What is year over year (YoY)?

Year over year (YoY) compares a figure for a period with the same period one year earlier, such as October 2026 against October 2025. Because both periods fall in the same season, YoY removes most seasonal effects.

How it is calculated

YoY change = this period - the same period last year

YoY % = YoY change / the same period last year × 100

You need at least 13 months of history for one YoY month, and 24 months to show YoY for every month of the year.

How it is used

YoY answers "is the business actually growing?" without the noise of the calendar. A retailer's December will always beat its November, so MoM says little. December against last December says a lot. Most monthly reviews show MoM, YoY and trailing twelve months side by side.

Worked example

RevenueAmountOctober 2026 change
October 2026$52,000
September 2026 (MoM)$58,000-$6,000 (-10.3%)
October 2025 (YoY)$45,000+$7,000 (+15.6%)

On MoM alone, October looks like a 10.3% drop. YoY shows October is up 15.6% on last year. September is simply this business's busiest month.

Common mistakes

  • Calendar differences. A month with an extra weekend, a moved holiday or an extra pay date can still distort YoY. Note it.
  • Reclassified accounts. If an expense moved to a new account this year, YoY shows a false jump on one line and a false drop on another.
  • Acquisitions and new locations. Growth from a purchase is not the same as growth in the existing business. Separate them where you can.
  • Different fiscal periods. Compare the same calendar month, or the same fiscal period, never a mix.

Related terms

  • Month over month (MoM): Month over month (MoM) compares a figure for one month with the same figure for the month before, as a dollar change and a percentage change.
  • Trailing twelve months (TTM): Trailing twelve months (TTM) is the total of a figure, such as revenue or net income, for the most recent 12 consecutive months.
  • Variance analysis: Variance analysis is the process of comparing actual results with a budget, forecast or prior period, measuring the difference and explaining why it happened.

Further reading