How to Calculate ROI (and Why the Number Misleads)

Two people can look at the same season of trading and quote 32.1% and 39.2%, and neither of them is lying. Nothing in the formula settles which costs belong on the bottom line, and nothing in it asks how long the money was tied up.

The formula, and the part people drop

Return on investment is the profit divided by what you spent to get it, expressed as a percentage. The trap is that the profit must be net of everything, not just the obvious purchase.

A market stall over one season takes $4,200. Stock cost $2,600, the pitch fees were $400 and packaging came to $180. Profit is $1,020. Against total costs of $3,180, that is an ROI of 32.1%.

Cost basis is a choice, and it shows

Now count only the stock as the investment and leave the fees and packaging out, on the grounds that they were operating costs. The same $1,020 against $2,600 reads as 39.2%. Seven points of difference, same season, same money, purely from where you drew the line.

This is why an ROI quoted at you without its inputs is close to meaningless. When you are comparing two options, the only thing that matters is that both were calculated the same way.

ROI without time is half a number

The plain formula has no clock in it. A 32.5% return sounds better than 20% until you learn the first took four years and the second took one.

Annualising fixes this. Take the total growth factor, in this case 1.325, and raise it to the power of one over the number of years. For four years that gives 1.073, so 7.3% a year. Against a one-year 20%, the choice is now obvious and it went the other way.

When annualising misleads too

The same maths runs the other direction and gets silly. A 3% return in a single month annualises to 42.6%, which is arithmetically correct and practically useless unless you can genuinely repeat the trade eleven more times. Treat annualised figures from very short holding periods with suspicion, particularly when someone else is quoting them at you. The mechanics of why time compounds so aggressively are covered in our piece on compound interest.

Using the ROI calculator

The tool wants three things, and the discipline is in sourcing them rather than estimating.

Amount invested. Every pound or dollar that left your account to make this happen, including fees, commission, delivery and any deposit. For shares, the cost basis column on your brokerage statement already has it. For a project, add up the invoices instead of remembering.

Amount returned. Everything that came back, including dividends, rent or a resale value. If you deduct tax from the return, deduct it from the comparison case too, or the two are not comparable.

Holding period. Enter the actual dates or the number of years. This is the input people skip, and it is the one that changes the ranking.

A second example makes the time point concrete. A £3,000 professional course raises your salary by £2,400 a year. At the end of year one you are down £600, an ROI of minus 20%. At the end of year two you have had £4,800 back on £3,000, an ROI of 60%, or 26.5% annualised. Same course, three completely different verdicts depending on when you stop counting.

Common questions

What counts as a good ROI? It depends entirely on the alternative and the risk. The honest benchmark is what the same money would have done in a broad index fund over the same period with none of your time, and anything below that is a hobby you are paying for.

Is ROI the same as profit margin? No, and they are routinely confused. Margin is profit divided by revenue, while ROI is profit divided by cost. The stall above has a margin of 24.3% and an ROI of 32.1%. Our explainer on margin versus markup covers the related mix-up.

Should I include my own time as a cost? If the alternative use of that time was paid work, yes, otherwise you are comparing a side project against an index fund while pretending your weekends are free. Price them at a rate you would actually accept.

Can ROI be negative? Yes, down to minus 100%, which means the investment returned nothing at all. It cannot go below that unless you were liable for more than you put in, which is why borrowed money and margin accounts are a different conversation.