Month-over-month formula
Subtract the previous month's value from the current month's value, divide by the previous month's value, and multiply by 100. A rise from 8,000 to 10,000 is a change of 2,000, or 25%. A fall from 10,000 to 8,000 is -20%.
Use comparable months
Wait until the current month is complete before comparing it with a full previous month. Keep the same time zone, currency, measurement method, and reporting cut-off. If the previous month was zero, report the new value and absolute change rather than claiming infinite growth.
Look beyond one month
Month lengths, holidays, promotions, and seasonality can move a metric even when the underlying trend is flat. Show the absolute change and several months of history. For a seasonal business, also compare with the same month a year earlier.