Sending inventory to FBA can feel like packing for a trip with uncertain weather. Bring too much and you pay for heavy bags. Bring too little and you’re stuck wearing yesterday’s clothes.
In 2026, the “right” amount of amazon fba inventory is the amount that keeps you in stock through lead time, stays inside your capacity limits, and avoids aged inventory fees. The goal isn’t a full warehouse, it’s a steady flow.
This guide gives you a simple, spreadsheet-friendly way to decide shipment quantities, plus a worked example you can copy.
What changed in 2026 that should shape your shipment size
First, inventory planning is tighter because costs add up faster when you over-send. Amazon adjusted fee schedules again in 2026, including fulfillment fee changes (effective mid-January). Before you lock in margins or decide on larger shipments, confirm your current per-unit fees using Amazon’s official guidance on FBA fulfillment fees. If you sell cheaper items, also check the low-price FBA fees because small per-unit changes can flip profitability.
Next, capacity limits still matter. Your account’s restock limits are tied to storage type and performance signals (often discussed around IPI). If you’re close to your cap, the “how much to send” question becomes “how much can I send this week without getting blocked.” Amazon explains the mechanics in Restock limits by storage type (FAQ). Check your limits before creating shipments, not after.
Finally, storage is not a flat cost. You pay more in peak months, and you can also get hit with aged inventory charges once units sit too long (commonly after 181 days). If you want a quick way to sanity-check how storage can stack up, this FBA storage fees calculator overview is a helpful reference point. Still, always confirm the exact 2026 rates in your own Seller Central fee tables.
Big idea: In 2026, “send more to be safe” often becomes “pay more to be sorry.”
A practical restock formula for Amazon FBA inventory (with spreadsheet-style formulas)
You don’t need perfect forecasting. You need a repeatable method that updates every week.
Step 1: Estimate daily demand from recent sales
Use 30, 60, and 90-day sales to smooth out noise. A weighted average keeps it grounded in recent velocity.
Step 2: Calculate true lead time to “available for sale”
Include supplier prep, freight, customs (if any), Amazon receiving, and the delay between “delivered” and “checked in.”
Step 3: Add safety stock in days, not vibes
Pick a number of “buffer days” based on volatility. For stable items, 7 to 14 days is common. For seasonal or spiky SKUs, go higher.
Step 4: Choose a review period (how often you replenish)
If you can send shipments every 2 weeks, you don’t need to hold 90 days at FBA. A shorter review period reduces risk and storage exposure.
Here’s a compact variable table you can drop into a spreadsheet. Adjust names to match your sheet.
| Variable | What it means | Spreadsheet-style formula example |
|---|---|---|
| Sales_30 | Units sold last 30 days | (enter value) |
| Sales_60 | Units sold last 60 days | (enter value) |
| Sales_90 | Units sold last 90 days | (enter value) |
| Daily_Demand | Weighted units per day | 0.5*(Sales_30/30)+0.3*(Sales_60/60)+0.2*(Sales_90/90) |
| LeadTime_Days | Days until sellable at FBA | Supplier_Days+Ship_Days+CheckIn_Days |
| Review_Days | Days between shipments | (example: 14) |
| Safety_Days | Your buffer in days | (example: 10) |
| LeadTime_Demand | Units needed during lead time | Daily_Demand*LeadTime_Days |
| Safety_Stock | Units for the buffer | Daily_Demand*Safety_Days |
| Target_Level | What you want available total | Daily_Demand*(LeadTime_Days+Review_Days)+Safety_Stock |
| Send_Qty | Units to send now | MAX(0,Target_Level-(FBA_OnHand+Inbound_Units)) |
After you calculate Send_Qty, round up to case-pack quantities and re-check your restock limit. If your limit blocks the full quantity, shorten Review_Days and send smaller replenishments more often.
Worked 2026 example (30/60/90-day sales, lead time, safety stock, shipment quantity)
Assumptions (keep yours explicit like this):
- Sales_30 = 300 units, Sales_60 = 540 units, Sales_90 = 720 units
- Supplier production = 18 days
- Shipping to Amazon + appointment time = 12 days
- Amazon check-in lag = 7 days
- Review period = 14 days (you plan to replenish every two weeks)
- Safety buffer = 10 days
- Current FBA on-hand = 210 units
- Inbound already created = 80 units
1) Daily demand (weighted)
- 30-day rate = 300/30 = 10.0/day
- 60-day rate = 540/60 = 9.0/day
- 90-day rate = 720/90 = 8.0/day
- Daily_Demand = 0.5(10.0)+0.3(9.0)+0.2(8.0) = 9.3/day (rounded)
2) Lead time days
LeadTime_Days = 18+12+7 = 37 days
3) Safety stock
Safety_Stock = 9.3*10 = 93 units
4) Target level (what you want covered)
Target_Level = 9.3*(37+14)+93
= 9.3*51 + 93
= 474 + 93
= 567 units (rounded)
5) Shipment quantity to send now
Available = FBA_OnHand + Inbound = 210+80 = 290 units
Send_Qty = 567-290 = 277 units
If your case pack is 24, round to 288 units. That’s your practical shipment quantity for this cycle.
How to avoid over-sending in 2026 (and paying for it later)
Over-sending usually shows up as rising storage costs, aging inventory, and stuck listings. Keep an eye on these signals: weeks of cover creeping up, sell-through slowing, and a growing “stranded” count.
When you do need to clear inventory, price cuts aren’t the only tool. Removals and disposals can be faster, but they have per-unit fees. Verify current charges on Amazon’s official removal and disposal fees page. Those fees can sting on oversized items, so run the math before you send 6 months of stock.
Also, remember that inbound shipping costs rose over time across carriers. Even if you’re sending less, shipment planning still matters. This 2026 guide to shipping to Amazon FBA is a useful refresher on shipment setup and cost drivers.
How to avoid under-sending (the quiet profit killer)
Stockouts don’t just pause sales, they often cause a slow restart. You can lose rank and ad momentum, then spend weeks buying it back with higher ad costs.
To reduce that risk, treat lead time as “time until sellable,” not “time until delivered.” In busy periods, check-in delays stretch, so Safety_Days should rise.
Q4 planning: change the buffer, not the whole system
For Q4, keep the same formulas, but adjust inputs:
- Increase Safety_Days (often 2x your normal buffer).
- Re-check LeadTime_Days (carriers and FC receiving can slow down).
- Shorten Review_Days if you can replenish faster from a 3PL.
- Stage backup inventory outside FBA when limits are tight. If you use Amazon Warehousing and Distribution (AWD), compare AWD storage costs and transfer timing against your 3PL, then choose the cheaper “waiting room.”
Conclusion
In 2026, the best shipment size is the one that covers lead time plus a realistic buffer, while staying inside your restock limits and avoiding aged inventory risk. Use the Target_Level formula, update it weekly, and keep your replenishment rhythm steady. Before you commit cash, confirm current fees and policies in Seller Central because Amazon can update rates and limits during the year. If you build the habit now, Q4 feels less like a gamble and more like a plan.