Many Amazon sellers don’t ship straight from the factory to Amazon. They hold bulk stock in their own warehouse or a 3PL, sell other channels from it, and send stock into FBA in smaller batches. That’s sensible: it keeps storage costs down and gives you a buffer if Amazon limits what you can send.
It also makes planning harder. You now have two decisions per product: when to send stock into FBA, and when to order more from your supplier. Get the link between them wrong and you either run out on Amazon or order twice as much as you need.
Treat FBA as its own warehouse
Stock in your warehouse can’t be sold on Amazon FBA until it’s been prepped, shipped and received by Amazon. So for planning, treat FBA as a separate warehouse with:
- Its own demand: Amazon FBA orders only.
- Its own stock: units available and inbound at Amazon.
- Its own lead time: prep + carrier transit + Amazon receiving.
- Its own source: your warehouse or 3PL, rather than the supplier.
Your main warehouse then has:
- Demand from your other channels (Shopify, eBay, TikTok Shop, wholesale and so on).
- Plus planned transfers into FBA.
- Supplier lead time.
If you sell in more than one Amazon region (for example the UK and EU), each region is its own FBA warehouse.
The double-counting trap
Here’s the mistake that leads to overstock.
Suppose Amazon sells 20 units a day and Shopify sells 10 a day, all from one product. A spreadsheet might add up “total demand = 30 per day” and size the supplier order from that. Separately, it plans FBA transfers based on the 20 per day Amazon demand. Then someone adds the FBA safety stock on top again.
The right way is a chain:
- Forecast Amazon demand → plan transfers into FBA.
- Those planned transfers become demand on the main warehouse, alongside Shopify and other channels.
- Size supplier orders from the main warehouse’s demand only.
Amazon demand is counted once, through the transfers. The FBA forecast is never added to the main warehouse on top. This is how OmniRestock models it: FBA transfer suggestions count as demand on the source warehouse, so supplier POs cover them.
Step 1: work out the FBA transfer point
FBA transfer point = (Amazon daily demand × FBA lead time) + FBA safety stock
FBA lead time is:
Prep days + carrier transit days + Amazon receiving days
Use your own recent inbound history. Amazon receiving times vary by fulfilment centre and season, so look at how long your last few shipments took from dispatch to “received”. OmniRestock defaults to 14 days and you can change it.
Worked example
- Amazon UK sales: 20 units/day (in-stock days only)
- Prep 2 days + transit 3 days + receiving 7 days = 12 days
- FBA safety stock: 1.645 × σ (6) × √12 ≈ 1.645 × 6 × 3.46 ≈ 34 units
FBA transfer point = (20 × 12) + 34 = 274 units
When FBA available + inbound falls to 274, plan a transfer.
How much to send
Decide how many days of cover you want at Amazon once the shipment lands. Shorter cover means lower storage fees and less risk of aged inventory, but more frequent shipments.
Transfer qty = Amazon demand × (FBA lead time + target FBA cover) + FBA safety stock − FBA available − FBA inbound
With 30 days of target cover, 274 available, 0 inbound:
20 × (12 + 30) + 34 − 274 = 840 + 34 − 274 = 600 units
Then apply two caps:
- What your warehouse can spare. Don’t strip your main warehouse below what other channels need.
- Amazon’s capacity limit for that storage type (see below).
Step 2: work out the supplier order for your warehouse
Your main warehouse’s demand is now:
Other channels’ demand + planned FBA transfers
Worked example (continued)
- Shopify + eBay: 10 units/day
- Planned FBA transfers: roughly 20 units/day on average (600 units every 30 days)
- Total main-warehouse demand: 30 units/day
- Supplier lead time: 50 days (manufacturing 20 + sea 24 + customs 3 + receiving 3)
- Main-warehouse safety stock: 150 units (from its own variability)
Reorder point = (30 × 50) + 150 = 1,650 units (available + on order)
Notice the 30 per day here is built from transfers, not from adding the FBA forecast again. If you also added FBA safety stock and the Amazon forecast on top, you’d be ordering hundreds of units too many.
Step 3: check against Amazon capacity limits
Amazon sets FBA capacity limits that cap how much inventory you can send and store, measured by volume for each storage type. Amazon publishes your limits in the FBA Capacity Monitor in Seller Central and, per that help page, sets them monthly with estimates for the following months. Open shipments count towards your usage.
