31.25% ROI sounds like a hedge fund flex until you realize it came from flipping a $2,400 project into a $750 profit. Percentages lie upward when the denominator is small. I still love ROI for quick comparisons - I just don’t let a big % seduce me without looking at dollars risked and dollars back.
The version that fits on a sticky note
Cost = cash you put in for this specific bet. Gain = what you walk away with minus that cost (after fees if you’re being decent). ROI% ≈ (gain / cost) × 100. Time can be layered on later; don’t let perfect be the enemy of a useful snapshot.
Work the numbers once, slowly
Parts and labor ran $2,400. You sold for $3,150 net after marketplace fees. Gain = $750. Divide by $2,400 → 31.25%. Punch it into the ROI calculator if you want the machine to agree with you.
The mistake I still see in Slack threads
People type revenue as if it were gain. Revenue is the top line; gain is what’s left after the $2,400 (or whatever you spent). Confuse those and you’ll walk into a meeting defending a number that dissolves the second someone asks for a P&L line.
Afterthoughts
Negative ROI isn’t shameful - it’s information. If you annualize, say so; a 40% return in 9 days is not the same species as 40% over four years.
For funnel-shaped spend, the marketing ROI calculator and the business ROI write-up on this blog use vocabulary closer to ad dashboards - same muscle, different costume.