What is the formula for email open rate?

Email open rate = (Unique opens ÷ Emails delivered) × 100. For example, 2,400 opens from 12,000 delivered emails = 20% open rate.

How do I calculate email click-through rate (CTR)?

Email CTR = (Unique clicks ÷ Emails delivered) × 100. If 480 people clicked from 12,000 delivered emails, your CTR is 4%.

What is click-to-open rate (CTOR) and how is it calculated?

CTOR = (Unique clicks ÷ Unique opens) × 100. It measures how compelling your email content is to people who actually opened — a better content quality signal than CTR alone.

How do you calculate email ROI?

Email ROI = ((Revenue generated – Campaign cost) ÷ Campaign cost) × 100. For example, £12,000 revenue from a £500 campaign = 2,300% ROI.

What is revenue per subscriber (RPS)?

Revenue per subscriber = Total email revenue ÷ Total subscribers. It tells you the average monetary value each contact generates and helps evaluate acquisition costs.

How do I calculate subscriber lifetime value (SLV)?

Subscriber lifetime value = Revenue per subscriber × Average subscriber lifespan. If each subscriber generates £3/year and stays for 4 years, SLV = £12.

What is a good email open rate?

Average email open rates vary by industry, ranging from around 15% to 45%. B2B typically sees higher open rates (25–35%) while e-commerce averages 15–25%. Always benchmark against your own industry.

What is the email delivery rate formula?

Delivery rate = (Emails delivered ÷ Emails sent) × 100. Or alternatively: Delivery rate = ((Sent – Bounces) ÷ Sent) × 100. A healthy delivery rate is 97% or above.

How can Email Calculator help with these formulas?

Email Calculator automatically applies all these formulas to your campaign data the moment you enter or sync your metrics — no spreadsheets needed. You get open rate, CTR, CTOR, delivery rate, quality score, and more calculated instantly.

Email Marketing Formulas Every Marketer Should Know

Most marketers track email metrics.

Very few understand the math behind performance.

Open rates and click-through rates are useful — but they don't answer the real questions:

Is my email program profitable?
How much is a subscriber actually worth?
What does my list cost vs. what does it generate?
When does sending more emails start hurting revenue?

This is the complete reference guide to email marketing formulas — from basic delivery metrics to advanced revenue calculations.

Bookmark it. Use it every time you analyse a campaign.

Quick Reference: All Email Marketing Formulas

Metric	Formula
Open Rate	(Unique Opens ÷ Delivered) × 100
Click-Through Rate (CTR)	(Unique Clicks ÷ Delivered) × 100
Click-to-Open Rate (CTOR)	(Unique Clicks ÷ Unique Opens) × 100
Delivery Rate	(Delivered ÷ Sent) × 100
Bounce Rate	(Bounces ÷ Sent) × 100
Unsubscribe Rate	(Unsubscribes ÷ Delivered) × 100
Spam Complaint Rate	(Spam Reports ÷ Delivered) × 100
Conversion Rate	(Conversions ÷ Delivered) × 100
Email ROI	((Revenue – Cost) ÷ Cost) × 100
Revenue Per Email (RPE)	Total Revenue ÷ Emails Delivered
Revenue Per Subscriber (RPS)	Total Revenue ÷ Total Subscribers
List Value	RPS × Total Subscribers
Subscriber Lifetime Value (SLV)	RPS × Average Subscriber Lifespan
Cost Per Open	Campaign Cost ÷ Total Opens
Cost Per Click	Campaign Cost ÷ Total Clicks
List Growth Rate	((New Subscribers – Unsubscribes) ÷ Total List) × 100
Profitability Threshold	(Revenue Per Subscriber × Recipients) – Campaign Cost

Why Email Marketing Formulas Matter

Email marketing generates an average return of £36–42 for every £1 spent — the highest ROI of any digital marketing channel.

But that average hides enormous variation.

Some email programs generate 10× that. Others lose money on every send.

The difference is usually measurement.

When you understand the underlying formulas, you can:

Calculate exactly what each campaign earns
Know how much you can spend to acquire a subscriber
Identify which segments are worth targeting
Prevent over-mailing from destroying long-term value
Report performance in business terms, not just percentages

Metrics show activity. Formulas show business impact.

