Shopify Inventory Forecasting: Never Run Out of Stock Again

Stockouts Cost More Than You Think

A stockout costs you the immediate sale, but the downstream effects are worse: the customer who couldn’t buy may leave and never return, your Google Shopping ads stop showing for that product (no inventory = no ad), and if you’re in a subscription model, stockouts trigger cancellations. Research by IHL Group found stockouts cost retailers $1.1 trillion globally annually. Inventory forecasting — predicting what you’ll need and when — is the discipline that prevents this.

Sales Velocity: Your Forecasting Foundation

Sales velocity is the rate at which a product sells. Calculate: units sold ÷ days in period = daily sales velocity. Example: 150 units sold in 30 days = 5 units/day velocity. Your reorder point = (daily sales velocity × lead time in days) + safety stock. If your supplier takes 14 days to deliver and you sell 5/day, reorder when you have 14 × 5 = 70 units remaining, plus safety stock (e.g. 20 units) = reorder at 90 units.

Seasonal Demand Adjustment

Sales velocity changes with seasons. A summer garden furniture store selling 5 units/day in August may sell 0.5/day in January. Build seasonal multipliers: review last year’s monthly sales data, calculate each month’s sales as a percentage of annual total, and apply these multipliers to your base velocity forecast. For a new store without historical data, use Google Trends to identify seasonal demand patterns for your product category.

Inventory Forecasting Apps for Shopify

• Inventory Planner — purpose-built for Shopify, connects to your sales data, generates automated purchase orders based on demand forecasts. £99+/month. Best mid-market option
• Cogsy — demand planning with subscription and replenishment tracking, clean UI
• Stocky — Shopify’s own free inventory management app, basic forecasting, integrates with Shopify POS
• Brightpearl — enterprise OMS + inventory management, connects multiple channels

Safety Stock and Supplier Reliability

Safety stock is buffer inventory held above your calculated reorder point to protect against demand spikes and supplier delays. Calculate safety stock: (maximum daily sales − average daily sales) × maximum lead time days. Adjust your safety stock level based on supplier reliability — an unreliable supplier with variable 14–28 day lead times needs more safety stock than a consistently reliable 7-day supplier. Review and adjust your safety stock quarterly.

Need inventory management setup for your Shopify store? Our operations team implements forecasting systems and demand planning tools. Get in touch.