How much should I send to FBA, and when?

Why FBA stock runs low

Stock runs low at Amazon for a handful of plain reasons. Sales rose and your reorder point did not. A supplier or a shipment ran late. You hit your capacity limit and could not send more. Stock was sent but is still being checked in. Or you hold plenty, but it is stranded or reserved and so not counted as available. Check the last two in the FBA Inventory page before you assume you need to order. My guide to FBA inventory problems covers them.

What you need to know first

• Daily sales. Units sold in the last 30 days divided by 30. For a seasonal product use last year's same month as well.
• Total lead time. Days from placing the order with your supplier until the units show as available at Amazon: making time, shipping, and check-in at the warehouse.
• Safety days. Extra days of cover for delays and sales spikes. Fourteen is a common start. Raise it for suppliers who run late.
• Your stock position. Units available at Amazon, plus inbound, plus on order with your supplier.
• Unit size. Cubic feet per unit, since Amazon measures storage and capacity in volume.

The restock formula

1. Reorder point = daily sales x (total lead time + safety days).
2. When your stock position falls to the reorder point, place an order.
3. Order quantity = daily sales x (lead time + safety days + days of cover you want) minus your stock position.
4. Check the result: stock after it lands should be under 90 days of supply, since Amazon treats stock as excess above that, and under your capacity limit.

A worked example (made-up numbers)

You sell 12 units a day. Making takes 25 days, sea freight 30 days, and Amazon check-in 7 days, so total lead time is 62 days. You add 14 safety days.

• Reorder point = 12 x (62 + 14) = 912 units.
• Today your position is 900 units, so it is time to order.
• You want 45 days of cover after the safety days. Order quantity = 12 x (62 + 14 + 45) minus 900 = 1,452 minus 900 = 552 units.
• Check: by the time the order lands, 62 days of sales have used 744 units, leaving 156. Add 552 and you hold 708 units, which is 59 days of supply. That is under 90, so you are safe on excess stock.
• Space: at 0.05 cubic feet a unit, 708 units fill about 35 cubic feet. At Amazon's standard-size storage rate of $0.78 per cubic foot from January to September that is about $28 a month. At the October to December rate of $2.40 it is about $85 a month. Both rates are from Amazon's supply chain page as of November 2026.

A calculator layout

Build one sheet with a row per SKU and these columns: SKU, daily sales, production days, transit days, check-in days, safety days, total lead time, reorder point, units available, units inbound, units on order, stock position, days of supply, order quantity, cubic feet per unit, and cubic feet after landing. Add a flag column that says "ORDER NOW" when the stock position is at or below the reorder point and "TOO MUCH" when days of supply is over 90. Color those two flags and you have a working alert system in a spreadsheet. My FBA inventory management guide has a longer layout that adds your other channels.

Capacity limits and lead times

Amazon limits how much you can send. Its 2023 announcement describes a single monthly capacity limit measured in cubic feet, announced in the third full week of each month in the Capacity Monitor, with estimates for the next two months. Amazon says the limit depends on your IPI score (the Inventory Performance Index, a 0 to 1,000 inventory health score), forecasts, shipment lead time and warehouse capacity. It also lets sellers request more space by naming a reservation fee. Amazon's current FBA page still says restock limits and warns that going over them can raise storage costs. Check your Capacity Monitor each month and plan orders around the next two months' estimates.

If you keep hitting limits, Amazon Warehousing and Distribution (AWD), a bulk storage service, may help. Amazon says AWD costs $0.48 per cubic foot per month, and that AWD auto-replenishment sends stock to FBA for you with no FBA capacity limit worries and no low-inventory charges. It adds a second storage step, so read the terms first.

Amazon's own restock tool

The Restock Inventory tool in Seller Central gives recommendations from your sales history and demand and seasonality forecasts. I use it as a second opinion. It only sees Amazon sales, so if you also sell on your own site, your true demand is higher than it shows. Compare its number to your formula. If they differ a lot, find out why before you order.

Tools with prices

• Amazon Restock Inventory. Inside Seller Central. Covers Amazon only.
• InventoryLab, a stock and shipment tool for Amazon resellers. $69 a month, or $58 a month billed yearly. Its Restocking Insights add-on, which tells you how much and when to reorder from sales trends and stock, is $25 a month, as of November 2026.
• sellerboard, a profit and inventory dashboard. From $19 a month for up to 3,000 orders a month. Its plans list inventory management and listing change alerts.
• Cogsy, a demand planning tool, has been discontinued, so skip it if an older article recommends it.

For a wider view of the tools around a store, see tool stack for an ecommerce store.

Set up alerts so you hear before you run out

1. Pick the reorder point for each SKU from the formula.
2. In your sheet, flag any SKU at or under the reorder point.
3. Send yourself the list every Monday. A reminder or a short automation does it. See business process automation.
4. Add a second flag for stock over 90 days of supply.
5. Review the reorder points every quarter, and before Q4 when storage costs jump.

Common mistakes

• Counting only available units and forgetting inbound and on-order stock, which causes double orders.
• Using a 30-day average in a month when demand is about to jump.
• Ignoring check-in time at the warehouse.
• Sending a full year's stock in one go and paying for it all through Q4.

How I'd approach this

Start with your top 10 SKUs, since they decide most of your risk. Set safety days by how late each supplier really is, not by habit. Use Amazon's tool as a check, not a boss. Move to a paid tool when you have enough SKUs that the sheet takes hours to maintain. If you sell on Amazon and other channels, forecast on total demand, not Amazon alone. That needs one view of all your stock, which is the case for a small connected system when no ready-made tool fits.