Know your real FBA profit before you source a single unit.

Every failed FBA launch has the same autopsy line: "I thought my margin was 35%." The selling price was right. Amazon's fees were right. The cost was wrong — or incomplete. The calculator did its job; the human skipped half the ledger.

Landed cost means what you truly pay to have one sellable unit in an Amazon box, ready to scan at the fulfillment center. Not factory price. Not "about $4." Landed. If you treat COGS as the invoice from your supplier, you are calculating a fantasy margin that will evaporate the week your first shipment lands.

Rule operators repeat until it sticks: if it is not in landed cost, it is not in margin. Customs, duties, inbound freight, labeling, inspection rejects, and payment fees all belong in that single dollar figure — per unit, not per container averaged in your head.

Payment terms matter more than beginners think. A supplier offering Net 30 sounds friendly until you are paying Amazon storage on inventory you have not sold through yet — but that is cash flow, not margin. For margin math, include the financing cost if you are putting the PO on a card or line of credit: even 1% per month on a 60-day cycle is real money on a $15,000 order.

Another quiet killer is yield loss. Order 500 units, receive 480 sellable after damage and QC fails. Your landed cost per sellable unit is total spend ÷ 480, not ÷ 500. Experienced importers add a 3–5% yield haircut on new SKUs and 1–2% on repeat runs. Type the adjusted figure into the calculator every time.

Finally, resist the urge to "round down" cost to make the margin look acceptable in a sourcing meeting. The calculator is a truth machine. Give it the ugly number. If the margin dies, you just saved four months and twelve thousand dollars — which is the entire point of running this check before you wire a deposit.

The calculator uses packaged shipping weight — what Amazon measures when the unit arrives at the fulfillment center, not the bare product on your kitchen scale. That distinction is worth repeating because it is the single most common input error we see: sellers weigh the product, forget the polybag, insert, box, and padding, then wonder why Fee Preview shows $0.80 more per unit.

Packaging is a profit center. Shaving two ounces can drop you into a cheaper fulfillment bracket on small standard — or delay the jump to large standard. Industrial designers on mature brands obsess over this; solo sellers should too.

Auto-detect vs manual tier. The calculator's auto tier maps weight to Small Standard (≤1 lb), Large Standard (≤20 lb), then bulky. That covers most catalog items. If your product is physically large but light — hollow decor, empty storage bins, oversized packaging for a small item — dimensional weight may push you into a higher tier than actual weight suggests. When in doubt, select the tier manually and use the higher fee. Optimistic tier selection is how "profitable" products become return labels.

Unit conversion trips up international sellers daily. The weight field accepts oz, lb, g, and kg — but mental math errors are common. 1 lb = 16 oz exactly. A product you think is "15 oz" on a metric scale reading 425 g is actually 15.0 oz — fine. 480 g is 16.9 oz — you may have crossed into large standard if boxed weight pushes past 16 oz. When sourcing from metric suppliers, convert in the calculator; do not round in your head.

Multi-pack listings deserve their own row in your vetting sheet. A 3-pack sold as one ASIN has the combined shipping weight of the bundle. Fulfillment on a 3×12 oz bundle is not the same as three single units — and referral is on the bundle price once. Run the calculator as the customer buys it.

The calculator shows net margin before advertising, storage, returns, and inbound placement. That is intentional — those costs vary by seller maturity and strategy. But beginners treat "25% margin" as spending money. Operators treat it as 25% minus TACoS minus return drag minus storage.

ROI vs margin: the calculator shows both. Margin is profit ÷ price — what you keep per dollar of revenue. ROI is profit ÷ cost — what you earn on capital tied in inventory. A low-cost, low-price gadget can show modest margin but huge ROI; a heavy $80 item can show healthy margin but slow capital turn. Use ROI when comparing against other uses of the same cash; use margin when judging ad spend headroom.

Experienced sellers rarely launch below 15% calculator margin in competitive niches unless they have a verified path to lower COGS at scale or organic ranking without paid ads. Thin margin plus paid launch equals charity work.

Inventory aging is the silent partner to margin. A 30% margin product that sits 180 days pays monthly storage and possibly aged inventory surcharges — not modeled here yet, but real. High margin plus slow turn can lose to moderate margin plus fast turn. After you vet unit economics here, run total order size through the Inventory Breakeven Calculator to see how many units you must sell to recover the PO.

1. 1

   Base case — honest inputs

   Expected sell price, true landed cost, correct category, packaged weight, surcharge on. Save the margin and ROI. This is your anchor.

2. 2

   Bear price — minus 15%

   Competitors run coupons; new sellers race to the bottom. If margin sub-10% at −15% price, you need a defensible moat (brand, bundle, patent) or walk away.

3. 3

   Cost shock — plus 10% COGS

   Factories requote, tariffs shift, freight spikes. If +10% cost erases profit, negotiate volume breaks or pass — do not hope.

4. 4

   Weight creep — plus 4 oz

   Simulate heavier packaging or supplier drift. If fulfillment tier jumps, fix packaging before the PO — not after the first FBA receipt.

5. 5

   Category pessimism — higher referral category

   Re-run with the pessimistic category if your listing could be reclassified. If it still works, sleep better.

Five minutes, five runs, one decision. This is faster than opening a Seller Central session, faster than a paid suite for pure margin questions, and more honest than a supplier's "estimated FBA fee" cell in a quote sheet.

Without an ASIN: that is the point of this tool. Amazon's Revenue Calculator wants a live catalog item. You are often vetting from a competitor link, a factory spec sheet, or a trade show sample. Weight estimate from a similar ASIN plus category from the browse node is enough for a directional answer — then tighten inputs when samples arrive.