What a periodic review system does for FBA sellers
A periodic review system is an inventory replenishment method where you check stock levels at fixed time intervals and order enough units to bring inventory up to a predetermined target (the order-up-to level). Instead of monitoring every SKU in real time and placing an order the moment stock drops to a threshold, you batch reviews on a schedule: every Monday, every other Friday, the first of each month.
For Amazon FBA sellers running 50-500+ active ASINs, the periodic review system is often the practical default. Most operators already do some version of it without naming it: they sit down weekly, pull their inventory reports, and decide what to reorder. The question is whether they are doing the math correctly or just eyeballing it.
The tradeoff of a periodic review system vs. a continuous review (fixed order point system) is straightforward: periodic review is simpler to manage across many SKUs, but it requires more safety stock because you are exposed to demand variation during the entire review interval plus lead time.
The periodic review system formula
The key difference from a fixed-order-point formula: the protection period is R + L, not just L. That longer exposure window is why the periodic review system demands higher safety stock buffers.
Example: weekly periodic review
You sell a stainless steel water bottle at 15 units/day. Your supplier in Guangdong has a 60-day lead time, and FBA receiving typically adds 14 days. You review inventory every 7 days. Daily demand standard deviation is 4 units; service level z-score for 95% is 1.65.
- Safety stock: 1.65 x 4 x √81 = 1.65 x 4 x 9 = 59 units
- Order-up-to level: 15 x (7 + 74) + 59 = 1,215 + 59 = 1,274 units
At Monday’s review, you have 820 units on hand and 300 on order. Order quantity: 1,274 − 820 − 300 = 154 units. Next Monday, inventory is 710 with 154 still in transit: 1,274 − 710 − 154 = 410 units. The variable order size is a hallmark of the periodic review system.
FBA-specific considerations
The periodic review system maps well to how most FBA businesses actually operate, but Amazon’s infrastructure creates wrinkles the textbook version ignores.
Receiving delays inflate L. Add 14-21 days to your supplier lead time for FBA check-in processing. During peak seasons (August through October), receiving can stretch to 30+ days, which blows up your order-up-to calculation if you use a static lead time.
Restock limits cap your order. Even if the periodic review system formula says order 800 units, Amazon may only allow 400 into FBA. You need a contingency plan: split shipments over multiple weeks, use AWD as overflow, or shorten the review interval.
Storage limits penalize overshooting. The order-up-to level must stay within your capacity allocation. Ordering up to a theoretical target that exceeds your FBA storage cap means rejected shipments and stockout risk on the back end.
Common mistakes
- Using the same review interval for all SKUs. Fast-moving A-class products need weekly or even twice-weekly reviews. Slow-moving C-class items can safely use biweekly or monthly intervals. A flat weekly schedule wastes time on tail SKUs and underserves your top sellers.
- Forgetting to include on-order inventory. The formula subtracts both on-hand and in-transit stock. Sellers who only look at current warehouse levels double-order consistently, creating excess that triggers aged inventory surcharges.
- Not adjusting for seasonality. A periodic review system using a static average daily demand will under-order heading into Q4 and over-order in January. Recalculate d and SS at least quarterly, monthly if you sell seasonal products.