9 metrics to measure email marketing ROI

Email marketing is still one of the highest-return channels around. Done right, it can deliver $36–$42 for every $1 spent. But here’s the problem: you can’t just send campaigns and hope the ROI is there. You need hard numbers to prove it — to yourself, your boss, and the budget holders.

And no, “our open rate went up 5%” isn’t enough to declare victory. Measuring ROI means connecting activity to business outcomes. That requires going beyond vanity metrics to see which campaigns actually move revenue, retention, and customer lifetime value.

Let’s break down the 9 most important metrics to measure email marketing ROI — and how to actually use them.

1. Conversion rate

Your open and click rates tell you interest; your conversion rate tells you impact. It’s the percentage of recipients who took the action you wanted after opening the email — whether that’s buying a product, booking a demo, or downloading a resource.

Formula:
(Number of conversions ÷ Number of delivered emails) × 100

Example: You send 5,000 emails promoting a webinar, 400 people register — your conversion rate is 8%.

Why it matters: High clicks but low conversions usually mean the email is fine, but the landing page or offer needs work.

2. Revenue per email (RPE)

RPE is one of the cleanest ways to see how much money each email generates. Instead of looking at revenue in bulk, it breaks it down per send, which is useful for comparing different campaigns.

Formula:
Total campaign revenue ÷ Number of delivered emails

Example: A campaign that makes $4,500 from 9,000 emails has an RPE of $0.50.

Why it matters: It’s a quick reality check on whether a campaign was worth the effort, especially when comparing one-off sends to automated flows.

3. Customer acquisition cost (CAC) from email

If you’re using email to generate new customers, you need to know what you’re paying for each one. This isn’t just ad spend — include your email platform fees, creative costs, and any incentives you offered.

Formula:
Total campaign cost ÷ Number of new customers acquired via email

Why it matters: Without this, you might think your email campaigns are profitable, but in reality, they’re costing more per customer than your average purchase value.

4. Return on investment (ROI)

Yes, ROI itself is a metric — and the ultimate one. It tells you how much you got back for every dollar you put in. For email, include costs like software, design, copywriting, list management, and promotions.

Formula:
((Total revenue – Total cost) ÷ Total cost) × 100

Example: If a $2,000 campaign generates $8,000 in revenue, ROI is 300%.

Why it matters: This is the number that makes budget conversations easier — or harder.

5. Click-to-open rate (CTOR)

CTOR looks at clicks as a percentage of opens, showing how compelling your email content is after it gets opened.

Formula:
(Unique clicks ÷ Unique opens) × 100

Why it matters: It cuts through inflated open rates caused by curiosity or accidental opens. A low CTOR often means your offer or CTA wasn’t strong enough.

6. Average order value (AOV) from email traffic

Some email campaigns drive smaller, impulse purchases; others push higher-value buys. Tracking AOV from email-driven sales shows you what kind of customer behaviour your campaigns are influencing.

Formula:
Email-driven revenue ÷ Number of email-driven orders

Why it matters: If AOV is consistently low, consider using segmentation and upsell tactics in your emails to nudge bigger purchases.

7. List growth rate

Your ROI will be hard to sustain if your list isn’t growing. This metric shows the rate at which you’re adding (or losing) subscribers.

Formula:
((New subscribers – Unsubscribes – Bounces) ÷ Total list size) × 100

Why it matters: It’s not just about size — it’s about keeping a steady pipeline of fresh, engaged subscribers who can convert into revenue later.

Pro Tip: Tools like ReferralCandy can fuel list growth automatically by turning your customers into advocates. When existing subscribers refer friends through a referral program, you not only grow your list but also acquire warmer leads who are more likely to convert.

8. Customer lifetime value (CLV) from email-acquired customers

Email isn’t just a sales channel; it’s a relationship builder. CLV measures the total revenue a customer brings in over their entire relationship with your brand — and tracking this for email-acquired customers tells you how valuable the channel really is long-term.

Why it matters: A campaign might look modest on short-term revenue but be a big win if those customers stick around and keep buying.

9. Campaign attribution

Attribution shows you how email fits into the bigger marketing picture. Maybe someone clicks an email, browses your site, leaves, then returns a week later via organic search to buy. Without multi-touch attribution, you’d think the email didn’t contribute.

Why it matters: Email often plays a role in nurturing and influencing purchases, even if it’s not the final click before conversion.

Wrapping it up

Email marketing ROI isn’t about counting opens and patting yourself on the back. It’s about connecting your sends to measurable business results: revenue, acquisition, retention, and lifetime value.

The smartest teams don’t track every metric under the sun — they pick a core set that aligns with their goals. A campaign designed to boost average spend will measure success differently than one meant to grow the list.

Track the right numbers consistently, and you’ll have the clarity (and the proof) to keep your email budget healthy — and your results even healthier.