Amazon FBA profit per unit: the real number after fees, storage, and ads

Amazon’s own dashboard tells you revenue, units sold, and maybe an estimated payout. None of those is profit per unit, and none of them includes the ad spend that got the sale in the first place. A seller who checks “sales” every morning and “profit” once a quarter is flying with half the instruments dark. The fix is a single number, recomputed per ASIN: what is actually left after every fee Amazon takes, every dollar you paid to make and ship the unit, and every dollar of PPC spend it took to sell it.

The formula

Profit per unit = price − referral fee − FBA fulfillment fee − monthly storage fee (allocated per unit) − inbound shipping per unit − COGS − PPC spend per unit sold

Each term is a real line item you can find in Seller Central or your own supplier invoices, not an estimate. The referral fee is Amazon’s cut for the sale, usually 15% of price for most categories, though it varies by category and is worth checking for yours. The FBA fulfillment fee depends on the size tier: a small standard item might run $3.50, a large standard item $5.00–$6.00, and oversize items considerably more. Storage is billed monthly per cubic foot and needs to be divided by units sold that month to become a per-unit cost. Inbound shipping is what you paid to get the unit into an Amazon warehouse, divided by units in that shipment. COGS is your manufacturing or wholesale cost. PPC per unit is total ad spend for that ASIN divided by units sold — not units clicked, units sold.

A worked example

Take a $27.99 kitchen gadget. The referral fee at 15% is $4.20. The item is large standard size, so FBA fulfillment runs $5.20. Monthly storage for this SKU works out to $0.35 per unit at current inventory levels. Inbound shipping from the supplier to the Amazon warehouse averaged $0.60 per unit across the last shipment. COGS is $6.50 per unit landed. PPC spend for the month on this ASIN was $412 against 68 units sold, so $6.06 per unit.

Add it up: $4.20 + $5.20 + $0.35 + $0.60 + $6.50 + $6.06 = $22.91 in total cost per unit. Profit per unit is $27.99 − $22.91 = $5.08, or about 18% of price. That is the number that matters, not the $27.99 on the listing page and not the “estimated payout” Amazon shows before ad spend is subtracted.

Change one input and the picture shifts fast. If PPC creeps to $8.00 per unit because competition on the keyword heats up, profit per unit drops to $3.14 — an 11% margin. If it drops to $3.00 per unit because the listing starts ranking organically, profit jumps to $8.06, nearly 29%. The same product, the same price, a completely different business depending on how hard the ad account has to work.

Why “margin after ads” per ASIN is the number

Most sellers track ACOS (advertising cost of sales) as a percentage and stop there. ACOS tells you what share of ad-attributed revenue went to ads; it does not tell you whether the unit was profitable, because it says nothing about referral fees, fulfillment cost, or COGS. A 25% ACOS looks fine on a slide but can still mean a loss if the product’s non-ad costs already eat 70% of price. Profit per unit after every cost, including ads, is the only number that answers the question a seller actually cares about: did this sale make money.

Running it per ASIN rather than per account matters for the same reason a store shouldn’t judge every product against one blended ROAS. A $12 accessory with a $2.50 fulfillment fee behaves nothing like a $60 kitchen appliance with a $9 fulfillment fee and heavy storage. Blend them into one “average margin” and you will keep funding the loser because the winner is covering for it in the total.

The storage-fee trap for slow movers

Storage fees are charged per cubic foot per month, and they do not care whether the unit sells. A slow-moving SKU that takes four months to sell through a batch pays storage for all four months, and after 365 days in a fulfillment center, Amazon adds long-term storage fees on top of the regular monthly rate — fees steep enough to erase a thin margin entirely. A unit that looked like $5.08 profit at a two-week sell-through can look very different at a four-month sell-through once storage is allocated honestly across the actual time it sat on the shelf, not the month it happened to ship.

The practical fix is to allocate storage per unit using real days-in-warehouse, not a flat monthly guess, and to flag any SKU whose sell-through has slowed enough that storage is climbing toward the long-term threshold. A product that was profitable at launch can quietly become a net loser six months later purely because inventory sat too long, with no change to price, fees, or COGS. Checking profit per unit only at launch and never again is how sellers keep running dead SKUs for a year.

Turning this into a weekly check

The routine that catches problems before they show up in a shrinking bank balance: pull price, referral fee, fulfillment fee, current storage rate, and this month’s PPC spend and units sold for every active ASIN; compute profit per unit and margin percent for each; sort by margin, not by revenue, so the SKUs quietly losing money surface at the top instead of hiding behind big sales numbers; flag anything under your target margin, and separately flag anything with storage age climbing past 200 days regardless of margin, since that is where the long-term fee trap starts.

The Profit Ritual — Amazon FBA Edition automates exactly this: paste your Seller Central data (price, fees, PPC spend, units) per ASIN and it computes profit per unit, margin percent, and a verdict for every SKU, with a separate storage-age warning so slow movers get flagged before long-term fees hit. Even without the spreadsheet, the formula travels: price minus referral fee, fulfillment fee, allocated storage, inbound shipping, COGS, and PPC per unit — run it per ASIN, every month, and check margin percent before you check revenue.