Inventory Forecasting Basics for Growth-Stage E-Commerce

Running out of a bestseller during a growth push, or sitting on dead stock from a slow season, both usually trace back to the same root cause: forecasting by feel instead of by data. Neither mistake is about bad luck, both are usually visible in advance in your own sales history.

Sales Velocity Is the Starting Point

Average units sold per week, per SKU, over the last 90 days gives a baseline. From there, adjust for known seasonality and any planned promotions, rather than guessing at a round number. This single number is the input for almost everything else in inventory planning.

Reorder Points, Not Reorder Panic

A reorder point, the inventory level that triggers a new order, should account for supplier lead time plus a buffer. Without one, reordering becomes reactive instead of planned, and reactive reordering is where stockouts on bestsellers usually come from.

Illustrative example: a store selling 20 units a week with a 3-week supplier lead time needs roughly 60 units in stock just to cover the lead time, plus a safety buffer on top for demand variance. Without doing this math, it’s easy to reorder either too late (stockout) or too early (cash tied up in inventory sitting on a shelf).

Slow Movers Deserve a Decision, Not Neglect

SKUs that consistently underperform should get an active decision: discount them out, bundle them, or discontinue them. Letting them sit ties up cash that could fund a bestseller’s next order.

How RevenueTHESIS Approaches This

Inventory forecasting connects directly to the seasonal pattern work covered in our seasonal slowdown guide, both are about using your own historical data to plan ahead instead of reacting. It’s foundational, unglamorous system-building work, and it’s usually one of the fastest wins available in a Growth Systems Architecture engagement.