Definition
Inventory forecasting is the process of predicting how much stock a business will need over a future period, based on historical sales data, demand trends, seasonality, lead times, and expected promotions or events. The goal is to have the right amount of stock available at the right time, avoiding both stockouts (running out of a product) and overstock (holding excess inventory that ties up cash and storage space).
Starting with Historical Sales Data
The starting point for inventory forecasting is historical sales data at the SKU level. How many units of each product sold in the equivalent period last year? What was the week-on-week trend leading up to peak seasons? These patterns form the baseline from which adjustments are made for expected changes in demand.
Factors That Affect Forecast Accuracy
Several factors can cause actual demand to deviate from historical patterns. New product launches, competitor activity, changes in pricing, marketing campaigns, and macroeconomic shifts all affect demand. A business that is running a 20% promotion on a product next month needs to build that uplift into its forecast and order accordingly.
Lead Times and Safety Stock
Lead time, the time between placing an order with a supplier and receiving the stock, is a critical variable in inventory forecasting. A product with a 6-week lead time requires the business to forecast demand further ahead than a locally sourced product that can be replenished within a week. Building safety stock protects against both forecast inaccuracy and lead time variability.
Reorder Points and Automated Forecasting
Reorder point calculation formalises this process. The reorder point is the stock level at which a replenishment order should be placed, calculated as the expected demand during the lead time plus the safety stock buffer.
Modern POS and inventory management systems can automate much of this process. By tracking real-time sales velocity and comparing it against reorder points, the system can generate purchase order recommendations without requiring manual intervention. For businesses with dozens or hundreds of SKUs, this automation is essential.