ROI vs margin: the same profit divided two ways

ROI and margin use the same profit and different denominators

"I made 160% on that!" and "my margin is 40%" can describe the same sale. Return on investment divides profit by what you spent; margin divides profit by what the buyer paid. Both are useful, but mixing them up leads to bad decisions, especially when setting a buying rule like "only buy at 3x" or "aim for 50% margin".

This guide defines both, works one sale through them, and shows when each one matters.

The two formulas

Profit here means sale price minus item cost, fees, shipping and packaging. Definitions vary; what matters is that you use the same one every time.

A worked example

In this chosen example, a vintage lamp costs $10 at a sale and sells for $40. Fees are $8, and shipping and packaging come to $6.

• ROI: $16 ÷ $10 = 160%.
• Margin: $16 ÷ $40 = 40%.

Same sale, same profit, two very different-looking percentages.

Why ROI can mislead

ROI looks spectacular on cheap items. In this second chosen example, a $1 paperback sells for $6 with $4 in fees and postage, leaving $1 profit: 100% ROI. But $1 is barely worth the time to list and pack. ROI says nothing about the absolute amount or the time it took. The true hourly tool is a better check on whether small wins are worth it.

Why margin can mislead

Margin looks modest on items that tie up a lot of money. A $150 camera that sells for $250 with $40 in fees and shipping leaves $60 profit: 24% margin, 40% ROI. That might be an excellent use of $150 if it sells in two weeks, and a poor one if it takes a year. Margin ignores speed. The sell-through guide and the cash flow vs margin guide cover the time side.

Step 1: Pick one profit definition and stick to it

Write down which costs go into profit: item cost, fees, shipping, packaging, and perhaps a return allowance. The return allowance guide explains how to include returns. Comparisons only work if every item uses the same definition.

Step 2: Use ROI for buying decisions

When standing at a sale table, ROI tells you how hard your money works. A buying rule like "at least 100% ROI after fees and shipping" protects you from paying too much. The sourcing ceiling tool works backward from a likely sale price to the most you should pay.

Step 3: Use margin for pricing decisions

When setting a price or accepting an offer, margin tells you how much room you have. If your margin at full price is 40%, a 15% discount takes a large share of it. The offer room tool shows how offers eat into margin.

Step 4: Add a minimum dollar profit

Both percentages can look fine on tiny amounts. Add a minimum profit per item, such as $8 or $10 in your own business, so low-value items don't pass just because the percentages look good.

Step 5: Review the numbers by category

Once a month, average ROI and margin for each category you sell. In your own records you might find that books average a high ROI on tiny dollar profits, while furniture averages a lower margin but large profits per sale. Neither is wrong; the averages simply show where your time and money earn most, and which buying rule fits each category.

Converting between the two

If you know one, you can find the other with the sale price and cost:

The mini calculator

The calculator on this page subtracts costs from the sale price to show what's left. Divide the result by your item cost for ROI, or by the sale price for margin.

Common mistakes

• Calling ROI "margin" or the reverse.
• Leaving fees or shipping out of profit so both look higher than they are.
• Chasing high ROI on items too cheap to be worth the time.
• Rejecting good, fast-selling items because their margin percentage looks modest.
• Using different profit definitions for different items.

Know which question you're asking

ROI tells you how well your money worked; margin tells you how much room each sale has. Use ROI when buying, margin when pricing, and a minimum dollar profit for both.