Demand forecasting tells you how much you will sell. Inventory forecasting answers the question that actually keeps you in stock: given that demand, how much stock do I need to hold and reorder, and when? It is where a forecast turns into a buying decision.
Inventory forecasting predicts the stock you need over a period by combining a demand forecast with lead time and a safety buffer. It is what sets your reorder point.
Inventory forecasting vs demand forecasting#
They are related but not the same.
- Demand forecasting predicts customer demand: units you expect to sell.
- Inventory forecasting takes that demand, adds your lead time and safety stock, and predicts the stock you need to have on hand and to reorder.
Demand forecasting is the input. Inventory forecasting is the output you act on.
The inputs you need#
A useful inventory forecast pulls together:
- Demand forecast per SKU, per channel (the units you expect to sell).
- Current stock: on-hand plus in-transit.
- Lead time, the full clock from reorder to sellable, not just shipping.
- Safety stock to cover the variability the forecast misses.
- Known events: promotions, launches, and seasonality.
The core formula#
At its simplest, the stock you need to cover a period is expected demand during the lead time plus a buffer:
That is the same logic as a reorder point: when on-hand stock falls to this number, you reorder. Inventory forecasting just runs it forward across your planning horizon so you can see the reorders coming.
If a SKU sold zero last Tuesday because it was out of stock, that is not low demand, it is missed demand. Leaving stockout days in your history teaches the forecast to under-order the exact SKUs that already sell out.
Forecast per SKU and per channel#
This is the mistake that quietly costs the most. A single blended forecast hides the channel that is heating up. Your warehouse, Amazon FBA, and TikTok Shop each sell at different speeds and replenish on different timelines, so forecast each one on its own. A climbing Amazon run rate with a 10-day FBA inbound needs a different reorder trigger than a steady warehouse.
It also connects to inventory turnover: a sharper forecast lets you hold less of what is slow and enough of what is fast, which lifts turnover without raising stockout risk.
Forecast stock needs per SKU and channel, updated daily
The bottom line#
Inventory forecasting turns a demand forecast into the stock you actually need: expected demand over the lead time, plus safety stock, per SKU and per channel. Clean your history, keep lead times honest, and forecast each channel separately. That is the calculation Enough Stock runs continuously, so your reorder points and order quantities stay right as demand moves.