Start with the decision
Storage is tied to inventory and time, but a quick profitability comparison needs a per-sold-unit allocation. Slow sales can make that allocation larger. A low storage input based on rapid sell-through is not a promise that inventory will move.
Worked calculation
If your modeled storage expense for a period is 60 and you sell 120 units during that period, the allocation is 0.50 per sold unit. At 30 units sold with that same modeled expense, it becomes 2.00.
Compare fast and slow sell-through allocations
For each fulfillment route, start from the same selling price and subtract the referral assumption, landed product cost and route-specific fulfillment, inbound, storage or dispatch costs. The comparison is only as complete as the inputs; record exclusions alongside the result.
A period allocation is useful only when the period and units sold match. Divide the estimated storage expense for a defined period by units sold in that period, then label it as an average. If stock arrives late or sells unevenly, a simple average can conceal the months when inventory sat longest.
Check the changed-cost scenario
Run a faster and slower sell-through case. Slower sales can increase storage per unit and keep cash tied up longer even when the per-unit purchase cost is unchanged. Use the platform’s current reports and fee estimate for actual charges; a planning input is not a storage quote.
| Input or result | Value |
|---|---|
| Modeled period storage expense | 60 |
| 120 sold units | 0.50 per sold unit |
| 30 sold units | 2 per sold unit |
| Contribution difference per unit | 1.50 |
Both denominators must refer to sales in the same defined period as the storage expense. Costs also depend on actual inventory characteristics and timing, so this allocation does not replace an Amazon fee estimate or forecast how quickly stock will sell.
Use the example with your own inputs
Actual storage fees can change with inventory characteristics and timing. Use Amazon reports for applicable costs; the tool neither forecasts sell-through nor automatically applies the current fee schedule.
Amazon FBA vs FBM Profit Calculator | PourMarginScope and source checks
These are planning examples, not actual seller results. Keep currency, tax basis and cost allocation consistent. Enter your own figures and reconcile with your records; tax, demand, refunds and unlisted business costs can change the outcome.
Source: Amazon fee and cost estimates. This article uses illustrative manual costs, not current category fee quotes.