The Complete Guide to Digital Marketing · Measurement

Return on Marketing Investment Explained

If you cannot measure it, you cannot judge it. Here is what return on marketing investment means, plus attribution, cost per acquisition, customer lifetime value, proving ROI and A/B testing.

Updated: July 2026
Written by: Andrew Odgers, Managing Director
Reading time: 8 minutes
The short answer

Return on marketing investment (ROMI) is a way of measuring what you get back from marketing compared with what you spend. Attribution works out which touchpoints contributed to a sale. Cost per acquisition is what it costs to gain one customer; customer lifetime value is the total value a customer brings over time, and comparing the two shows whether acquiring them is worthwhile. You prove ROI by tracking activity through to real outcomes and comparing value against cost. A/B testing compares two versions to see which performs better, improving results over time. No result is guaranteed.

Spend versus what it returns

What ROMI is, and attribution

Return on marketing investment, or ROMI, is a way of measuring what you get back from your marketing compared with what you spend, so you can judge whether your marketing is paying off. It relates the value generated to the cost of generating it; because it connects spend to results, ROMI is the headline measure of whether marketing is worthwhile, though it depends on measuring both sides accurately. Attribution is working out which marketing touchpoints contributed to a sale or lead, so you can credit the channels and campaigns that actually helped rather than guessing. Because customers often interact with several channels before buying, attribution tries to share the credit fairly; because it shows what is really driving results, attribution is central to measuring return accurately, even though it is rarely perfect.

RM 01

Value vs cost

The core of ROMI.

RM 02

CPA and CLV

Cost to gain vs value over time.

RM 03

Test and prove

Evidence, not guesswork.

CPA, CLV, proving ROI and testing

Cost per acquisition and lifetime value, proving ROI, and A/B testing

Cost per acquisition is what it costs, on average, to gain one customer, while customer lifetime value is the total value a customer is expected to bring over the whole time they stay with you. Comparing the two shows whether acquiring a customer is worthwhile; because a customer worth far more than they cost to acquire is a good investment, these two metrics together are key to judging marketing return. You prove the ROI of digital marketing by tracking marketing activity through to real outcomes, such as leads and sales, using analytics and clear goals, then comparing the value produced against the cost, honestly and consistently. Good tracking and sensible attribution make the case credible, which starts with the goals in your marketing plan; because decision-makers want evidence, connecting spend to measurable outcomes is how you demonstrate the return, while being clear that no result is guaranteed. A/B testing is comparing two versions of something, such as an email, ad or web page, by showing each to a portion of your audience to see which performs better. It replaces guesswork with evidence about what works; because small improvements found through testing can compound over time, A/B testing helps improve results and, in turn, the return you get from your marketing, and it works alongside your budget decisions.

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Frequently asked

Return on marketing investment

What is return on marketing investment?
Return on marketing investment, or ROMI, is a way of measuring what you get back from your marketing compared with what you spend, so you can judge whether your marketing is paying off. It relates the value generated to the cost of generating it. Because it connects spend to results, ROMI is the headline measure of whether marketing is worthwhile, though it depends on measuring both sides accurately.
What is attribution in digital marketing?
Attribution is working out which marketing touchpoints contributed to a sale or lead, so you can credit the channels and campaigns that actually helped rather than guessing. Because customers often interact with several channels before buying, attribution tries to share the credit fairly. Because it shows what is really driving results, attribution is central to measuring return accurately, even though it is rarely perfect.
What are cost per acquisition and customer lifetime value?
Cost per acquisition is what it costs, on average, to gain one customer, while customer lifetime value is the total value a customer is expected to bring over the whole time they stay with you. Comparing the two shows whether acquiring a customer is worthwhile. Because a customer worth far more than they cost to acquire is a good investment, these two metrics together are key to judging marketing return.
How do you prove the ROI of digital marketing?
You prove ROI by tracking marketing activity through to real outcomes, such as leads and sales, using analytics and clear goals, then comparing the value produced against the cost, honestly and consistently. Good tracking and sensible attribution make the case credible. Because decision-makers want evidence, connecting spend to measurable outcomes is how you demonstrate the return, while being clear that no result is guaranteed.
What is A/B testing and how does it help ROI?
A/B testing is comparing two versions of something, such as an email, ad or web page, by showing each to a portion of your audience to see which performs better. It replaces guesswork with evidence about what works. Because small improvements found through testing can compound over time, A/B testing helps improve results and, in turn, the return you get from your marketing.