This Google Ads formula library explains the common calculations behind campaign reports. It keeps formulas separate from benchmark judgments, because a number can only be judged after the campaign goal, attribution setup, bid strategy, and conversion quality are clear.
Core Google Ads Formulas
| Metric | Formula | Use carefully because… |
|---|---|---|
| CTR | Clicks / impressions x 100 | It measures click response, not conversion quality. |
| Average CPC | Cost / clicks | Actual CPC depends on auction conditions and Ad Rank. |
| CPM | Cost / impressions x 1,000 | Best for impression-cost context, not sales performance alone. |
| Conversion rate | Conversions / interactions x 100 | It depends on which conversion actions are counted. |
| Cost per conversion | Cost / conversions | Do not compare lead conversions and purchase conversions as if they are equal. |
| ROAS | Conversion value / cost | Value tracking and attribution rules matter. |
| Impression share | Impressions received / eligible impressions x 100 | Eligibility changes with targeting, budget, bids, and quality. |
Quality Score and Ad Rank
Quality Score is a diagnostic rating for Search campaigns based on expected CTR, ad relevance, and landing page experience. It should not be treated as the only auction input or a performance forecast. Google Ads says Ad Rank considers bid, ad and landing page quality, thresholds, competition, search context, and the expected impact of assets and formats.
Business Formulas Around Google Ads
- CPA = ad cost / conversions.
- CAC = acquisition-related costs / new customers acquired.
- Break-even ROAS = 1 / gross margin.
- ROI = profit or return / investment cost x 100, using a clearly defined cost scope.
These broader business formulas may use Google Ads data, but they are not always the same as platform-reported metrics. The Marketing KPI Formula Library, ROAS Calculator, and CPA Calculator are useful companion resources.
Sources and Methodology
This version removes unsupported “good” benchmarks from the formula library and keeps the article focused on definitions, calculations, and caveats.
