Two people can look at the same account, quote the same figure, and mean different things. It happens constantly, and it makes performance conversations impossible.
Here is the distinction, why it matters more than it sounds, and how to configure your account so the number you report is the number you meant.
The definitions
Cost per lead is spend divided by leads. A lead is someone who has given you their details and asked to be contacted – a completed form, a phone call. It is a top-of-funnel measure. It tells you what it costs to get someone to raise their hand.
Cost per acquisition is spend divided by acquisitions, and an acquisition is a customer. Someone who bought. It is a bottom-of-funnel measure, and it includes every lead that went nowhere.
If you spend £4,000, generate 80 leads and close 10 of them, your cost per lead is £50 and your CPA is £400. Same spend, same month, eight times the difference. Quote the wrong one in a board meeting and you will have an interesting conversation.
Why the confusion is so persistent
Google Ads reports a column called “Cost / conv.” – cost per conversion. A conversion is whatever you configured it to be. So if your only conversion action is a form submission, Google’s cost per conversion is your cost per lead, even though the label says something more official.
People read “cost per conversion”, think “acquisition”, say “CPA”, and a lead-generation number gets discussed as though it were a sales number. Nobody is being careless – the platform’s own vocabulary invites it.
Which one should you optimise to?
It depends on how long your sales cycle is and how good your lead quality data is.
Use cost per lead when your sales cycle is short, lead quality is consistent, or you are early in a campaign and need volume to learn from. It is responsive – you see the effect of a change within days – and Smart Bidding has enough data to work with.
Use CPA when your sales cycle is long enough that lead volume misleads, or when lead quality varies sharply by source. A campaign producing cheap leads that never close is a campaign losing money, and only CPA reveals it.
A worked example makes the difference concrete. Say two campaigns each spend £2,000 in a month. Campaign A returns 50 leads at £40 each; campaign B returns 25 leads at £80 each. On cost per lead, A wins comfortably and the obvious move is to shift budget towards it.
Now add the close rate. A’s leads close at 6%, giving 3 customers and a CPA of £667. B’s close at 20%, giving 5 customers and a CPA of £400. The campaign that looked twice as expensive is the one actually producing customers, and the budget shift would have cost you money.
In practice most accounts should track both: cost per lead as the operational metric you optimise weekly, CPA as the commercial metric you review monthly. The trap is switching between them mid-conversation.
Setting it up so the numbers are real
Count only qualified actions. Form submissions and phone calls over a meaningful duration – thirty to sixty seconds, so enquiries count and wrong numbers do not. Mark everything else secondary. Page views and button clicks should never be primary conversion actions.
Set the counting method correctly. For lead generation, count one conversion per click, not every. Otherwise a single person submitting twice inflates your numbers.
Deduplicate calls and forms. If someone fills in a form and then rings, that is one lead. Without deduplication your cost per lead looks better than it is.
Import offline conversions if you can. This is what turns cost per lead into true CPA. When a lead becomes a customer in your CRM, send that back to Google Ads. Bidding then optimises towards leads that actually close, not leads that merely arrive. It takes setup effort and it is the single highest-value tracking improvement available to most accounts.
Write the definition down. Put it at the top of your monthly report: what counts as a conversion, which metric this document uses, and the date the definition last changed. It sounds bureaucratic. It ends the arguments.
The short version
Cost per lead measures interest. CPA measures revenue. Both are useful, neither substitutes for the other, and the worst outcome is using them interchangeably – because then you cannot tell whether the account is getting better.
Pick your definition, configure conversions to match it, and keep it stable long enough to see a trend.
Not sure what your account is counting?
Our free audit checks exactly this – what is set as a conversion, whether it should be, and what your real cost per lead looks like once the noise is stripped out.