Your GA4 data source does not have a Cost per Conversion field. Your Google Ads connector does not have a ROAS column. Your blended data source does not calculate what percentage of total sessions each channel represents.
These are not gaps in the data — the raw numbers are all there. They just need to be combined. Calculated fields are how you do that.
What a Calculated Field Is
A calculated field is a custom metric or dimension you define using a formula. It does not exist in the original data source — you create it inside Looker Studio and it is calculated on the fly when the chart renders.
You can create calculated fields in two places:
In the data source editor. These fields are available across all charts in any report that uses that data source. If you build Cost per Conversion as a data source field, every report connected to that GA4 or Google Ads source has access to it automatically.
In a specific chart. These are local to that chart only. Useful for a one-off calculation that does not need to be reused across the report.
For metrics you use regularly — ROAS, CPL, conversion rate variations — create them at the data source level. For ad hoc calculations, use the chart-level option.
Creating a Calculated Field
In the data source editor, scroll to the bottom of the field list and click Add a Field. Give it a name, write the formula, and save.
In a chart, open the Metrics or Dimensions panel in the properties sidebar and click Add Metric, then Create Field.
Common Formulas for Marketing Dashboards
Cost per Conversion
Cost / Conversions
Straightforward division. Watch out for division by zero — if a campaign has zero conversions, this field will show an error or blank for that row. Wrap it in a NULLIF to handle this:
Cost / NULLIF(Conversions, 0)
NULLIF(Conversions, 0) returns null when conversions is zero, which causes the division to return null rather than an error. The cell will appear blank, which is cleaner than an error message.
ROAS (Return on Ad Spend)
Revenue / Cost
Where Revenue is your conversion value field. In GA4 this is typically the purchase_revenue metric or a custom conversion value you have configured. In Google Ads it is the conversion value field.
Conversion Rate
Conversions / Sessions
Or for a click-based conversion rate from Google Ads:
Conversions / Clicks
Percentage of Total
To show what percentage of total sessions each channel represents, use:
Sessions / SUM(Sessions)
The SUM() function without a grouping dimension calculates the grand total across all rows in the chart, so dividing each row’s value by the sum gives you the percentage share.
Click-Through Rate
Clicks / Impressions
Format the field as a percentage in the field settings.
Text and Dimension Calculated Fields
Calculated fields are not limited to numbers. You can create calculated dimensions using string functions.
Consolidate campaign name variations:
CASE
WHEN REGEXP_MATCH(Campaign, ".*Brand.*") THEN "Brand"
WHEN REGEXP_MATCH(Campaign, ".*Competitor.*") THEN "Competitor"
ELSE "Non-Brand"
END
This uses a CASE statement to bucket campaigns into simplified categories based on whether their name contains certain text. Useful when you have dozens of campaign names but want to report at a higher level.
Extract a value from a URL parameter:
REGEXP_EXTRACT(Page Path, "utm_campaign=([^&]+)")
Extracts the UTM campaign value from a page path if it is present in the URL.
Truncate a long string:
LEFT(Campaign, 30)
Returns the first 30 characters of the campaign name. Helpful when long campaign names overflow table columns.
Number Formatting
After creating a calculated field, set its type and format. A field calculated as a ratio (Conversions / Sessions) will default to a raw decimal like 0.043. Format it as a percentage and it displays as 4.3%.
In the data source editor, click the type dropdown next to the field name and select the correct type: Number, Percent, Currency, Duration, or Text.
For currency fields, also set the currency symbol. Looker Studio does not automatically inherit the currency from the underlying data source.
Common Mistakes
Creating the same field in multiple charts instead of the data source. If you create ROAS in five different charts, you have five separate formulas to maintain. Create it once in the data source and reference it everywhere.
Mixing metrics from different data sources in one field. A calculated field in GA4 can only reference GA4 fields. You cannot write a formula that divides Google Ads Cost by GA4 Sessions — those fields live in separate data sources. For cross-source calculations, you need data blending (covered in the next post).
Forgetting that aggregation matters. When you write Cost / Conversions in a chart that shows data by campaign, the formula divides each campaign’s cost by that campaign’s conversions. If you use that same field in a scorecard showing totals, it divides total cost by total conversions. The formula is the same but the result is different because aggregation changes what the variables represent. Make sure the formula behaves correctly at every level of aggregation you use it.
Not naming fields clearly. A calculated field named “Field 1” or “Metric A” creates confusion six months later. Name it descriptively: “Cost per Conversion”, “ROAS”, “Channel Share of Sessions”. Anyone editing the report after you should understand what the field is without reading the formula.
Calculated fields are one of the highest-leverage features in Looker Studio. A well-built data source with the right calculated fields already defined means every report built on top of it has immediate access to the metrics you actually use — not just the ones the platform decided to export.
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Building Your First Looker Studio Dashboard: A Step-by-Step Guide
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Looker Studio Chart Types Explained: When to Use Each One
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