Short answer
Buy on CPM only when your click-through rate is above the break-even CTR: CPM ÷ (10 × CPC), in percent. At a $40 CPM and a $5 CPC that is 0.8%. Below it, CPC is cheaper per click and the platform carries the risk of a weak ad. Start on CPC, learn the real CTR per ad group, and move only proven ad groups to CPM.
One formula
On CPM you pay for impressions, so the cost of a click depends on how many people click:
| Metric | Formula |
|---|---|
| Effective CPC when buying on CPM | CPM ÷ (1,000 × CTR) |
| Break-even CTR | CPM ÷ (10 × CPC), in % |
| Highest CPM worth paying at your CTR | CPC × CTR × 10 |
Two worked examples
| Example (illustrative prices) | CPM | CPC | CTR | Effective CPC on CPM | Break-even CTR | Cheaper |
|---|---|---|---|---|---|---|
| ChatGPT ads, decision questions | $40 | $5.00 | 0.66% | $6.06 | 0.80% | CPC |
| Same ad group, after better copy | $40 | $5.00 | 1.00% | $4.00 | 0.80% | CPM |
| LinkedIn sponsored content | $45 | $9.00 | 0.50% | $9.00 | 0.50% | Break-even |
The 0.66% CTR comes from the one published B2B ChatGPT ads pilot, which reported $5.68 a click. OpenAI added cost-per-click buying in 2026, so most advertisers can now choose.
Who carries the risk
- CPC: the platform carries the risk of a weak ad. If nobody clicks, you pay nothing.
- CPM: you carry it. A strong ad gets cheap clicks; a weak one gets expensive ones.
- So CPC suits new ad groups, new creative and unknown audiences. CPM suits proven ad groups and reach goals.
A switching rule
- Launch every new ad group on CPC.
- After two to four weeks, compare each ad group's CTR with its break-even CTR.
- Move only ad groups that held above break-even for the whole period, and keep a CPC control running.
- Move back if CTR drops below break-even for two weeks running.
Check the lead maths too: the highest CPC your cost-per-lead target allows is the target × landing page conversion rate. If both prices are above it, fix the landing page before the bid. The max CPM calculator works it back from deal value.
Beyond price
CPC optimisation leans towards people who click; CPM leans towards delivery. In B2B the cheapest click isn't always the best company, so compare the accounts each model brings before you settle. More on bidding in ChatGPT ads bidding for B2B.
Questions
What is the break-even CTR between CPC and CPM?
CPM divided by (10 × CPC), expressed in percent. Above it, buying on CPM gives a lower cost per click.
Is CPM ever better even when it costs more per click?
Yes, when the job is reach to a defined audience rather than clicks, for example keeping a target account list aware of you. Then judge it on reach and account engagement.
Does the bidding model change who sees the ad?
It can. Platforms optimise CPC campaigns towards people likely to click and CPM campaigns towards delivery. Compare the companies each brings, not only the price.
What CTR should I assume for ChatGPT ads?
Use your own after two to four weeks. The one published B2B pilot reported 0.66%, which is a reference point, not a target.
Facts checked against the sources below on 19 October 2026. Ad platforms change quickly: if something here no longer matches what you see, tell us and we will update it.
