ROI equals the gain minus the cost, divided by the cost, multiplied by 100 to give a percentage.
TL;DR: Open the ROI calculator and enter what you spent and what you got back. Spend $2,000 and return $4,000, and your ROI is 100%.
The formula and a worked example
The formula is:
ROI = (gain − cost) ÷ cost × 100
Suppose you spend $2,000 on a campaign and it brings in $4,000. The gain over cost is $4,000 − $2,000 = $2,000. Divide that by the $2,000 you spent and you get 1, or 100% once you multiply by 100. You doubled your money. An ROI of 0% means you broke even, and a negative figure means you lost money.
ROI vs ROAS, and annualizing the return
ROI and ROAS get mixed up often. ROI counts profit against your full cost, so it reflects what you actually kept. ROAS counts revenue against ad spend alone, so a $4,000 return on $2,000 of ads reads as a 2x or 200% ROAS even before other costs come out. Use ROI when you want the honest picture.
Time matters too. A 100% ROI earned in three months is far better than the same 100% earned over three years. To compare projects of different lengths, annualize the return. For a return earned over six months, the rough annual equivalent is higher because the money turned over twice in a year.
For investment growth and CAGR across multiple years, use the investment return calculator instead, since it handles compounding over time. For a single-project return, the ROI calculator is the right tool.