Small ecommerce businesses in the UK can use basic spreadsheet techniques and free online tools to forecast inventory demand and reduce the risk of stockouts or excess stock. These methods rely on historical sales data, seasonal patterns and straightforward calculations that any founder can apply without specialist software.
By tracking orders, identifying trends and adjusting for known variables such as promotions or holidays, you can make more informed purchasing decisions. The following guide sets out practical steps using tools that are either already on your computer or available at no cost.
Why accurate inventory forecasting matters for UK ecommerce
Running out of popular items leads to lost sales and disappointed customers, while holding too much stock ties up cash and risks obsolescence, especially for fashion or seasonal goods. Good forecasting helps balance these risks and supports smoother fulfilment operations.
In the UK, many small sellers manage inventory across multiple channels including their own website, Amazon, eBay and Etsy. Each channel can have different demand patterns, so a single forecast rarely fits all. Starting with a central record of sales data from all sources gives a clearer overall picture.
Gather and organise your historical sales data
Open a spreadsheet in Google Sheets or Microsoft Excel. Create columns for date, product SKU, units sold, sales channel, price, and any notes such as whether a promotion ran. Export order reports from your platforms and combine them into one sheet.
Sort the data by date and product. Fill any gaps with zero values rather than leaving cells blank so that calculations remain accurate. Keep at least 12 months of data where possible; shorter histories can still be useful but will be less reliable for spotting trends.
Calculate average monthly demand
For each product, use the AVERAGE function over the past three, six or twelve months depending on how stable sales have been. In a new column labelled "Avg Monthly Demand", enter a formula such as =AVERAGE(B2:B13) where those cells contain monthly totals.
Adjust the average for obvious seasonality. If you know December sales are usually double the rest of the year, create a separate "Seasonal Multiplier" column and multiply the average by that factor for December forecasts. Record the reasoning next to each product so you can revisit assumptions later.
Apply simple trend adjustments
Use a moving average to smooth out random fluctuations. In Google Sheets, add a column that calculates the three-month rolling average with a formula such as =AVERAGE(C2:C4) and drag it down. Compare this with the most recent month to see whether demand is rising or falling.
For products showing consistent growth, add a simple percentage increase based on the change between the last two quarters. If sales grew by 15 percent from Q1 to Q2, multiply the latest average by 1.15 for the next quarter forecast. Document each adjustment so the forecast remains transparent and easy to update.
Factor in lead times and reorder points
Find out the typical time between placing a supplier order and receiving stock; this is your lead time. In the spreadsheet, add a column for "Lead Time in Days" and another for "Daily Demand" calculated by dividing monthly average by 30.
Calculate reorder point as daily demand multiplied by lead time in days, then add a small safety buffer of extra days based on how variable past sales have been. When stock on hand falls to or below this reorder point, it is time to order again. Update the buffer periodically as you learn more about supplier reliability and demand variation.
Use free tools to visualise and refine forecasts
Google Sheets charts help spot patterns quickly. Select your date and sales columns, insert a line chart, and add a trendline via the chart editor. This visual can highlight seasonal peaks that a simple average might miss.
Free templates are available on the Google Sheets template gallery and from some accounting platforms. Import your data into one of these, review the automatically generated forecasts, and manually override any figures that do not match your knowledge of upcoming marketing campaigns or market changes. Cross-check against supplier minimum order quantities to ensure the forecast is commercially realistic.
Review and improve the process regularly
Set a calendar reminder to update the spreadsheet at the end of each month. Compare actual sales against the forecast and note the difference. Over time you will see which methods work best for different product types.
Keep the spreadsheet version-controlled by saving copies with the month in the filename. As the business grows you may add more sophisticated methods, but the core principles of recording data, calculating averages, adjusting for known events and reviewing outcomes remain the same.
Frequently asked questions
What is the simplest way to forecast inventory without paid software?
Combine all sales data into a Google Sheet, calculate average monthly sales for each product, adjust for obvious seasonality or promotions, then set reorder points based on supplier lead times. Update the sheet monthly and compare forecasts with actual results to improve accuracy.
How far ahead should I forecast stock needs?
Most small UK ecommerce businesses find a three-month rolling forecast practical. It covers typical supplier lead times while remaining responsive to changes in demand. Extend to six months for slower-moving or custom-made items.
How do I account for seasonal peaks in my spreadsheet?
Create a separate column for seasonal multipliers based on previous years. Multiply your baseline average demand by the multiplier for peak months such as November and December. Review and adjust the multipliers each year as your customer base evolves.
What should I do if my sales data is limited?
Start with whatever history you have and supplement with channel-specific trends. Use conservative estimates and a larger safety buffer until you have at least six months of consistent data. Focus first on your best-selling products.
When should I consider upgrading from spreadsheets?
Spreadsheets remain effective for most businesses selling fewer than 50 active products. Move to integrated inventory tools only when the time spent maintaining multiple sheets exceeds the benefit or when you add significant product variety and international shipping complexity.
Related reading: Using DHL Parcel for UK ecommerce shipping explained, Is Evri a good shipping partner for small UK ecommerce brands?, How much does warehouse storage cost for small UK ecommerce brands?. Browse all fulfilment & logistics guides or apply to Loomworks.