How to Forecast Demand for Your Shopify Store

What Is Demand Forecasting?

Demand forecasting is the process of estimating how much of each product you’ll sell over a future period. It’s the foundation of good inventory management: if you know what’s going to sell, you can order the right amount at the right time.

For most Shopify merchants, forecasting doesn’t require machine learning or a data science degree. It requires looking at your historical sales data and using it to make informed decisions about the future.

Start with Sales Velocity

Sales velocity is the simplest and most useful forecasting metric. It tells you how many units of a product you sell per day (or week, or month) on average.

How to calculate it: Take total units sold over a period and divide by the number of days. For example, if you sold 300 units of a product in the last 30 days, your velocity is 10 units per day.

Use velocity as your baseline forecast. If you’ve been selling 10 units per day, you can reasonably expect to sell around 10 units per day next month, absent any major changes.

Choose the Right Time Window

The time window you use to calculate velocity matters. A 7-day window is responsive to recent trends but noisy. A 90-day window is stable but slow to reflect changes. Most merchants find a 30-day window strikes the right balance.

However, if your products are seasonal, you should also look at the same period from the prior year. Selling 50 scarves per day in October doesn’t mean you’ll sell 50 per day in April.

Account for Trends

Velocity tells you the average, but trends tell you the direction. Is a product’s sales velocity increasing, decreasing, or flat?

Compare the most recent 30-day velocity to the prior 30 days. If a product went from 5 units/day to 8 units/day, it’s trending up — and your next order should reflect the higher run rate, not the average of both periods.

Factor in Stockout Days

One of the biggest forecasting mistakes is including days when a product was out of stock in your velocity calculation. If a product was out of stock for 10 out of 30 days, your average is artificially low — you’re underestimating demand.

The correct approach is to calculate velocity only for days when the product was in stock. If you sold 200 units in 20 in-stock days, your true velocity is 10/day, not the 6.7/day you’d get by dividing by 30.

From Forecast to Reorder

Once you have a demand forecast, turning it into a reorder decision is straightforward:

1. Calculate how many days of stock you have: Current inventory ÷ daily velocity = days of supply.
2. Compare to your lead time: If you have 15 days of stock and your supplier takes 10 days to deliver, you need to order within 5 days.
3. Add safety stock: Build in a buffer for demand variability and supplier delays.
4. Calculate order quantity: (Daily velocity × days until next order) + safety stock − current inventory = order quantity.

Common Forecasting Pitfalls

• Forecasting new products. You have no history for a new product. Start with conservative estimates, monitor closely, and adjust as data comes in.
• Ignoring promotions. A flash sale or social media feature can spike demand 5–10x. Factor upcoming promotions into your forecast manually.
• Over-relying on automation. Automated forecasts are a starting point, not gospel. Review the numbers and apply your knowledge of the business.

How StockrHub Helps

StockrHub’s demand forecasting engine runs these calculations automatically for every product in your catalog. It analyzes your Shopify order history, calculates velocity, adjusts for stockout periods, and generates reorder recommendations with specific quantities.

You get a clear view of which products need reordering, how many units to buy, and when to place the order. From there, you can generate a purchase order with a single click.