Use Amazon inventory age to make a safer reorder decision
Inventory age explains how long units have occupied fulfilment capacity. It does not tell you, by itself, whether the next purchase order should be larger, smaller or cancelled. Reorder logic needs age, demand, sellable stock, inbound stock, lead time and safety coverage together.
What inventory age can and cannot tell you
Age bands expose slow-moving units and the risk of aged-inventory charges. They are especially useful when recent sales hide an old layer of stock or when a seasonal item still has units left after demand has passed. Age does not measure future demand, supplier lead time or units already moving through the inbound network, so it cannot be the only reorder signal.
Amazon's FBA Inventory tool includes inventory-age and sell-through views. Amazon defines sell-through as units sold and delivered during the previous 90 days divided by average sellable units in fulfilment centres. Read the current definition and available controls in the official FBA Inventory tool guide.
Build one marketplace-level stock position
Calculate each SKU-marketplace independently before combining a regional total. Demand, fulfilment availability and inbound timing can differ by country. A global stock number may look comfortable while one marketplace is approaching a stockout and another is carrying old inventory. Keep sellable, reserved, unsellable and inbound units separate.
| Input | Question it answers | Common mistake |
|---|---|---|
| Sellable units | What can satisfy demand now? | Including reserved or unsellable stock |
| Inbound units | What is expected to become available? | Treating every created shipment as confirmed supply |
| Velocity | How quickly is demand consuming stock? | Using lifetime average for a seasonal SKU |
| Lead time | How long until replenishment is sellable? | Counting supplier production but not transit and receiving |
| Inventory age | How much stock is becoming costly or obsolete? | Assuming old stock means zero future demand |
Calculate coverage before reorder quantity
Days of cover equals usable units divided by a representative daily sales velocity. Use a recent window that matches the product's demand pattern and remove periods where the SKU was out of stock, because zero recorded sales during a stockout are not zero demand. Compare the result with total replenishment lead time plus a deliberate safety-stock window.
Inbound stock should enter the projection on its expected availability date. A shipment that has not moved, a shipment in transit and units being received carry different confidence. Apply a conservative arrival assumption rather than adding all inbound units to today's sellable balance.
Use age as a constraint on the order
- Healthy age and low projected cover support a normal reorder.
- Old inventory and falling velocity support a smaller order or no order.
- Young inventory with weak sales may indicate a new launch that needs more evidence.
- Old inventory in one marketplace should not block a justified reorder in another without transfer analysis.
- Unsellable or second-hand units should not inflate replenishment supply for a new-condition offer.
Amazon notes that aged-inventory charges apply after defined storage periods and that sound inventory health can reduce those costs. Fee thresholds can change, so avoid embedding one permanent surcharge table into reorder logic. Use the current marketplace fee source and keep the decision model focused on units, timing and margin impact.
Protect against two opposite errors
The first error is ordering from gross sales without subtracting current and confirmed inbound stock. That creates excess inventory. The second is refusing to reorder because some units are old even though the current selling rate and long lead time will exhaust usable stock before replacement arrives. A projection should show both the expected stockout date and the age risk.
Reorder when projected usable stock falls below demand through lead time plus safety stock, unless age, margin or lifecycle evidence provides a documented reason to constrain the order.
Review supplier orders before sending them
A proposed order should show supplier, SKU, marketplace, velocity window, current sellable units, confidence-adjusted inbound units, lead time, safety stock and recommended quantity. Exclude deleted, archived, dormant, second-hand and non-replenishable products before the table is produced. If a required source is missing, mark the row incomplete instead of converting an unknown value to zero.
Operate a repeatable weekly loop
- Refresh inventory, inbound and recent sales for every connected marketplace.
- Classify source completeness and remove ineligible SKUs.
- Calculate velocity, days of cover and projected stockout date.
- Overlay lead time, safety stock, age bands and unit economics.
- Review exceptions before approving supplier quantities.
- Track expected and actual inbound dates to improve future lead-time assumptions.
Stock Control combines those inputs into marketplace-aware reorder decisions. Continue with the Amazon FBA stockout guide for the detailed coverage and safety-stock workflow.