ROI or margin: which should I judge a supplier list on?
Both come from the same profit figure and both rank a supplier list — differently. Four lines, one fixed budget, and the sort order that actually produces the most money.
The short answer
If you have a fixed amount of cash and you want the most profit out of it, sort by ROI. Margin answers a different question — how far the price can fall before the line stops working — and profit per unit answers a third: what each piece of handling is worth. On a real supplier list the three rankings disagree, so the sort order you pick decides what you buy.
The two numbers, defined
Both start from the same profit figure: the shelf price, less Amazon's fees, less the VAT you hand over, less what the unit cost you to buy and prepare.
- ROI = profit ÷ the money you put in (cost plus prep). "For every pound I spend, how many pence come back?"
- Margin = profit ÷ the shelf price. "Of what the customer pays, how much is mine?"
The denominators are the difference, and that is why they rank differently. ROI measures the money you had to find. Margin measures the price you had to charge.
One list, three rankings
Four invented supplier lines, all Toys and Games, all FBA, £0.20 of prep per unit, seller VAT registered. Fee figures are Amazon's published UK rates from the rate card our calculator uses.
| Line | Cost | Sells at | Amazon fees | Profit | Margin | ROI | Break-even |
|---|---|---|---|---|---|---|---|
| A | £8.40 | £24.99 | £6.60 | £5.62 | 22.5% | 65.3% | £16.27 |
| B | £3.10 | £11.99 | £4.30 | £2.39 | 19.9% | 72.4% | £8.47 |
| C | £22.00 | £54.99 | £11.53 | £12.09 | 22.0% | 54.5% | £37.20 |
| D | £14.00 | £34.99 | £8.35 | £6.61 | 18.9% | 46.5% | £25.27 |
Sort that table three ways and you get three different answers:
- By ROI: B, A, C, D
- By margin: A, C, B, D
- By profit per unit: C, D, A, B
B is the best line and the third-best line, depending on which column you read. That is not a paradox; the columns are answers to different questions.
What the same £600 buys
Give each line the same £600 of buying money — cost plus prep, whole units, one buying cycle — and the ambiguity disappears.
| Line | Units for £600 | Actually spent | Profit on the lot |
|---|---|---|---|
| B | 181 | £597.30 | £432.59 |
| A | 69 | £593.40 | £387.78 |
| C | 27 | £599.40 | £326.43 |
| D | 42 | £596.40 | £277.62 |
The order is exactly the ROI order, and it has to be: ROI is profit per pound spent, and this is pounds spent × profit per pound. The margin ranking would have sent you to A, which is £45 worse on the same money. Sorting by profit per unit would have sent you to C, which is £106 worse.
What ROI does not know
- How much work 181 units is. Every unit is a label, a bag, a line on a box content form and a slot on a shelf. C reaches three quarters of B's profit on 27 units. If your Saturday is the constraint rather than your bank balance, C is the better line.
- Storage. Fulfilment fees are per unit; storage is per cubic metre per month. A slow-moving bulky line pays rent that no ROI figure at the point of purchase can see.
- Sell-through. 72% on stock that takes six months to clear is worse than 46% that turns twice in three months. Neither ROI nor margin knows how fast anything sells; that comes from your own sales history, or from an honest guess you write down.
- Concentration. 181 units of one ASIN is one price war away from being 181 units of a problem.
What margin is actually good for
Margin is a resilience measure. Compare each line's price with its break-even and you get the room a price can fall into before the sale stops paying:
- A: £24.99 against £16.27 — 34.9% of the price to give away
- C: £54.99 against £37.20 — 32.4%
- B: £11.99 against £8.47 — 29.4%
- D: £34.99 against £25.27 — 27.8%
That is the margin ranking again, in different clothing. So use margin for the question it answers well: how much competitive pressure this line survives. On a crowded ASIN where the price will be tested, the higher-margin line is the safer buy even at lower ROI — and it is the number that decides where your price floor can sit.
A decision path
- Filter first, sort second. Drop everything below the minimum ROI you will accept, and everything below the margin you need on a contested ASIN. Sorting an unfiltered list just puts the riskiest rows on top.
- Sort what survives by ROI if cash is the constraint, by profit per unit if handling time is.
- Check the fee band. A price near £10 or £20 sits next to a fee step, and a small price change can move the whole line — how UK fee thresholds work.
- Check what you already sell. A restock priced against a cost you already know is a better bet than a new ASIN at the same ROI.
- Write down the sell-through you are assuming. If you would not say it out loud, do not buy on it.
Where Eve helps
Eve Scout prices a whole supplier file rather than the first fifty rows: map the columns once, and every line comes back with the live selling price, the referral and fulfilment fees, profit per unit, ROI and margin as sortable columns, with minimum ROI and minimum margin filters over the top. Lines are matched by barcode against your own catalogue first, so a restock is obvious as a restock and keeps the SKU you already have.
The definitions are the same ones in this article, and the same ones Performance uses on orders you have already shipped — the fee maths applied forward instead of backwards. A line worth buying takes a pick quantity and becomes an inbound Workflow, so the analysis and the shipment are not two separate pieces of work.
Common questions
Is ROI or margin more important for Amazon FBA?
ROI, if you have to choose one, because it ranks purchases by what they return on the cash you have. Margin still matters as a second filter: it tells you how far the price can fall before the line stops paying.
What ROI should I aim for?
There is no correct figure, because it depends on how fast your stock turns and what your overheads are. Set it as a rule you can defend — a minimum you will not buy below — rather than adopting a number from someone else's business, and raise it when your cash is the constraint.
Should ROI be calculated on cost alone or cost plus fees?
On the money you actually put in before the sale: cost of goods plus prep and inbound. Amazon's fees are deducted from the proceeds, not funded by you up front, so they belong in the profit figure rather than in the denominator. Whichever convention you use, use one — two conventions on one list is how a buying rule stops meaning anything.
Can a line have good ROI and still be a bad buy?
Easily. High ROI on many cheap units can mean a lot of handling, a lot of shelf space and a slow sell-through, and neither ROI nor margin measures any of those. Both are point-in-time figures at one assumed selling price.
Sources and scope
Checked 10 September 2026. Fulfilment and referral fees are Amazon's published UK figures for the rate card effective 1 July 2026, as used by our FBA calculator; current rates are on Amazon's UK selling fees page. The four supplier lines, their costs and their prices are invented; every derived figure was computed with the calculator's own arithmetic rather than rounded by hand. Profit is contribution after stock, prep, Amazon's per-order fees and output VAT for a registered seller — it excludes storage, returns, advertising and overheads. The £600 comparison assumes one buying cycle in which everything sells, which is an assumption, not a forecast.