Start with last year's Q4 sales as a seasonal index (each peak month relative to your yearly average), apply that index to this year's baseline forecast, and order a full lead time before the ramp begins. Exclude any days you were out of stock last Q4, or the peak will read artificially low and you will under-build.
When should I order Q4 inventory?
A full lead time before demand climbs. If your total lead time is 80 days and your peak is late November, the purchase order goes out in late summer. Ordering when sales rise means the stock lands after the season has already started, and Q4 check-in delays make late stock even later.
Andrew Erickson is the founder of Inventory Hero. He has spent years working with Amazon FBA sellers on demand forecasting, restock planning, and the cash flow side of running a private-label brand. Inventory Hero exists because every spreadsheet-based inventory system he tried eventually broke — usually right before Q4.
How much Q4 inventory should I carry into January?
As little as possible. Q4 storage rates are higher while you hold the build through the peak (the elevated rate runs October to December), but the real January problem is slow demand: leftover peak stock sells slowly, ties up cash, and edges toward aged-inventory surcharges. Plan the drawdown so the seasonal SKU exits the season near zero, easing price down through December rather than dumping it in January.
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To forecast Q4 inventory for Amazon FBA, build from last year's seasonal index times your current baseline, order a full lead time before the ramp, and plan the January drawdown so you exit near zero. The short version: Q4 is the highest-stakes forecast of the year because the peak is large and the platform amplifies both mistakes, surcharges punish overstock and limits plus slow check-in punish understock. Below is how to build the number, when to order, and how to land the drawdown.
A flat average forecast misses the entire point of Q4. Build it the seasonal way:
Find last year's Q4 index. Divide last year's peak-month sales by that year's monthly average. If last November sold 640 units against a 400 monthly average, the November index is 1.6. The seasonality guide covers building indices in depth; here you just need the Q4 index.
Set this year's baseline. Forecast the non-seasonal run rate with a weighted average or trend method, say the SKU now averages 500 a month.
Multiply. This November's forecast is 500 x 1.6 = 800 units. The baseline captures your growth; the index captures the peak.
The single biggest Q4 forecasting trap is a stockout in last year's peak. If you ran out for two weeks last December, that month reads low and your index tells you December is weak, exactly backward, so you under-build this year and run out again. Pull last year's units-ordered history from Business Reports, use the Inventory Ledger report to find the zero-stock days, and drop them before computing the index. It is the same out-of-stock correction as sales velocity, and in Q4 one missed peak poisons the whole plan.
One year is the minimum, because you cannot see a Q4 pattern in less than a year. Two years is much better: it separates a real, repeating peak from a one-off (a viral moment, or a competitor stockout that sent you extra demand) that happened to land in Q4 once. If you have only one year, lean conservative on the index and watch early-season velocity closely, so you can adjust the order while there is still lead time left to act. If the SKU has also been trending hard, apply the index to the trended baseline, not last year's level, or you will plan the peak at last year's smaller size.
Work backward from the peak. If the ramp starts in early November and your total lead time is 80 days, the order goes out in mid-August. Two FBA realities make the timing even tighter:
Q4 check-in slows down. The leg you control least, Amazon check-in, commonly stretches to two or three weeks during peak, so stock you cut close lands after the ramp.
Capacity and restock limits tighten. The window when you most need to send inventory in is when restock limits are tightest, so a big seasonal build can hit the ceiling. Confirm your limit has room before you commit the full order.
Set the order date in the reorder point calculator by backing peak demand out through the full lead time, and place it early rather than perfectly.
Building for the peak is half the job. The other half is selling through. Q4 storage rates are higher while you hold the build through the peak (the elevated rate runs October to December), and once January arrives the rate drops back but so does demand, so leftover stock becomes slow dead stock that ties up cash and ages toward surcharges.1 Say you built to 800 units for November and December still runs hot at an index of 1.3 (about 650 units). January is the trap: its index is usually well below 1, often around 0.4, so a 500 baseline implies roughly 500 x 0.4 / 30, about 7 units a day. At that rate you clear only around 200 units in January, so anything much above that sits and ages. Ease the price down through December rather than dumping it in January, and aim for a seasonal SKU to exit the season near zero.
If you over-built, the held reserve should be the part you can redirect cheaply: stock at a 3PL or in AWD never paid FBA's peak storage and is easier to liquidate or carry than units stranded in a fulfillment center.
Flat-averaging the peak away. A yearly average forecast is always too low for November and December.
Letting last year's stockout depress the index. Exclude out-of-stock days or you under-build the peak.
Ordering when sales climb. The most expensive Q4 error: by then the lead time has already passed.
Forgetting January. Unsold peak stock no longer pays the Q4 rate once January hits, but it sits in slow post-peak demand, tying up cash and aging toward surcharges.
Forecasting Q4 inventory means building from last year's seasonal index on top of this year's baseline, excluding last Q4's stockout days, ordering a full lead time before the ramp, and planning the drawdown to exit near zero. It is the highest-stakes forecast you make all year, so make it deliberately, in summer. For the wider system, see Amazon inventory forecasting and restock planning.