The formula, and worked examples
ROI is one of the few finance formulas that fits in a sentence. Net profit is the amount returned minus the amount invested. ROI is that profit divided by the amount invested, expressed as a percentage. That is exactly what this calculator does, with no adjustments layered on top.
With the values it starts on, 1,000 invested and 1,500 returned, the profit is 500.00 and the ROI is 50.00 percent. Put 2,500 in and take 3,100 out and the profit is 600.00 at an ROI of 24.00 percent, which shows how a larger absolute profit can be the weaker return.
Losses work the same way. Invest 8,000 and get 6,800 back and the profit is negative 1,200.00 at an ROI of negative 15.00 percent. The calculator does not hide a loss or clamp it to zero, which is the correct behavior and the reason to check it against a spreadsheet rather than a gut feeling.
- Net profit = amount returned - amount invested
- ROI = net profit / amount invested x 100
- 1,000 in and 1,500 out gives 500.00 profit at 50.00 percent
Getting the two inputs right
Most wrong ROI figures come from a wrong input rather than wrong arithmetic. The amount invested should be everything you actually spent to get the return: the purchase price plus fees, shipping, setup, agency costs, and the platform cut. Leaving costs out inflates the percentage in a way that feels good and misleads whoever reads it later.
The amount returned is the other half of the same discipline. It should be what actually arrived, net of the costs of realizing it, so a sale price after commission rather than before, and revenue after refunds rather than gross. For a marketing spend, the honest figure is usually margin rather than revenue, because revenue counts money that goes straight back out as cost of goods.
Be consistent about which convention you use, and say which one you used when you share the number. Two people can compute a defensible ROI for the same campaign and get very different answers purely from where they drew the line, and that ambiguity is why the metric gets argued about.
What ROI does not tell you
The big one is time. This calculation has no notion of duration, so a 50 percent return earned in one month and a 50 percent return earned over five years produce the identical figure. In reality those are wildly different investments. Whenever you compare two options, compare the period alongside the percentage, or annualize both before you put them next to each other.
It also has no notion of risk, of the money you could have made doing something else, or of inflation eating the value of the return while you waited. A high ROI on a small, risky bet is not obviously better than a lower ROI on a large, safe one, and no percentage carries that information.
Finally, this is simple ROI rather than a discounted cash flow. It assumes one amount in and one amount out. If money went in and came out in stages, a proper internal rate of return or net present value calculation is the right tool, and squeezing staged cash flows into two boxes will give an answer that looks precise and is not.
- No time dimension: one month and five years look identical
- No adjustment for risk, opportunity cost, or inflation
- Simple ROI only, not an IRR or a net present value