FBA Replenishment Math: How Much to Order from China and When to Trigger the PO (2026)

If you sell on Amazon FBA and source from China, the classic dilemma is between waiting too long to reorder and ordering too much too early. Both hurt: a stockout costs ranking and recovery; overstock triggers storage and aged-inventory fees. This guide gives you a simple replenishment math framework and a weekly routine you can run with your operations team—so each purchase order (PO) is triggered at the right time, and your cash is not trapped in idle inventory.

Split your lead time into five stages

The first step is to stop thinking of “China to FBA” lead time as a single number. Break it into five stages, each with its own owner and normal range. That way, when a delay happens, you know exactly where it happened and what to correct.

Add your own actual numbers to this table. The total is usually eight to ten weeks for sea, but your reality could be shorter or longer. Use a range, not a fixed number, and update it monthly.

Reorder point: how much and when

For each SKU, the reorder point (ROP) answers “when should I place the next PO?” A simple formula is:

ROP = (Average daily sales × Total lead time in days) + Safety stock

Safety stock covers the upside case: your sales spike, or the next shipment is delayed. It is not “extra” inventory; it is the minimum buffer you need to avoid a stockout while still keeping your inventory below Amazon’s restock limits and fee thresholds.

Let’s look at two examples with different lead times. These are hypothetical, so use your own numbers.

Example A – Sea freight (total lead time 60 days)

• Daily sales: 20 units/day
• Safety stock: 300 units
• ROP = (20 × 60) + 300 = 1,500 units

When your available inventory plus in-transit units falls below 1,500, you trigger a new PO.

Example B – Air freight (total lead time 20 days)

• Daily sales: 20 units/day
• Safety stock: 150 units
• ROP = (20 × 20) + 150 = 550 units

With air, you can run with a lower ROP because the lead time is shorter. But air costs more per unit, so you need to decide when to use sea, when to use air, and when to combine both in a “sea base load + air top-up” plan.

The in-transit deduction

Always apply the in-transit deduction before triggering. Your available inventory in Amazon is not the whole picture. Add units that are already on the water or in the air. Then compare that total to the ROP. Example:

• Available in FBA: 1,100 units
• In transit: 250 units
• Total: 1,350 units
• ROP: 1,500 units

Since 1,350 is still below 1,500, you trigger the next PO now. If the in-transit count were 500, total would be 1,600, and you could wait another few days.

The 2026 fee corridor — why “more safety stock” is not automatically better

In 2026, Amazon’s inventory policies push you to keep a “just right” buffer rather than a big one. Three mechanisms matter:

• Low-inventory fee applies to SKUs with fewer than 28 days of sell-through inventory (when historical inventory is below a certain threshold). It penalizes running too lean.
• Aged inventory surcharge applies to inventory aged 181 days or more. The longer stock sits, the more expensive it becomes.
• Restock/storage limits cap how much total inventory you can hold in Amazon’s fulfillment network, often around five months of predicted sales.

Together, these create a “fee corridor”: your safety stock should be large enough to avoid low-inventory fees, but small enough to avoid aged-inventory surcharges. There is no single number — you need to model your own sales volatility.

Always verify current thresholds and rates with the official Amazon Seller Central pages for the specific year and marketplace. Go to Seller Central Help and search for “FBA inventory fees” or “Aged inventory surcharge.” These pages are the single source of truth.

How a China warehouse with SKU-level pre-stocking changes the trigger line

If you only ship directly from the supplier to FBA, your effective reorder point is locked to the full 8–10 week lead time. A China warehouse can change that by holding your finished goods before the first mile. Here’s how the math shifts:

• You ship finished goods from the supplier to a China consolidation warehouse (not directly to FBA).
• The warehouse receives, inspects, and optionally repacks/combines your SKUs.
• You then trigger FBA inbound shipments from that warehouse. Because the goods are already in China, the remaining lead time to FBA can be much shorter than the full supplier-to-FBA time.
• You can also ask the warehouse to hold “SKU-level pre-stock” — meaning you keep a small buffer of each SKU in China, ready to ship on demand.

This does not eliminate production lead time, and it may add a small handling/storage fee. It gives you a faster “second leg” for reordering: instead of waiting 8–10 weeks for new production, you can refill from the China warehouse in days. It is especially useful for SKUs with variable demand, or as a bridge between a sea shipment and the next production cycle.

The trade-off: you are paying for storage and handling in China. The mechanism is useful when the cost of that buffer is lower than the income you lose during a stockout, or when it lets you postpone a more expensive air shipment. It is not automatically “cheaper” — it is a risk-management tool.

A weekly replenishment routine + seasonal backward planning

Replenishment is not a once-a-month exercise. Run this 30-minute routine every week for your top SKUs:

1. Pull average daily sales over the last 14 and 30 days.
2. Update the in-transit quantity for every open PO.
3. For each SKU, compare: available in FBA + in-transit < ROP? If yes, trigger a new PO.
4. Check the 28-day sell-through metric to make sure a SKU is not about to enter low-inventory fee territory.
5. Check aged inventory: any unit older than 181 days? If yes, plan a promotional action or removal order.

For events like Prime Day or Q4, work backward from the delivery-window drop-dead date. Start with the FBA “ship by” date, subtract the five lead-time stages, and set a supplier PO date. Build this as a simple calendar and revisit it every two weeks.

Seasonal rule of thumb: if your peak season demand is 1.5× your normal rate, your safety stock formula should use the peak daily sales number for at least the eight weeks before the event.

Start with one SKU, then scale the routine

The math is not complicated, but it has to be repeatable. You don't need a huge buffer — you need a buffer that is just right for your demand, your lead time, and Amazon's fee structure. Start with a simple ROP model, split your lead time into stages, and update it weekly. When you have a grey area, or when a specific SKU keeps overstocking or stocking out, that's where a workflow review pays off.

To dig deeper: review the merchant fulfillment model, read the companion guide on FBA prep and labeling, or learn more about consolidation. If you want help setting up a replenishment rhythm around your own supply chain, request a workflow review and our operations team can look at your current PO process. When you're ready, create a merchant account and start with one SKU.