How to plan next month's ad budget, step by step

Usually in seven steps. Work out break-even ROAS from your margin, then list last month's spend and claimed sales for each channel. Check those claims against GA4 and Shopify, and look for headroom. Keep the base, move one small slice, then set daily budgets and book a test.

Step by step

You need three tabs open: Shopify, GA4 and Google Ads. Do the steps in this order, because each one changes how you read the next. The goal is not a perfect split. It is a split you can defend, plus one test that makes next month's split better.

1. Work out your break-even ROAS. Find your gross margin in Shopify's profit reports, then divide 1 by it. If your margin is 40%, break-even ROAS is 2.5x; below that, the ads lose money on the order. Path: Shopify admin > Analytics > Reports > Category filter > Profit Margin > Gross profit by product.
2. List last month's spend and claimed revenue per channel. One row per channel: spend, the revenue the platform claims, and the ratio between them. In Google Ads, show the All conv. columns so the numbers line up with GA4, as Google's attribution settings page advises. Do the same for Meta and every other platform, for the same dates. Path: Google Ads > Campaigns menu > Campaigns.
3. Check the claims against GA4 and Shopify. Compare Paid and organic last click with Data-driven for each channel. Then add up the platform claims and set the total next to Shopify's sales for the same dates. A channel whose revenue swings between models has credit that depends on the rule, not on the buyer. Path: GA4 > Advertising > Attribution > Attribution models, then Shopify admin > Analytics > Reports > Total sales over time.
4. Check the lag before you judge last month. Read Days to key event for your purchase key event. If slow buyers take weeks, part of last month's spend has not paid out yet, so judge it later. The Path length filter lets you look at multi-touch paths on their own. Path: GA4 > Advertising > Key events > Key event attribution paths.
5. Look for headroom the platforms report. A campaign marked Limited by budget missed 5% or more of its potential traffic last week. That is Google's definition, not a promise of profit. For bigger moves, build a plan for next month's dates, knowing it forecasts platform conversions. Path: Google Ads > Insights page > Budget pacing insights, and Tools menu > Performance Planner > plus icon.
6. Keep the base, move one slice. Hold every channel that clears break-even on the stricter view at last month's level. Move one slice from the weakest claim to the clearest headroom. Keep each campaign's change at or under 20%, since Google says bigger budget changes trigger a new learning period. Path: Google Ads > Campaigns menu > Campaigns > Budget column > pencil icon.
7. Set daily budgets, then book the check. Divide each campaign's monthly amount by 30.4, as Google's budget page instructs. Then book a holdout test for the channel that gained or lost the slice, so next month's split rests on evidence. Path: Google Ads > Campaigns menu > Campaigns > Budget column > pencil icon > Save.

A worked example

For illustration, with round invented numbers: next month's budget is €30,000, your margin is 40%, and last month looked like this.

Say your Shopify total for the month is €70,000. If the platforms claim €81,000, at least €11,000 of credit is counted twice. At a 40% margin, break-even is 1 / 0.40 = 2.5x, the same arithmetic as the Break-even sheet in one store's export.

Now read the rows. Google Search clears break-even on its own claim and on GA4's view. Meta sits on the line by its own count and well below it in GA4. It holds its budget and gets the next holdout test. TikTok is under the line on both views. If your buyers watch it more than they click it, click reports undercount it, so trim it rather than switch it off.

Say Budget pacing shows Search as Limited by budget. Say you move €1,000 from TikTok to Search: TikTok falls by about 17% and Search rises by about 8%, both under Google's 20% line. For illustration, the new split is €13,000 for Search, €12,000 for Meta and €5,000 for TikTok.

For illustration, Search at €13,000 a month becomes €13,000 / 30.4 = about €428 a day. In one store's Journeys sheet, two-to-three-touch journeys took 12.5 days to buy. If yours look similar, judge each move two weeks after month end.

Next month, the test decides the next slice. If Meta clears break-even on the sales it caused, it keeps its budget and can bid for more. If it does not, Meta is where the next slice comes from.

What to check when the numbers look wrong

• GA4 and Google Ads disagree about Google's own sales. Start with GA4's attribution settings: Admin > Data display > Events > Attribution settings. GA4's default lookback window is 90 days for most key events. GA4 uses last click for every Google Ads conversion built on a GA4 key event. Each tool also reports in its own time zone.
• Shopify's total is higher than GA4's revenue. Shopify's Total sales include taxes, shipping and pending orders, and GA4 only sees tracked visits. Compare like with like, or keep the gap steady month to month.
• The platform claims beat your real sales. Expected, not a bug. Each platform counts the sales it touched, so shared buyers show up as revenue that exists twice on paper.
• Shopify's profit report shows less revenue than the sales report. Profit only counts products that had a cost recorded when they sold. Add cost per item to your best sellers: Products > product name > Price section > Cost per item.
• Performance Planner will not forecast a campaign. Campaigns that changed bid strategies in the last 10 days, or that sit in an experiment, are ineligible. Wait, or plan them by hand.
• Spend overshot a daily budget. Google allows it. Most campaigns can spend up to two times the average daily budget on a day, but no more than 30.4 times it in a month.

What to do this week

1. Fill in cost per item for your best sellers. In Shopify, go to Products, open one, and fill in Cost per item under Price. Pass: Gross profit by product now covers most of your net sales. Fail: big gaps remain, so your margin, and with it your break-even, is a guess.
2. Write down your GA4 attribution settings. In Admin, under Data display, click Events, then Attribution settings. Pass: you know the reporting model and the lookback window, and you judge every channel on them. Fail: nobody knows who changed them, and Google notes a model change rewrites history too.
3. Draft next month in Performance Planner. Go to Performance Planner in the Tools menu, select the plus icon, and enter next month's dates. Pass: your main campaigns are forecastable. Fail: some are ineligible, for example after a recent bid strategy change, so plan those by hand this month.

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: Profit reports (Shopify Help Center). Select attribution settings (Google Analytics Help). Key event attribution models report (Google Analytics Help). Key events attribution paths report (Google Analytics Help). About budget pacing insights (Google Ads Help). Create and edit a plan with Performance Planner (Google Ads Help). About Performance Planner (Google Ads Help). Implement campaigns for geo experiments (Google Ads Help). Set and change an average daily budget for your campaign (Google Ads Help). About average daily budgets (Google Ads Help). Sales reports (Shopify Help Center).

Related answers

• All Budget & channel mix questions
• How do I decide where to spend next month's ad budget?
• Marketing mix modeling, explained
• Data-driven attribution, in the glossary