
Practical Examples
Hiring a virtual assistant. You pay a VA £1,500 per month. The VA handles tasks that would otherwise take you 15 hours per week. You value your time at £100 per hour. That is £6,000 per month in time freed up. The cost is £1,500. The net gain is £4,500 per month. ROI is 300% per month, assuming you use that freed time productively.
Buying a piece of equipment. A piece of machinery costs £15,000 and reduces production time by 8 hours per week, saving £40 per hour in labour. That is £320 per week, £16,640 per year in labour savings. After 1 year, your ROI is (£16,640 - £15,000) / £15,000 x 100 = 10.9%. Over 3 years with the same savings rate, the cumulative return is £49,920 and the ROI is 233%.
Running a paid ads campaign. You spend £2,000 on Google Ads. It generates 40 conversions. Average order value is £120 with a 50% gross margin. Revenue is £4,800, gross profit is £2,400. Net gain is £400. ROI is 20%. Acceptable but not exceptional. You would want to optimise before scaling.
Use the Calculator
Run your own scenarios using the ROI Calculator. Try a few different versions of the same investment: optimistic, realistic, and conservative returns. The realistic version should be the basis for your decision. If the conservative scenario still produces acceptable ROI, the investment is more robust. If it only makes sense under optimistic assumptions, be cautious.
Good ROI analysis is not about finding reasons to make an investment. It is about understanding the range of outcomes and making a decision with clear eyes.