What that means for planning:
- Convert units to volume. A transfer of 600 units at 0.05 cubic feet each is 30 cubic feet. Store each product’s carton dimensions so you can do this sum.
- Prioritise when capacity is tight. Send your fastest-selling, highest-margin products first. Leave slow sellers in your own warehouse.
- Cancel shipments you won’t send. Open shipments use capacity.
- Keep a buffer at home. If capacity tightens, your own warehouse stock keeps other channels going and lets you top up FBA later.
Read more in Amazon FBA restock and capacity limits explained.
Step 4: create the inbound shipment
Once you’ve decided what to send, you need to create the shipment in Seller Central (Send to Amazon) with:
- SKUs and quantities
- Box contents (which units are in which carton)
- Carton weights and dimensions
- Labels for units (if required) and cartons
This is where errors creep in when it’s done by hand. OmniRestock turns approved transfer suggestions into inbound shipments through Amazon’s API, with box contents and labels, so you’re not copying numbers between screens. Nothing is created until you approve it.
Timing: order-by dates for both legs
The most useful number isn’t the reorder point, it’s the date you need to act by.
FBA send-by date = FBA stockout date − FBA lead time − (FBA safety stock ÷ Amazon daily demand)
Supplier order-by date = warehouse stockout date − supplier lead time − (safety stock ÷ daily demand)
Worked example
- FBA has 500 units, Amazon sells 20/day → stockout in 25 days
- FBA lead time 12 days, safety stock 34 units ≈ 1.7 days
- Send-by: 25 − 12 − 1.7 ≈ day 11
If day 11 has already passed, you’re late: even if you send today, there may be a gap before stock lands. OmniRestock shows this as Late, with the expected number of days out of stock, so you can decide whether to air-freight, pull forward a shipment, or accept the gap.
Where most FBA replenishment goes wrong
- Counting FBA demand twice in supplier orders.
- Using the factory lead time for FBA transfers, or the FBA lead time for supplier orders.
- Ignoring stockout days. If you were out of stock on Amazon for a week, your Amazon sales history understates demand. Exclude those days.
- Not linking multipacks. If you sell a 2-pack on Amazon that uses two units of the product you buy, each sale uses two units from the main warehouse.
- Sending everything. Overloading FBA increases storage fees and the risk of aged inventory charges.
FBA replenishment checklist
- Set up FBA as a separate warehouse per Amazon region.
- Link Amazon SKUs to your master products, with pack quantities.
- Record prep, transit and receiving days.
- Forecast Amazon demand (excluding stockout days).
- Calculate FBA transfer points and quantities.
- Cap by spare warehouse stock and Amazon capacity.
- Add planned transfers to main-warehouse demand.
- Size supplier POs from main-warehouse demand only.
- Create inbound shipments with box contents.
- Review weekly, or let software recalculate daily.
Frequently asked questions
Should I send stock straight from my supplier to FBA?
It can be cheaper per unit, but you lose flexibility. If Amazon capacity is tight, stock has nowhere to go. Many sellers send part of each shipment to FBA and keep the rest in their own warehouse or 3PL.
What’s a sensible FBA lead time?
Use your own data. Measure from when you start prepping to when Amazon marks the shipment received for your last several shipments. OmniRestock starts at 14 days until you set your own.
How many days of cover should I hold at FBA?
There’s no single right answer. Shorter cover reduces storage fees and aged-inventory risk; longer cover reduces how often you ship. Many sellers balance this against their capacity limits and how reliably Amazon receives.
Does Amazon Warehousing and Distribution (AWD) change this?
AWD adds another upstream location. The same logic applies: treat it as a warehouse with its own stock and lead time, and avoid counting the same demand twice.
Can I plan FBA from a 3PL like Mintsoft or ShipBob?
Yes. Connect the 3PL as the stock source for your main warehouse and the planning works the same way. See our Mintsoft and ShipBob integrations.
Next steps
- See how OmniRestock handles FBA restock planning.
- Read about multi-warehouse planning.
- Get a free stockout report for your Amazon and Shopify stock, with no card needed.