how do i calculate roas without a big budget?

Then compare it against your margin, not a number someone quoted online. 4x sounds great, but at a 20% margin your break-even ROAS is 5x, so 4x is losing money on every sale. Work out 1 divided by your margin first and you know the line you have to beat.

Revenue from the ads divided by what you spent on them. Two thousand back on five hundred spent is 4x, and you don't need a tool for that. The bit that trips people up is which revenue goes on top - it has to be what those ads actually drove, not your total sales for the month, or every campaign you run will look like a winner. Two things worth doing once you have the number. Work out your break-even ROAS from your margin first: if you keep a quarter of each sale, 4x is standing still, not success. And expect the platform's reported figure to be friendlier than your bank statement, because it takes credit for people who were going to buy anyway.