The measured percentage decline in a specified content metric over a defined period.
Content decay rate expresses a decline in a chosen performance metric over a defined comparison. State what is being measured, which content is included and the two periods being compared. The term has no single universal reporting window or metric that every team must use.
One simple percentage-decline calculation is (baseline value minus comparison value) divided by baseline value, multiplied by 100. It requires a positive baseline. If the baseline is zero, this percentage is undefined; report the absolute values instead. Under this convention, a negative result represents growth rather than decline.
Use comparable measurement conditions. Different date lengths, changed tracking or a seasonal peak can distort interpretation. This calculation summarizes the difference between two values; it is not automatically a daily decay constant, a compounded rate or evidence that the content itself caused the change.
Takeaway: Keep the metric and time base attached to every percentage. Use the figure to prioritize investigation rather than label a page unsuccessful without context.
Example
A guide records 400 organic search clicks in one 28-day period and 300 in a comparable 28-day period. The calculation is (400 minus 300) divided by 400, multiplied by 100: a 25% decline. That arithmetic alone does not explain whether demand, visibility or another factor changed.
What to check
Show both raw values beside the percentage, especially for small baselines. If the periods are not comparable, explain the limitation or choose a better comparison. Do not silently average page-level percentages with different baseline sizes.
For more detail, see Debugging drops in Google Search traffic.