Part 1: Delivery & Engagement Metrics

These are the foundational formulas — the ones you should calculate for every single campaign.

1. Delivery Rate

Delivery rate measures the percentage of emails that actually reached a recipient's inbox (or junk folder). It's the most fundamental indicator of list health and sender reputation.

Formula

Delivery Rate = (Emails Delivered ÷ Emails Sent) × 100

Or equivalently:

Delivery Rate = ((Sent – Bounces) ÷ Sent) × 100

Example

Sent: 15,000
Bounces: 300
Delivered: 14,700

(14,700 ÷ 15,000) × 100 = 98% delivery rate

Benchmarks

Rating	Delivery Rate
Excellent	99%+
Good	97–99%
Concerning	95–97%
Poor	Below 95%

Why It Matters

A low delivery rate means your list contains invalid or inactive addresses. This hurts your sender reputation, which compounds over time. Clean lists aren't optional — they're a business requirement.

2. Open Rate

Open rate measures the percentage of delivered emails that were opened. It's the most commonly reported email metric.

Formula

Open Rate = (Unique Opens ÷ Emails Delivered) × 100

Example

Delivered: 14,700
Unique opens: 3,234

(3,234 ÷ 14,700) × 100 = 22% open rate

Important Caveat

Apple Mail Privacy Protection (MPP) — introduced in iOS 15 — automatically pre-loads email pixels for many Apple Mail users, inflating open rates. In 2026, open rates should be treated as a directional indicator, not an absolute measurement. Watch trends over time rather than comparing raw numbers across platforms.

Industry Benchmarks

Sector	Average Open Rate
Non-profit	35–45%
Government / Public Sector	30–40%
B2B Technology	25–35%
Finance	25–30%
Retail / E-commerce	15–25%
Travel	18–22%

Why It Matters

Open rate signals how well your subject line and sender name perform. It's the first filter — if subscribers don't open, nothing else matters.

3. Click-Through Rate (CTR)

Click-through rate measures the percentage of delivered emails that generated at least one click.

Formula

CTR = (Unique Clicks ÷ Emails Delivered) × 100

Example

Delivered: 14,700
Unique clicks: 588

(588 ÷ 14,700) × 100 = 4% CTR

Industry Average

Average email CTR across industries is typically 2–5%. B2B campaigns and highly segmented lists often exceed this significantly.

Why It Matters

CTR measures overall campaign effectiveness — the combined impact of subject line, content, design, and call-to-action. It's a reliable proxy for engagement intent.

4. Click-to-Open Rate (CTOR)

CTOR is one of the most underused — and most valuable — email metrics.

While CTR measures clicks against all delivered emails, CTOR measures clicks only against people who opened. This isolates the quality of your email content from the quality of your subject line.

Formula

CTOR = (Unique Clicks ÷ Unique Opens) × 100

Example

Unique opens: 3,234
Unique clicks: 588

(588 ÷ 3,234) × 100 = 18.2% CTOR

Why It Matters

If your open rate is high but CTOR is low, your subject line is working but your email body isn't compelling people to act. If CTOR is high but open rate is low, you have a subject line problem, not a content problem.

CTOR lets you diagnose exactly where campaigns break down.

Benchmark

A CTOR of 10–20% is considered good across most industries.

5. Bounce Rate

Bounces occur when emails cannot be delivered. There are two types:

Hard bounces: Permanent failures (invalid or non-existent addresses). Remove these immediately.
Soft bounces: Temporary issues (full mailbox, server downtime). ESPs typically retry automatically.

Formula

Bounce Rate = (Total Bounces ÷ Emails Sent) × 100

Example

Sent: 15,000
Bounces: 300

(300 ÷ 15,000) × 100 = 2% bounce rate

Benchmarks

Rating	Bounce Rate
Healthy	Below 2%
Acceptable	2–5%
Concerning	Above 5%

Why It Matters

High bounce rates damage your sender reputation with ISPs. Most ESPs will suspend accounts with consistently high bounce rates. A clean, regularly validated list is essential for long-term deliverability.

