Year-over-year growth formula
Subtract the previous period from the current period, divide by the previous period, then multiply by 100. A metric rising from 8,000 to 10,000 changed by 2,000, or 25%. A fall from 10,000 to 8,000 is a change of -2,000, or -20%.
Compare like with like
Use a full year against the preceding full year, or the same month or quarter in two different years. Do not compare a partial current period with a complete previous one. Keep currency, accounting treatment, customer definition, and data source consistent. If a business changed its reporting method, explain the break in comparability.
Interpret a small baseline carefully
A jump from 1 sale to 3 sales is 200% growth but only 2 additional sales. A very small previous value can make a percentage look dramatic. Report the absolute difference beside it. If the previous value was zero, describe the current value as a new activity rather than an infinite percentage increase.