How much inventory to send to Amazon FBA
Updated October 2026 · By Artem Stepanenko, founder of SCAIT
Short answer. Send enough to cover FBA sales until your next shipment is live, plus a few safety days, minus what Amazon already holds and what is already inbound. As a formula, units to send = daily FBA sales × (days between shipments + Amazon receiving days + safety days) − (FBA available + FBA inbound). Then check it against your capacity limit.
Why FBA needs its own calculation
Most brands keep the bulk of their stock in a 3PL or their own warehouse and send FBA a slice every week or two. The question is not how much to buy from the supplier but how much to move from your warehouse to Amazon. Two Amazon rules push in opposite directions:
- Too little: Amazon charges a low-inventory-level fee on standard-size products when both the 30-day and the 90-day historical days of supply are below 28 days.
- Too much: Units that sit in FBA for 181 days or longer pick up an aged inventory surcharge, and everything you send counts against your capacity limit.
Step 1. Daily FBA sales
Use FBA sales only, averaged over the in-stock days of the last 4 to 8 weeks. If a promotion or Q4 is coming, use your expected rate, not last month's.
Step 2. Days the shipment has to cover
Add the days until your next shipment, the days Amazon needs to receive and check in a shipment (use your own recent history) and a few safety days.
Step 3. Subtract FBA stock and inbound
Subtract available units in FBA and units already in transit or being received.
Worked example
- FBA sales: 25 units a day
- You ship every 14 days; Amazon takes about 10 days to receive; 7 safety days
- Days to cover = 14 + 10 + 7 = 31 days → 25 × 31 = 775 units
- FBA available 300 + inbound 150 = 450 units
- Units to send = 775 − 450 = 325 units
With about 31 days of cover you stay above the 28-day line without parking months of stock at Amazon. If the Capacity Monitor shows you can't send 325, send what fits and plan the next shipment sooner.
Amazon rules change often. Check the current fee pages and the Capacity Monitor in Seller Central before you plan a shipment. Sources for the rules above: low-inventory-level fee overview; Amazon seller forum notice on the aged inventory surcharge.
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Related: How to calculate how much inventory to order · How to prevent stockouts and overstock
Questions founders ask.
How many days of inventory should I keep at Amazon?
Enough to stay above 28 days of supply so the low-inventory-level fee doesn't apply, and well below the 181-day mark where aged inventory surcharges start. For most brands that means a few weeks at Amazon and the rest in a 3PL or upstream storage.
What counts against my FBA capacity limit?
Stock already in Amazon's fulfillment centers and shipments on the way. Check the Capacity Monitor on the FBA dashboard before you create a shipment.
Should I send everything before Q4?
No. Send in waves based on expected Q4 sales and Amazon's published holiday inbound deadlines, so you don't hit capacity limits or pay for stock that sits past the season.
Is this the same as how much to order from my supplier?
No. This is the transfer from your warehouse to Amazon. How much to buy from the supplier is a separate calculation with the full production and freight lead time.