A year-on-year (YoY) comparison measures a figure against the same period twelve months earlier: October 2026 against October 2025, or this week against the matching week last year. Because both periods share the same season, it shows whether the business is genuinely growing or shrinking, rather than simply moving with the calendar. Year on year is the usual UK phrasing; year over year is the American version.
How a year-on-year comparison works
The sum is simple: take this period’s figure, subtract last year’s, and divide by last year’s. If a firm received 1,150 enquiries this quarter against 1,000 in the same quarter last year, that is 150 more, or 15% growth year on year.
The care goes into lining the periods up. Calendar months contain different mixes of weekdays and weekends, and for many businesses a Saturday behaves nothing like a Tuesday. For weekly or daily figures, compare against the period 52 weeks earlier, so Mondays sit against Mondays. GA4, Search Console and the ad platforms all let you add a comparison period to a date range; check which option you have chosen before reading the result.
Then look for anything that makes the two periods unlike each other. In the UK the usual suspects are:
- Moving holidays. Easter falls in March some years and April in others, and bank holidays shift between weeks.
- Different school holidays across England, Scotland, Wales and Northern Ireland.
- Retail events such as Black Friday, which falls on a different date in late November each year.
- Weather. A cold snap in one October and a mild one in the next changes demand for heating engineers, roofers and garden centres.
- Changes to your own measurement Such as a new consent banner, a tracking fix, or a move from Universal Analytics to GA4 if your history goes back that far.
Why it matters
Many UK businesses have a strong seasonal pattern, and month-on-month figures mix that pattern up with real change. A tax accountant sees enquiries fall every February, straight after the 31 January Self Assessment deadline. Read month on month, February looks like a crisis. Read year on year, it may be the best February the practice has had.
The same applies to search. Organic clicks to a garden landscaper fall every winter; comparing this January with last January shows whether the site is gaining ground or simply following the weather. For paid search, comparing cost per click with the same month last year separates your account’s performance from seasonal changes in competition.
Common mistakes
- Comparing across a tracking change. If your consent set-up or conversion tracking changed during the year, part of the movement is measurement, not customers.
- Percentages on small numbers. Going from 4 sales to 8 is 100% growth, but it could easily be chance.
- Revenue without volume. Price rises can push revenue up year on year while the number of customers falls. Report both.
- Ignoring the market. If demand across your sector fell, holding steady may be a good result.
- Not writing down what happened. A year later, nobody remembers that the site was down for two days or that a big campaign ran.
How to act on it
Report year-on-year change next to the raw figures and next to the month-on-month view, so readers can see all three. Keep a running log of anything that could affect the numbers: launches, price changes, tracking changes, outages, algorithm updates. In GA4 an annotation on the date does the job, and a shared spreadsheet works for everything else.
For organic search, Google Search Console keeps sixteen months of data, which is enough for one full year-on-year comparison; export it regularly if you want a longer history. Where tracking has broken, use a source that did not change, such as your CRM or your bank statements, to anchor the comparison. If you want an independent read of whether your search performance is really growing, a full SEO audit includes a year-on-year review of organic traffic by page and query.