How do I decide where to spend next month's ad budget?
Keep most of next month's budget where it is. Move a small slice toward the channel with the best evidence that extra spend adds sales above break-even. Judge it on what each channel added, not on platform ROAS, and test big moves first.
By Joris van Huët, Founder & CEOUpdated 6 min read
Usually, keep most of next month's budget where it is. Move a small slice toward the channel with the best evidence that more spend adds sales above break-even. Judge that on what each channel added, not on last month's platform ROAS, and test any big move before you bet the month on it.
What one store's data shows
One store's anonymised GA4 export, 1 January 2024 to 21 August 2026. It holds shares of revenue only: no ad spend, no order counts.
| What the export shows | Value | Source cell |
|---|---|---|
| Direct, in last click, first click and touched views | 57.7% of revenue | Channels sheet, Direct row |
| All channels added up in the touched view | 110.4% of revenue | Channels sheet, Touched column total |
| Journeys with ten or more touches (16.0 days to buy) | 3.0% of revenue | Journeys sheet, ten-or-more touches row |
| Break-even ROAS at a 40% margin (1 / 0.40) | 2.5x | Break-even sheet, 40% margin row |
The touched view of that store's Channels sheet adds up to 110.4%, by design: a journey that touched two channels counts in both. Ad platforms overlap the same way. Each one counts the sales it touched, so their claims can add up to more than your actual sales. Total last month's platform reports and you may be splitting money that was only earned once.
Direct holds 57.7% of revenue in every view of that store's export. Click-based reports leave that money with Direct. Whatever started those journeys, a video, a newsletter or a friend's tip, earns no credit when you split the budget. A channel whose buyers never click, then come back direct, looks idle in every click report.
The slow lane is small, but it matters for timing. Journeys with ten or more touches took 16.0 days to buy in that store's Journeys sheet. Money you move on the first of the month shows up partly in the month after. Judge it at month end and you cut it off mid-sentence.
In the Break-even sheet, the 2.5x row is arithmetic at a 40% margin, not a measured result. It is the bar a channel has to clear on sales it caused, not on sales it claims.
What the export cannot do is rank channels by return, because it holds no spend. It shows where credit lands, which is exactly what you are trying to see past.
Why does chasing last month's best ROAS mislead?
- ROAS is an average, and you are buying the next euro. Last month's number includes the easy wins: people searching your brand name, warm retargeting. The next euro buys colder auctions, so it returns less than the average promised.
- Each platform grades its own homework. Google Ads counts what Google touched, and Meta counts what Meta touched. Even Google's Performance Planner forecasts the conversions in your Google Ads Conversions column, not the sales the ads added.
- The models disagree, and that is information. GA4 gives Direct credit only when a path is all direct visits. Its last-click model ignores direct traffic and credits the last channel clicked. If a channel's revenue swings when you switch models, its credit is a choice, not a fact.
- Last month is not finished. If your slow buyers take weeks, late-month spend has not paid out yet. Judge it early and you cut the channel that was halfway through working.
- Last month was a different month. A sale, a launch or the run-up to a holiday lifts every channel at once. The channel with the most budget then looks like the hero, whatever it caused.
Most of these errors lean the same way: toward the channel that closes the sale and away from the one that opened it. Left alone, next month's budget follows the closer.
What can last month's numbers not tell you?
What happens at a different budget. Google's Performance Planner forecasts are refreshed daily and based on the last 7 to 10 days, adjusted for seasonality. That helps for nudges inside the range you already spend. It says little about doubling a channel.
Whether the sales would have happened anyway. A channel can claim buyers who were already on their way to you. Only an experiment settles that, and a geo holdout test is the version you can run yourself.
What your margin can carry. Platform ROAS is revenue over spend, and your bank cares about margin over spend. The bar comes from your own costs, not from any dashboard.
So the decision has two parts. The base stays where the evidence holds up. The slice goes where the evidence is strongest, and gets checked before it grows.
What to do this week
- Add up what the platforms claim. Total last month's claimed revenue across your ad platforms. Set it next to Shopify's Total sales over time for the same dates, under Analytics > Reports. Pass: the claims sum to less than your sales. Fail: they sum to more, so discount the loudest claimer before you feed it.
- Compare two models in GA4. Click Advertising, then Attribution > Attribution models, and compare Paid and organic last click with Data-driven. Pass: a channel's revenue barely moves between the two. Fail: it swings hard, so keep its budget flat until a test settles it.
- Find where Google says demand is being turned away. In Google Ads, open the Insights page and read Budget pacing insights. Pass: a campaign is Limited by budget and clears your break-even even on GA4's last-click view, so it gets the slice. Fail: nothing is limited, so extra money buys weaker auctions; hold.
Check the homework. Your GA4 Attribution paths export already holds the evidence. Causality Engine reads that one file and shows what each channel caused next to what last-click gave it, in 1 to 2 minutes, for €99 once (excluding VAT), refundable within 30 days. Check the homework
Sources, 1 October 2026: Get started with attribution (Google Analytics Help). Key event attribution models report (Google Analytics Help). About Performance Planner (Google Ads Help). About budget pacing insights (Google Ads Help). Sales reports (Shopify Help Center). Implement campaigns for geo experiments (Google Ads Help).
Related answers
Frequently asked questions
How much of my budget should I move in one month?
A slice small enough to read. Google notes that a budget change greater than 20% sets off a new learning period, so smaller moves disturb less. Test bigger shifts with a holdout before you commit a whole month to them.Should I cut the channel with the lowest ROAS?
Not on platform ROAS alone. Channels people watch more than they click get little credit in click-based reports, whatever they cause. Check the channel against break-even on your own sales first, then confirm with a holdout before you switch it off.Is blended MER enough to set next month's budget?
It is a guardrail, not a steering wheel. Total revenue over total ad spend tells you whether the whole mix pays, not which channel to feed. Use it to cap total spend, and use channel tests to split it.
Go deeper: Causal attribution, explained.
Sixty-second versions of these ideas: Causality Engine on YouTube Shorts.
Keep reading
Terms in this article
- AttributionAttribution identifies user actions that contribute to a desired outcome and assigns value to each. It reveals which marketing touchpoints drive conversions.
- Attribution ModelAn Attribution Model defines how credit for conversions is assigned to marketing touchpoints. It dictates how marketing channels receive credit for sales.
- Causal AttributionCausal Attribution uses causal inference to determine which marketing touchpoints genuinely cause conversions, not just correlate with them.
- Direct TrafficDirect Traffic refers to website visitors who arrive by typing the URL directly into their browser or through bookmarks. They do not come from search engines or referrals.
- ExperimentsExperiments are scientific procedures that test hypotheses or demonstrate facts. In marketing, experiments like A/B tests determine the causal effect of campaign changes, enabling data-driven decisions.
- Google AnalyticsGoogle Analytics is a web analytics service that tracks and reports website traffic.
- Holdout TestA holdout test is an experiment where a portion of the audience does not see a campaign. This measures the campaign's true incremental impact.
- MER vs ROASMER (marketing efficiency ratio) is total revenue divided by total ad spend; ROAS is revenue credited to a specific channel divided by that channel's spend. MER cannot be gamed by attribution but is blind to channels; ROAS is channel-level but only as honest as the attribution behind it.