You need four inputs: total budget, expected cost-per-click (CPC), expected conversion rate, and average order value (AOV). Pull realistic CPC and conversion rate estimates from a past campaign or platform benchmark rather than guessing, since the forecast is only as good as the assumptions behind it.
The calculator then projects customers acquired, revenue, CAC, ROAS and profit for that budget, letting you compare two or three budget scenarios side by side before committing spend.