How to use the seasonal sales variation calculator
Enter sales month by month for the past year and the calculator shows how far the peak season runs ahead of the low season, in both dollars and percentage terms. Only the months you actually fill in are counted, so three months of history is enough to get a reading.
The gap on this page means the highest month minus the lowest month. Dividing that gap by the monthly average gives the gap as a share of average, and the coefficient of variation adds a second view by measuring how far the months scatter around the average overall. It uses the population standard deviation of the months entered, divided by the monthly average. A peak-to-low gap can be thrown off by one unusual month, so reading both figures together is safer.
The seasonal index in the table is that month's sales divided by the monthly average. An index of 1.00 matches the average and 1.35 means the month ran 35% above it. Multiplying next year's plan by the low-season index is a quick way to find the month when cash will be tightest.
Frequently asked questions
There is no fixed line, since it varies by industry. As a rough read, under 10% means fairly flat monthly sales, while above 30% usually means you should set aside working capital to cover the slow months.
Enter 0 if the business really had no sales that month, for example during a closure. Only months with a value are counted, so a 0 pulls the average down while leaving the field blank drops that month from the calculation entirely.