Year over year comparison in analytics: same dates or same weekdays
YoY growth formula, when to compare 364 days back so weekdays line up, partial months, moving holidays, and what breaks a comparison with last year.
On this page
A year over year comparison sets a period against the same period a year earlier: growth = (this year − last year) ÷ last year × 100. For daily or weekly numbers, compare with the days 364 days back, so a Saturday lines up with a Saturday; for whole months and quarters, compare the same calendar dates.
Most pages on this stop at the formula. The formula is the easy part. The comparison goes wrong on weekday mix, half-finished months, holidays that move, and tracking that changed in between. This guide covers each, with the checks to run.
The formula
YoY change = (this period − same period last year) ÷ same period last year × 100.
| Last year | This year | YoY change |
|---|---|---|
| 1,200 orders | 1,380 orders | +15% |
| $48,000 | $45,600 | −5% |
| 0 signups | 40 signups | No percentage: nothing to divide by |
| 3.2% conversion rate | 3.6% conversion rate | +0.4 points, or +12.5% |
Two notes. A change from zero has no percentage; show the two counts instead. And a change in a rate can be written two ways: in points (3.2 to 3.6 is +0.4 points) or as a relative change (+12.5 percent). Say which, because "conversion rate up 12.5 percent" reads like a far bigger move than it is.
Same dates or same weekdays
A year is 365 days, 52 weeks and one day. So the same date falls one weekday later each year, two after February 29. Saturday, October 24, 2026 compares with Friday, October 24, 2025 by date, and with Saturday, October 25, 2025 if you go back 364 days (52 weeks exactly).
It matters whenever days of the week behave differently, which they do for nearly every business: stores sell more at weekends, B2B sites get their leads on weekdays.
| You are comparing | Use | Why |
|---|---|---|
| One day, or a run of days in a chart | 364 days back | Each day lines up with the same weekday |
| A week or a set of whole weeks | 364 days back | Same weekdays, same count of each |
| A calendar month, quarter or year | Same dates | That is what finance, tax and budgets use |
| A month for a weekday-heavy business | Both, and note the weekday mix | October 2026 has five Saturdays; October 2025 had four |
| Around a holiday tied to a weekday | 364 days back | Black Friday 2025 was November 28; 364 days later is Black Friday 2026, November 27 |
The holiday example works because US Thanksgiving is fixed in law as the fourth Thursday in November (5 U.S.C. 6103), so Black Friday always falls on a Friday and the 364-day shift lands on it. By date, November 27, 2025 was Thanksgiving itself, a very different day for most stores.
Holidays tied to a date (Christmas, July 4) do the opposite: same dates line them up, 364 days back moves them a day. Holidays tied to neither, such as Easter, which can move by weeks, line up under neither rule. Mark them on the chart so nobody reads the jump as a trend.
Google Analytics 4 compares with Previous year by the same dates; Google notes that its Previous year (match day of week) option has been removed (Google). To line up weekdays in GA4, set a custom comparison range 364 days earlier.
Partial periods: compare like with like
On October 23, "This month" is October 1 to 23. Comparing it with all of October last year makes this year look down by about a quarter, every time. Compare with October 1 to 23 last year instead.
The same holds for "This quarter" and "This year": compare the days elapsed with the same days last year, and leave the full-period comparison for when the period has ended.
What breaks a comparison with last year
The numbers can be honest and the comparison still meaningless. Check these before you trust a YoY change:
- Tracking changed. A new analytics tag, a cookie banner, a move from Universal Analytics to GA4, a new bot filter or a site redesign that dropped the tag from some pages. Each can move counts by more than the business did. Put a note on the chart on the day it happened.
- The data is not there. GA4's data retention setting (2 or 14 months for most properties) limits explorations and funnel reports, though not its standard reports (Google). A tool you started this year has no last year at all.
- The site changed. New products, new prices, a new checkout or a new domain change what a visit means. Compare the outcome (orders, revenue, leads) rather than the steps.
- The mix changed. Traffic up 40 percent from one viral post is not the same business as last year growing 40 percent. Split the change by source before celebrating.
- Currency. Revenue in a reporting currency moves with exchange rates. Compare at last year's rates, or say how much of the change is currency.
Year over year or previous period?
Both, for different questions:
| Question | Compare with |
|---|---|
| Are we growing, after seasonality? | Same period last year |
| Did last week's change work? | Previous period |
| Is this month normal for the time of year? | Same period last year, 364 days back for daily charts |
| Is something broken right now? | Previous period, at day level |
| Board or client report for a seasonal business | Same period last year, with the previous period beside it |
A seasonal store that only reads month over month will see a crash every January and a boom every November. Neither says anything about the business. See how to read a dashboard for an online store for where each comparison sits on a board.
Common questions
How do you calculate year over year growth?
Subtract last year's figure from this year's, divide by last year's figure, and multiply by 100. From 1,200 to 1,380 orders is (1,380 − 1,200) ÷ 1,200 × 100 = 15 percent.
Why compare 364 days back instead of 365?
364 days is exactly 52 weeks, so each day lines up with the same weekday a year earlier. 365 days back shifts every day by one weekday, which distorts daily charts for any business that is busier on some days.
How do I compare year over year in Google Analytics 4?
Open the date picker, turn on Compare, and choose Previous year. It compares the same dates. For weekdays to line up, choose a custom comparison range that starts 364 days before your range.
Should I compare this month to last month or to the same month last year?
To the same month last year for growth and seasonality. To last month for spotting a recent change. A report for a seasonal business should show both.
Can I compare year over year with less than a year of data?
No. Compare with the previous period, and keep the data so next year's comparison exists. If your old tool still holds last year, export or import it before it is deleted.
Sources
- Google Analytics Help: Change and compare date ranges in reports, Data retention. Checked October 2026.
- Legal Information Institute: 5 U.S. Code § 6103, Holidays. Checked October 2026.
- Calendar dates computed for 2025 and 2026. Example numbers are made up for illustration.
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