A fast-selling SKU can become a stockout long before it looks urgent in Seller Central. The reason is simple: the inventory you see is not the inventory you can sell tomorrow. To calculate FBA reorder quantities correctly, you need to account for forecast demand, inbound timing, Amazon receiving delays, supplier constraints, and the inventory already committed to your next purchase order.
That sounds more complicated than a basic sales-velocity formula. It is. But the alternative is worse: reacting to an FBA stockout after ranking, conversion, and ad efficiency have already taken a hit – or placing oversized orders that trap cash and create storage-fee exposure.
Start With the Right Reorder Question
Most sellers ask, “How many units should I order?” That is the second question.
The first is: “How much inventory will I need from the moment I place this order until the next order can arrive and be available for sale?” Your reorder quantity needs to cover that full period, not merely the next few weeks of sales.
For an FBA SKU, the calculation should reflect four realities:
- Forecasted demand during supplier lead time and Amazon inbound time.
- Safety stock needed to absorb demand volatility and timing delays.
- Inventory already on hand, inbound, and available through other fulfillment channels.
- Supplier rules such as minimum order quantities, case packs, order cadence, and unit cost.
When any of these inputs are wrong, the result can look mathematically clean while being operationally useless.
The Core Formula to Calculate FBA Reorder Quantities
A practical reorder formula is:
Reorder quantity = Forecast demand during coverage period + Safety stock – Inventory position
The coverage period is the number of days your new inventory must support. In a disciplined planning process, it usually includes supplier production time, transit time, check-in and receiving time, plus the days until your next planned reorder opportunity.
Your inventory position is more than FBA available inventory. It should include sellable inventory across FBA, AWD, FBM, and other connected channels, then add confirmed inbound units and subtract committed demand or allocations where relevant.
Here is a simplified example. A SKU is forecast to sell 18 units per day. Your supplier requires 35 days, transit takes 12 days, and you plan to reorder every 14 days. Your total coverage period is 61 days.
Forecast demand is 18 units multiplied by 61 days, or 1,098 units. You want 180 units of safety stock. You have 420 sellable units and 300 confirmed inbound units. Your inventory position is 720 units.
The calculation is 1,098 plus 180 minus 720. The initial reorder recommendation is 558 units.
If the supplier requires orders in 100-unit case packs, round the order to 600 units. If its MOQ is 1,000 units, the real decision is no longer just a demand calculation. You must decide whether the expected margin, carrying cost, and cash position justify buying the MOQ now or whether you need a different supply plan.
Use Forecasts, Not Yesterday’s Average
A trailing 30-day average is tempting because it is easy. It is also one of the fastest ways to under-order seasonal products and over-order products that have cooled off.
Your demand signal should reflect trends, seasonality, promotions, stockout history, and channel mix. If a SKU was out of stock for 10 days last month, the recorded sales are not its true demand. Treating those zero-sales days as normal demand will tell you to buy too little – and can create the next stockout.
The same issue appears when ad spend changes. A product supported by active campaigns may have a materially different sales rate than the same product with ads reduced or paused. Inventory and advertising decisions should not live in separate spreadsheets. If inventory cannot support current demand, reducing spend may protect your in-stock position. If replenishment is arriving on time, maintaining demand can be the more profitable move.
For stable, mature SKUs, a short recent history may be enough. For seasonal, promotional, or rapidly growing products, use a forecast that looks farther back and selects a model suited to the SKU’s demand pattern. One forecasting method for every SKU is convenient, not accurate.
Separate lead time from reorder cadence
Lead time is how long it takes inventory to become available after you place an order. Reorder cadence is how often you review or place orders. They are related, but they are not interchangeable.
If you place orders every two weeks, your inventory must last until the next order arrives, not just until the next review date. That extra cadence window is commonly missed in spreadsheet planning.
A seller with a 45-day total lead time and a 14-day order cycle may need to plan for nearly 59 days of demand, plus safety stock. Ignoring the order cycle can make an apparently healthy SKU run out before the next shipment is available.
Set Safety Stock by Risk, Not Habit
Safety stock is your buffer against the things that do not follow plan: a demand spike, a late supplier release, a delayed inbound appointment, or a forecast that was directionally right but not precise enough.
A flat “30 days of safety stock” rule is simple, but it is rarely capital-efficient. Fast, predictable SKUs may need a smaller buffer. Volatile SKUs with long lead times, high margins, or expensive stockout consequences may deserve more protection.
Start by considering demand variability and lead-time reliability. If your supplier consistently delivers in 30 days and the SKU sells steadily, your safety stock can be leaner. If lead time ranges from 30 to 50 days and demand swings sharply during promotions, the buffer should rise.
There is also a commercial decision behind safety stock. A high-margin hero SKU may justify more inventory because a stockout damages revenue, ranking, and customer acquisition efficiency. A slow-moving accessory with a low margin may deserve a tighter target, even if that means accepting a modest risk of running lean.
Count Inventory by Availability, Not by Location
FBA available units are only one part of the equation. A reliable plan brings together every unit that can satisfy demand, while recognizing when each unit will actually be usable.
For example, inventory in AWD may be valuable supply, but it does not carry the same immediate availability as FBA units. A purchase order in production is not the same as inventory that has shipped. An inbound shipment that Amazon has not received may arrive on schedule, or it may not. Treating all inventory as equally available creates false confidence.
Use clear inventory statuses in your model: sellable now, in transit, confirmed inbound, on order, and unavailable or reserved. Then assign timing assumptions to each status. This gives you a projected inventory balance by day or week instead of one misleading total.
For multichannel brands, subtract expected Shopify or FBM demand from shared inventory as well. FBA demand alone does not tell the whole story when the same SKU supports multiple sales channels.
Apply Supplier Constraints After the Demand Math
Forecasting tells you the economically sensible quantity. Supplier constraints determine whether that quantity is actionable.
Round recommendations to case packs. Enforce MOQs. Consider supplier-specific order calendars and whether consolidating related SKUs can improve freight economics. But do not let an MOQ automatically become the answer. If the minimum forces you to carry 10 months of inventory, the correct response may be to renegotiate terms, adjust the product plan, or reconsider the SKU’s contribution to cash flow.
This is where finance and operations need the same view. A purchase order can prevent a stockout while still being a poor cash decision. Review unit margin, landed cost, expected sell-through, storage exposure, and the opportunity cost of tying up capital in slow-moving inventory.
Build a Reorder Process You Can Trust
The formula is only useful if the inputs stay current. Sales velocity changes. Inbound dates move. Supplier lead times slip. A weekly reorder review may be enough for stable catalogs, while high-volume or volatile brands may need daily exception monitoring.
The practical goal is not to stare at every SKU every day. It is to surface the exceptions that need a decision: projected stockouts, inventory that will exceed target coverage, purchase orders that must be released, and products where advertising demand is outpacing supply.
Inventory Optimizer brings sales history, forecast demand, inventory across channels, inbound purchase orders, and supplier rules into one planning workflow. That matters because the final output should not be another spreadsheet calculation. It should be a clear action: how many units to order, from which supplier, when to release the PO, and whether the inventory plan supports current demand.
A good reorder quantity is not the biggest order that avoids a stockout. It is the smallest defensible order that protects availability, respects supplier reality, and keeps cash working where it earns the best return.
