From your own measured conversion-rate change in the equivalent past period, not from a guess — and only for short, predictable events. If you cannot point at a comparable historical window, do not set one.

On this page

• What are seasonality adjustments for?
• When should I not use a seasonality adjustment?
• How do I derive a seasonality adjustment percentage?
• Why does historical sale data overstate the adjustment?
• What if I have no history to derive an adjustment from?
• What matters more than the seasonality adjustment?

What are seasonality adjustments for?

Short, sharp, predictable changes in conversion rate. A flash sale, a known event, a product launch with a hard date. Days, not weeks.

They tell the bidder to expect a conversion rate different from what it has observed, so it can bid appropriately from hour one instead of learning the change after it is over.

When should I not use a seasonality adjustment?

• Slow seasons. Too long, and the bidder should be observing reality.
• Changes you are not confident about.
• Compensating for a tracking problem.
• Recurring, already-learned seasonality. The bidder has seen last year.

How do I derive a seasonality adjustment percentage?

1. Find the equivalent past period — same promotion, same event, ideally same time of year.
2. Compute conversion rate during it and in the fortnight before.
3. The percentage change is your starting adjustment.
4. Haircut it. Being wrong high is worse than being wrong low, because the bidder overbids on demand that does not arrive.

Why does historical sale data overstate the adjustment?

Conversion rate during a past sale is a mix statistic. It rises during a sale partly because the traffic mix changed — email sends, returning buyers, deal-seekers — not only because the offer converted better.

So the historical figure overstates what a bidder-relevant adjustment should be. Another reason to haircut.

What if I have no history to derive an adjustment from?

Do not set one. An unfounded adjustment is strictly worse than nothing: you have told the bidder something specific and false, and it will act on it immediately.

What matters more than the seasonality adjustment?

Make sure the promotion actually reaches the feed. A cart-level automatic discount never does, so Shopping quotes full price while the site is on sale — and no seasonality adjustment fixes a promo that is not visible to the channel.

Related questions

How do I work out what target ROAS to actually set for my store?

Derive it from contribution margin and your fixed per-order costs, not from a benchmark. The formula everyone uses — 1 ÷ gross margin — is a breakeven floor, not a target, and it ignores the per-order costs that decide whether an order is actually worth having. Read the full answer.

Something changed my Google Ads budgets and it wasn't me — how do I find out what did?

The commodity answer is "check Tools and Settings, then Change history." The version worth reading is narrower: it may not be a person. Scheduled scripts edit accounts on a timer, they can silently reverse your changes overnight, and if one is running then **no before-and- after read on those campaigns is clean. Read the full answer.

Is it a conflict of interest to pay a Google Ads agency a percentage of ad spend?

Yes, structurally — the agency earns more when it spends more of your money, regardless of what that spend returns. It is not necessarily disqualifying, but you should know what it does to incentives and what the alternatives are. Read the full answer.

Written by Usama Khan, who runs Google Ads for e-commerce brands and home-services companies at AdventurAds. Figures are from accounts under active management and are anonymised. Last reviewed 2026-08-26.