6. Unsubscribe Rate

Unsubscribe rate measures what percentage of recipients opted out after receiving your email.

Formula

Unsubscribe Rate = (Unsubscribes ÷ Emails Delivered) × 100

Example

Delivered: 14,700
Unsubscribes: 44

(44 ÷ 14,700) × 100 = 0.3%

Benchmarks

Rating	Unsubscribe Rate
Excellent	Below 0.1%
Good	0.1–0.3%
Concerning	0.3–0.5%
Poor	Above 0.5%

Why It Matters

A rising unsubscribe rate signals a mismatch: either you're emailing too frequently, your content isn't relevant, or your list quality has declined. It's also an early warning sign of spam complaint risk.

7. Spam Complaint Rate

Spam complaints occur when recipients mark your email as spam rather than unsubscribing. This is far more damaging to your sender reputation than unsubscribes.

Formula

Spam Complaint Rate = (Spam Reports ÷ Emails Delivered) × 100

Benchmarks

Rating	Spam Complaint Rate
Safe	Below 0.05%
Warning	0.05–0.10%
Dangerous	Above 0.10%

Why It Matters

Google and Yahoo's 2024 sender guidelines require spam complaint rates to stay below 0.10% to maintain inbox placement. Exceeding this threshold consistently can result in emails going to spam — or being blocked entirely.

8. Conversion Rate

Conversion rate measures the percentage of recipients who completed a desired action — a purchase, signup, booking, or download.

Formula

Conversion Rate = (Conversions ÷ Emails Delivered) × 100

Example

Delivered: 14,700
Purchases: 147

(147 ÷ 14,700) × 100 = 1% conversion rate

Why It Matters

Conversion rate is the revenue-driving metric. A 1% conversion rate on 14,700 emails at a £50 average order value generates £7,350 in a single send. Improving conversion rate by 0.2 percentage points on that same list adds £1,470 per campaign.

Conversion rate is where subject line, content, design, offer, and timing all come together.

Part 2: Cost & Revenue Metrics

Once you understand delivery and engagement, the next layer is financial performance.

9. Email ROI

The headline number every marketer should calculate for every campaign.

Formula

Email ROI = ((Revenue Generated – Campaign Cost) ÷ Campaign Cost) × 100

Example

Revenue: £14,700
Campaign cost (platform, design, copywriting): £600

((£14,700 – £600) ÷ £600) × 100 = 2,350% ROI

Why It Matters

Email routinely delivers the highest ROI of any digital marketing channel. Calculating it per campaign — not just annually — helps justify budget, prioritise email over paid spend, and identify underperforming sends early.

10. Revenue Per Email (RPE)

RPE measures how much revenue each individual delivered email generates. It normalises performance regardless of list size, making it easy to compare campaigns across different audiences.

Formula

Revenue Per Email = Total Campaign Revenue ÷ Emails Delivered

Example

Revenue: £14,700
Delivered: 14,700

£14,700 ÷ 14,700 = £1.00 per email

Why It Matters

RPE is a cleaner comparison metric than total revenue because it accounts for list size differences. A £30,000 campaign sent to 100,000 people (RPE £0.30) vs a £5,000 campaign to 5,000 people (RPE £1.00) — total revenue is useless for comparison, RPE tells the real story.

11. Revenue Per Subscriber (RPS)

One of the most strategic email metrics. Measured across your entire programme, not just one campaign.

Formula

Revenue Per Subscriber = Total Email Revenue ÷ Total Subscribers

Example

Total annual revenue from email: £48,000
Total subscribers: 30,000

£48,000 ÷ 30,000 = £1.60 per subscriber per year

Why It Matters

RPS answers the most important strategic question in email marketing: how much is each subscriber worth?

If your RPS is £1.60 and you can acquire a subscriber for £0.40 via paid ads, each new subscriber generates £1.20 net. That makes list growth a confident, high-ROI investment.

If RPS is lower than acquisition cost, you have either a monetisation problem or a targeting problem — both need fixing before scaling spend.

12. Email List Value

Your email list is a measurable financial asset, not just a marketing tool.

