How to forecast next December's sales in a spreadsheet

Creating accurate sales forecasts for the upcoming December in a spreadsheet can be achieved using the built-in Forecast Sheet tool in the osFoundry desktop app. Instead of relying on last year's sales number and a hopeful percentage, this method takes into account the seasonal patterns present in historical sales data.

To use the Forecast Sheet, first lay out the historical sales data in a two-column format, with dates in the first column and sales figures in the second. Ensure the dates are evenly spaced, such as the first day of each month, for the entire period you want to analyze. At least thirty-six months of data is recommended for capturing yearly seasonality effectively.

Once the data is organized, click on any cell within the data range, then navigate to the Data tab and select Forecast Sheet. In the resulting Create Forecast Worksheet dialog box, set the Forecast end date to the last month you wish to forecast, such as December 2026. The editor will automatically determine the seasonality, typically finding a twelve-month repeating pattern in this example data.

The forecast sheet will generate a new sheet with the original data, followed by three columns: Forecast, Lower Confidence Bound, and Upper Confidence Bound. The Forecast column provides the predicted sales value for each month, including December 2026. In this example, the forecasted sales for December 2026 are $20,654, while the sales in January 2026 are projected at $12,260.

The additional columns offer a range of possible outcomes, based on a 95 percent confidence level. This range, from $19,008 to $22,300 for December 2026, indicates the uncertainty surrounding the forecast. It is essential to consider this range when communicating the forecast to stakeholders, as it reflects the potential variability in sales.