Four terms trip people up: CPC (cost per click), CTR (click-through rate), conversion rate (orders ÷ clicks) and CPA (cost per sale). For eCommerce, only two are the scoreboard: CPA and ROAS, the money metrics. CPC, CTR and conversion rate are diagnostics that explain the money, not goals to chase. A high CTR with a bad conversion rate is not a win.
Google Ads throws a lot of acronyms at you, and it is easy to end up optimising the wrong one. This is a plain-English glossary of the five metrics that matter most for an online store, what each actually means, and, more usefully, which ones are the goal and which are just clues.
The five metrics, defined
| Metric | What it is | Role |
|---|---|---|
| CPC (cost per click) | What you pay for one click. Spend ÷ clicks. | Diagnostic |
| CTR (click-through rate) | How often people click when they see your ad. Clicks ÷ impressions. | Diagnostic |
| Conversion rate (CVR) | How often a click becomes a sale. Conversions ÷ clicks. | Diagnostic |
| CPA (cost per acquisition) | What it costs to get one sale. Spend ÷ conversions. | Goal |
| ROAS (return on ad spend) | Revenue for every euro spent. Revenue ÷ spend. | Goal |
How they connect (the chain)
These metrics are not independent, they are links in one chain from an ad being seen to money in the bank:
Impressions → (CTR) → clicks → (CPC pays for them) → (conversion rate) → sales → CPA & ROAS.
Read left to right, each metric explains the next. If your CPA is too high, the cause is upstream: either your CPC is high (clicks are expensive), your conversion rate is low (clicks are not turning into sales), or both. That is exactly why CPC, CTR and CVR are worth watching, not as targets, but as the diagnostic trail that tells you why your CPA and ROAS look the way they do.
Goals vs diagnostics: the distinction that matters
This is the whole point of the guide. CPA and ROAS are goals, they measure whether the account makes money. CPC, CTR and conversion rate are diagnostics, they help you understand and improve the goals, but they are not themselves the target.
Common misreadings
- “Our CTR is up, things are working.” Not necessarily. CTR up with conversion rate down can mean you are attracting more clicks that do not buy.
- “We got the CPC down, great.” Only if the cheaper clicks still convert. Cheap, low-intent clicks can raise your CPA even as CPC falls.
- “Conversion rate dropped, the ads are broken.” Maybe, but a conversion-rate drop is often a site, price or tracking issue, one of the usual suspects behind a falling ROAS, not an ad problem at all.
So what should you actually report?
Lead with the money: revenue, ROAS against your break-even, and CPA. Keep CPC, CTR and conversion rate as the second layer that explains movements in the first. If your reporting is built the other way round, with impressions and CTR as the headline, you are measuring effort rather than outcome, which is also a sign worth reading in an agency's reporting.
Want someone to translate your account's numbers into plain “is this making money, and why?” A free audit reads the whole chain, from CTR to CPA, and tells you where the profit is being made or lost.
