CTR, CPC, CPA and ROAS Explained, with Worked Examples
The formulas behind the main advertising metrics, how they relate, and how to read them together instead of one at a time.
The core formulas
Every advertising dashboard reports these numbers. Each is a simple ratio.
Suppose a campaign had 200,000 impressions, 4,000 clicks, 160 conversions, a spend of 20,000 and revenue of 80,000. The examples below use those figures.
- CTR (click-through rate) = clicks / impressions. Here 4,000 / 200,000 = 2%.
- CPC (cost per click) = spend / clicks. Here 20,000 / 4,000 = 5.
- Conversion rate = conversions / clicks. Here 160 / 4,000 = 4%.
- CPA (cost per acquisition) = spend / conversions. Here 20,000 / 160 = 125.
- CPM (cost per thousand impressions) = spend / impressions x 1,000. Here 100.
- ROAS (return on ad spend) = revenue / spend. Here 80,000 / 20,000 = 4, often written 4x or 400%.
How they connect
The metrics are linked. CPA equals CPC divided by the conversion rate, so 5 / 0.04 = 125. That tells you there are only two ways to lower CPA: pay less per click, or turn more clicks into conversions. Improving the landing page can cut CPA as much as improving the ad.
Worked example: two campaigns compared
Two campaigns can look similar in one metric and very different in another. Here is one on search and one on display, with real arithmetic for each.
| Metric | Search | Display |
|---|---|---|
| Impressions | 50,000 | 400,000 |
| Clicks | 2,500 | 2,000 |
| Conversions | 100 | 20 |
| Spend | 10,000 | 4,000 |
| Revenue | 30,000 | 6,000 |
| CTR | 5% | 0.5% |
| CPC | 4 | 2 |
| Conversion rate | 4% | 1% |
| CPA | 100 | 200 |
| CPM | 200 | 10 |
| ROAS | 3 | 1.5 |
Display looks cheaper on CPC and CPM, but each conversion costs twice as much because only 1 click in 100 converts. The cheaper click is not the cheaper customer. Always finish the chain down to CPA and ROAS before judging a channel.
Applying break-even ROAS to the example
Say the product has a 40% margin. Break-even ROAS is 1 / 0.40 = 2.5.
Search returns 30,000 in revenue, which is 12,000 in gross profit at a 40% margin. After 10,000 of ad spend, it makes 2,000. Its ROAS of 3 is above 2.5, so it is profitable.
Display returns 6,000 in revenue, which is 2,400 in gross profit. After 4,000 of ad spend, it loses 1,600. Its ROAS of 1.5 is below 2.5, so it loses money even though its ROAS is above 1.
This is the reason a ROAS above 1 does not mean a campaign is working. It only means revenue exceeded spend, and margin has not been paid yet.
Reading ROAS correctly
ROAS compares revenue to ad spend, not profit. ROAS compares revenue to ad spend, not profit. A ROAS of 4 sounds strong, but at a 20% margin every 1 spent on ads brings 4 in revenue and only 0.80 in gross profit, so you lose 0.20 on each unit spent before any other cost.
The break-even ROAS is 1 divided by your margin. At a 20% margin it is 5, and at a 50% margin it is 2. Compare your actual ROAS to that number, not to a general benchmark.
Common mistakes
- Judging a campaign by CTR alone. A high CTR with low conversions can still waste money.
- Mixing attribution windows. Compare platforms only when they use the same window.
- Ignoring returns and cancellations when counting revenue.
- Looking at tiny samples. A CPA based on five conversions moves a lot with one more.
The Marketing Metrics Calculator computes all of these from your totals, and the Break-even ROAS Calculator turns your margin into the number to beat.
Which number to fix first
- Low CTR with decent conversion rate: the ad or targeting is not attracting clicks. Test new headlines, creative and audiences.
- High CTR with low conversion rate: the ad promises something the page does not deliver, or the page is slow or confusing. Fix the landing page before spending more.
- High CPC with healthy conversion rate: you are paying a lot per click but it still works. Check CPA against what a customer is worth before cutting.
- Good CPA but low total volume: the campaign is efficient but limited by budget or audience size, so scaling is the question, not fixing.
Frequently asked questions
- What is a good CTR? It depends on the channel, the audience and the ad position, so there is no single number. Compare against your own past campaigns on the same channel.
- Is a higher ROAS always better? No. A very high ROAS often means you are spending too little and missing volume. The goal is the highest profit, not the highest ratio.
- What is the difference between CPA and CAC? CPA usually counts the ad cost per conversion, which may be a sale or a lead. CAC is a broader figure that can include sales, tools and salaries for winning a customer.
- Why does my CPA change when my CPC stays the same? CPA is CPC divided by conversion rate. If fewer clicks convert, CPA rises even when each click costs the same.
- How many conversions do I need before trusting the numbers? There is no fixed threshold, but a result built on a handful of conversions swings widely with each new one. Wait until adding one more conversion barely moves the figure.