"What's the ROI on this?" and "what's the margin on this?" sound like they're asking the same question. They're not, and mixing them up leads to bad comparisons between businesses, products, or investments that don't actually work the same way.
Profit margin: how much of each sale you keep
Profit margin measures profitability relative to revenue. In its simplest form:
Profit Margin = (Revenue - Costs) / Revenue x 100
If you sell something for $100 and it costs $70 to produce and deliver, your profit margin is 30%. This tells you how much of every dollar that comes in actually turns into profit, after costs. It's a measure of pricing power and cost control -- useful for comparing how efficiently two businesses convert sales into profit, or for spotting when rising costs are quietly eating into what looks like a healthy top line.
What margin does not tell you is how much capital you had to put in to generate that revenue in the first place.
ROI: what you got back relative to what you put in
Return on investment measures profit relative to the cost of the investment itself, not relative to revenue:
ROI = (Gain from Investment - Cost of Investment) / Cost of Investment x 100
If you put $10,000 into something and it's now worth $12,000, that's a 20% ROI -- regardless of how much revenue was involved along the way. ROI is the number you reach for when comparing whether it was worth tying up your money in this particular thing versus something else you could have done with it.
Where the confusion causes real problems
A business can have a thin profit margin and still be an excellent investment if it turns over inventory fast and requires little capital -- a grocery store might run on a 2-3% margin but generate a strong ROI because sales volume is high relative to the capital tied up. Meanwhile, a business with a fat 40% margin can be a mediocre investment if it required enormous upfront capital that's now sitting mostly idle.
The same split matters for a single purchase decision. Say you're comparing two pieces of equipment: one has a higher margin on each job it enables but cost far more upfront. Margin tells you about the ongoing sale; ROI tells you about the wisdom of the purchase. You need both, and they can point in different directions.
A word on time
Basic ROI also doesn't account for how long your money was tied up. A 20% return over one month is a very different result from a 20% return over five years, even though the ROI figure is identical -- this is why annualized ROI, which spreads the return out over time, is usually the more honest number to compare across options with different holding periods.
Use the right one for the question you're actually asking
If the question is "are we pricing this correctly and controlling costs," reach for margin. If the question is "was putting money into this worth it compared to the alternative," reach for ROI. Running the actual numbers instead of eyeballing them avoids the trap of assuming a high margin automatically means a good investment.
The ROI Calculator works out return on investment and annualized returns, and the Profit Margin Calculator breaks down profit margin from your revenue and costs -- worth running both when you're deciding whether something was actually worth doing.