You've planned a short sale and expect more visitors to buy. A Google Ads seasonality adjustment lets you tell Smart Bidding about that expected change in conversion rate. The useful part is putting a reasoned number and a clear end date on it.
I'd start with the evidence for the expected lift. A promotion being on the calendar is a reason to investigate, not enough on its own to choose a percentage.
When a seasonality adjustment makes sense
Google recommends this setting for substantial conversion-rate changes during short events, ideally one to seven days. Smart Bidding already handles normal seasonal patterns; adjustments can be less effective when they run longer than 14 days. Google's seasonality guidance.
Use the following as a planning check before opening the settings:
| Situation | What I'd check first |
|---|---|
| Three-day sale with evidence from comparable promotions | Expected conversion-rate change and affected campaigns |
| The usual increase in demand around a holiday | Whether conversion rate changes enough to justify an adjustment |
| A month-long campaign with a new offer | A broader budget and bidding plan |
| Missing conversions after a tracking failure | Tracking and measurement recovery |
The question is whether a visitor is more or less likely to convert during a defined period. More searches, a bigger budget and a higher conversion rate are different assumptions. Write down which one you expect to change.
Calculate the conversion-rate adjustment
Use this relative-change formula:
Adjustment (%) = ((expected event conversion rate ÷ baseline conversion rate) − 1) × 100
Here is a hypothetical example. Your comparable baseline is 2%, and you expect 3% during a three-day sale. The calculation is (3 ÷ 2 − 1) × 100 = 50%.
| Input | Hypothetical value |
|---|---|
| Baseline conversion rate | 2% |
| Expected sale conversion rate | 3% |
| Difference in percentage points | 1 percentage point |
| Relative conversion-rate change | +50% |
That distinction matters when entering the number. The change from 2% to 3% is a 50% relative increase, even though the difference is one percentage point.
A +50% adjustment describes your conversion-rate expectation. It is not an instruction to raise every CPC by 50%. Google describes it as information that Smart Bidding uses when optimising bids during the event. How the adjustment works.
Choose a baseline you can explain
For planning, I'd use a comparable period with the same conversion definition and a similar mix of campaigns. Check whether that period contains another promotion, a tracking issue or an unusual change in traffic before borrowing its conversion rate.
Avoid averaging weekly percentages when the weeks have very different traffic volumes. In a hypothetical click-based example, 20 conversions from 1,000 clicks and 9 from 100 clicks give a combined rate of 29 ÷ 1,100, or about 2.64%. Averaging 2% and 9% would give 5.5%, which describes neither the combined traffic nor a useful baseline for this example.
I'd keep the baseline dates, included campaigns and calculation together in one sheet. If someone asks where the number came from, the answer should take a minute to find.
Estimate the sale period from comparable evidence
A previous promotion can help, provided you compare it with its own normal period. Look at the relative change, then consider whether the upcoming offer, products and audience are sufficiently similar.
Suppose a previous sale moved conversion rate from 2% to 3%. That is evidence for a 50% relative lift in that particular event. It does not establish that every future sale will do the same. A weaker discount, different stock or a different traffic mix gives you a reason to revisit the assumption.
Record a range of plausible outcomes while planning. Then document why you selected the number you intend to use. A calculator can do the division; choosing comparable evidence is the part that needs your attention.
Set it up in Google Ads
Google's current instructions place this under Tools → Budgets and bidding → Adjustments → Seasonal. Create an adjustment, choose the conversion-rate type, name it, set its dates and scope, enter the expected percentage change, and review before saving. Google's setup instructions.
Eligibility depends on campaign type and bidding strategy. The same documentation lists supported combinations, including Search, Standard Shopping and Display with Target CPA or Target ROAS, plus Performance Max and App campaigns. Check that list for the campaigns you intend to include.
Before saving, I'd check these five details:
- The start and end times match the offer and the account's time zone.
- The scope contains the campaigns affected by that offer.
- The percentage agrees with the calculation in the planning sheet.
- The landing pages, stock and promotion details are ready.
- Someone owns checking the event and reviewing the result afterwards.
Give the adjustment a name that will still make sense later, such as the promotion and its dates. A note explaining the assumed baseline is more useful than “sale boost”.
What happens when the promotion ends?
Google says campaigns return to their pre-adjustment behaviour after the event. You do not need a negative adjustment simply to undo the completed promotion. Google's explanation of event completion.
Afterwards, compare the expected and observed conversion rates using the same campaign scope and conversion definition. Allow for conversions that arrive later before calling the assumption right or wrong. Google explains conversion delay here. Keep that review with the original calculation so the next promotion starts with better evidence.
If you are putting together the rest of your workflow, the free Google Ads tools guide covers a useful starting kit. For this setting, start with one promotion, one defensible baseline and a clear explanation of the expected change.
