The most common version of this comparison goes badly wrong immediately: pull the ROAS number from Meta Ads Manager, pull the ROAS number from Google Ads, compare them directly, and allocate more budget to whichever is higher. It is a plausible-looking analysis that is nearly always misleading, because the two numbers are not measuring the same thing using the same rules.
Getting a meaningful cross-channel ROAS comparison requires adjusting for at least three structural differences between Meta and Google. Not adjusting for them does not make the comparison neutral. It makes it biased in ways that almost always favor Google, which means teams systematically over-invest in Google and under-invest in Meta relative to what a corrected analysis would suggest. Sometimes. The opposite bias exists in other scenarios too, and understanding why matters more than knowing which direction the error usually runs.
Attribution Window Differences
Meta's default attribution setting is a 7-day click plus 1-day view window. Google's default is a 30-day click window for most campaign types (some use shorter windows). When you compare these numbers without adjustment, you are comparing a 7-day signal from Meta to a 30-day signal from Google.
For most product categories, conversion rates are not uniform across the days following an ad interaction. A significant share of conversions happen within 24 to 48 hours for impulse-adjacent categories (fast fashion, consumables). For considered purchases (home furnishings, electronics, premium apparel), the distribution extends further: a meaningful proportion of conversions happen 8 to 20 days after first ad exposure.
If your product has a longer consideration cycle, Google's 30-day window captures conversions that Meta's 7-day window misses for the same customer journey. Your Google ROAS looks higher not because Google performed better, but because it is capturing more of the eventual conversions from the same media exposure period. The comparison is not between two equivalent signals: it is between a partial attribution from Meta and a more complete one from Google.
The correction is to run Meta with a 1-day click window or pull the data using consistent windows across both platforms via the API. Most teams do not do this because it requires going outside the default dashboard view, and default dashboard views are what most allocation decisions get made from. The mismatch is built into the standard workflow.
Channel Intent and Conversion Probability
Google Search captures in-market intent. Someone searching "buy noise-cancelling headphones under $200" has signalled active purchase intent. The ad that appears in response to that query reaches a person who is already in a buying mindset. Conversion rates on branded and high-intent non-branded Search terms are structurally higher than on any push channel.
Meta reaches people who were not looking for your product at the moment they saw your ad. The conversion rate from ad impression to purchase is structurally lower because you are starting from a cold or warm audience rather than from declared intent. Meta's job is different: it creates awareness, consideration, and intent in people who may then go search for your product on Google. Meta's visible ROAS often looks lower than Google's not because Meta is performing worse but because Meta is doing different work in the funnel.
This creates a genuine comparison problem. A ROAS of 2.5x on Meta prospecting and 4.5x on Google Search are not automatically a signal to move budget from Meta to Google. If Meta is generating the initial intent that leads to the Google searches that generate the 4.5x ROAS, cutting Meta budget will eventually show up as degraded Google performance with a 3 to 6 week lag. By the time you see the signal, you have already committed three months of budget to a mix that is cannibalizing its own upper-funnel.
Variance, Stability, and Risk Profile
Google Search ROAS is relatively stable week over week for established accounts. The auction for most search terms does not change dramatically from one week to the next. Your ROAS on branded search might range from 6.0x to 8.0x over a quarter, with the variance driven mainly by seasonality and competitive bidding pressure.
Meta ROAS is structurally more volatile. Creative fatigue sets in within two to four weeks for most audiences. Algorithm changes affect delivery efficiency. Audience saturation in your core targeting builds over time. It is not uncommon for a Meta account to see ROAS swing from 3.5x in one week to 1.8x the following week when a creative set is fatiguing and the account has not refreshed it.
When you compare a stable 4.5x from Google to an average 3.2x from Meta that has variance from 1.8x to 4.5x, the right budget allocation decision depends heavily on your organization's risk tolerance and your ability to react when Meta underperforms. A team with weekly review cadence and the operational capability to refresh creative on a 2 to 3 week cycle can make the most of Meta's upside potential. A team locked into monthly reviews and a quarterly creative production schedule should probably hold a higher proportion of budget in the more predictable Google channel and treat Meta as a smaller, more managed allocation.
Lag Curve Differences and Time-Value of Money
There is a practical consideration that rarely shows up in ROAS comparisons but affects cash flow planning for smaller brands: the lag between spend and recognized revenue is different across channels.
Google Shopping revenue attribution is typically within 24 to 72 hours of the click. Google Search for lower-consideration products is similar. Meta, with its push channel dynamics and longer consideration cycles, can have a longer average time between ad exposure and purchase. For a brand spending at the level where cash flow timing matters (which includes most early-stage DTC operators running on lean working capital), the time-to-revenue profile of each channel affects how much of your cash budget can comfortably be allocated to it in a given week.
We are not saying this is a primary allocation driver. But it is a real factor that shows up in practice and is invisible when you look at ROAS numbers alone.
How to Actually Do This Comparison
A comparison that is honest about the structural differences requires three adjustments at minimum: standardize attribution windows (use 7-day click across both platforms), segment by funnel position (compare Google branded search to Meta retargeting separately from Google non-branded to Meta prospecting), and account for return rates if they differ by channel origin.
Even with these adjustments, the comparison is still incomplete because it does not account for the upstream contribution of awareness channels to lower-funnel performance. Getting the full picture requires incrementality testing, which is a separate workstream from ROAS analysis. The honest position is that cross-channel ROAS comparison, even done carefully, does not give you a complete picture of which channels are earning their allocation. It gives you a less misleading picture than the raw dashboard numbers do. That is a lower bar than it sounds, and it is still genuinely worth reaching.