Restocking by gut has two bad endings: you run out during peak demand, or you bury your capital in product that doesn’t move. A forecast doesn’t need to be a complex model; it needs to answer two questions: when to order and how much.

The three numbers you need

Sales velocity (units per day): your real average, ideally from recent weeks, not the year.
Lead time: days from ordering until the product is sellable (including production, transit and inbound to the channel).
Safety stock: a buffer for demand swings and delays.

When to restock: the reorder point

Reorder point = (Sales velocity × Lead time) + Safety stock

When your real available hits that number, it’s time to order. Not earlier (you tie up capital) nor later (you risk a stockout).

How much to restock

The simplest thing that works: cover the period between orders plus the lead time.

Quantity = Sales velocity × (Target coverage days + Lead time) − Current stock − In-transit

Subtract what’s already on the way so you don’t double the order.

Example

You sell 8 units/day, your lead time is 30 days and you want 45 days of coverage, with 5 days of safety:

Reorder point = (8 × 30) + (8 × 5) = 280 units
Quantity to order = 8 × (45 + 30) − current stock − in-transit = 600 − stock − transit

The details that ruin it

Using the annual average on a seasonal product.
Forgetting the inbound (the time Amazon/MELI take to receive).
Not subtracting what’s already in transit.
Computing on total stock instead of real available.

In short

A good forecast is real velocity × lead time + safety, applied to real available and net of in-transit. Doing it by hand per SKU is where it falls apart. iqseller computes your reorder point and how much to order using your real velocity.

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Inventory forecast: how much to restock and when (a simple method)