Formula

List Value = Revenue Per Subscriber × Total Subscribers

Example

RPS: £1.60
Subscribers: 30,000

£1.60 × 30,000 = £48,000 list value

Why It Matters

List value lets you track business growth in concrete financial terms. A list growing from 20,000 to 30,000 subscribers at £1.60 RPS adds £16,000 of measurable business value — before the additional revenue is even earned.

13. Subscriber Lifetime Value (SLV)

Not all value is immediate. SLV estimates the total revenue a subscriber generates over their entire relationship with your list.

Formula

SLV = Revenue Per Subscriber (per year) × Average Subscriber Lifespan (years)

Example

Annual RPS: £1.60
Average subscriber lifespan: 3.5 years

£1.60 × 3.5 = £5.60 lifetime value per subscriber

How to Estimate Average Lifespan

Average lifespan (years) = 1 ÷ Monthly unsubscribe rate ÷ 12

At 0.5% monthly unsubscribe rate: 1 ÷ 0.005 = 200 months ÷ 12 ≈ 16.7 years. In practice, use actual historical unsubscribe data.

Why It Matters

SLV transforms acquisition decisions. If each subscriber is worth £5.60 over their lifetime and you operate at a 40% gross margin, you could rationally spend up to £2.24 per subscriber to acquire them — far more than a short-term RPS figure suggests.

14. Cost Per Open

How efficiently you're generating attention from each email — useful for comparing campaign formats.

Formula

Cost Per Open = Campaign Cost ÷ Total Opens

Example

Campaign cost: £600
Total opens: 3,234

£600 ÷ 3,234 = £0.19 per open

15. Cost Per Click

How much each click costs you — critical for comparing email against paid channels.

Formula

Cost Per Click = Campaign Cost ÷ Total Clicks

Example

Campaign cost: £600
Total clicks: 588

£600 ÷ 588 = £1.02 per click

Why It Matters

Email CPC is typically dramatically lower than paid search or social advertising. Knowing your email CPC makes it easy to demonstrate the channel's cost efficiency — and to calculate a cost-per-acquisition when combined with your conversion rate.

16. List Growth Rate

How fast your audience is growing or shrinking each month.

Formula

List Growth Rate = ((New Subscribers – Unsubscribes – Spam Complaints) ÷ Total List at Start) × 100

Example

List at start of month: 30,000
New subscribers: 850
Unsubscribes: 300
Spam complaints: 20

((850 – 300 – 20) ÷ 30,000) × 100 = 1.77% monthly growth

Why It Matters

A list growing at 1.77% per month will nearly double in size within 4 years through organic growth alone. Negative list growth rate means you're losing subscribers faster than gaining them — a structural problem that optimisation alone can't fix.

17. Breakeven Send Frequency

More emails = more revenue? Not always.

Beyond a certain frequency, each additional send produces diminishing returns as engagement drops and unsubscribes accelerate.

Conceptual Formula

Additional Revenue per Extra Send = Subscriber Loss Cost from That Send

Where:

Additional Revenue = extra campaign income from sending once more per period
Subscriber Loss Cost = lost lifetime value from the extra unsubscribes that send causes

Worked Example

Average revenue per campaign: £2,000
Extra unsubscribes from sending once more per week: 50
SLV per subscriber: £5.60

Revenue gained:  £2,000
Value lost:      50 × £5.60 = £280
Net gain:        £1,720  ✓ Worth sending

As frequency increases further, unsubscribe rates accelerate and the net gain shrinks — eventually turning negative.

Why It Matters

Most marketers either under-send (leaving revenue on the table) or over-send (eroding list value faster than they realise). Breakeven frequency gives you a financial answer rather than relying on guesswork.

18. Send Profitability Threshold

Every campaign has a point where it stops being profitable. Know it before you hit send.

Formula

Profit = (Revenue Per Subscriber × Recipients) – Campaign Cost

Example — Engaged Segment

Revenue per recipient: £0.60
Recipients: 10,000
Campaign cost: £800

(£0.60 × 10,000) – £800 = £5,200 profit  ✓

Example — Cold Re-engagement Segment

Revenue per recipient: £0.06
Recipients: 10,000
Campaign cost: £800

(£0.06 × 10,000) – £800 = –£200 loss  ✗

Why It Matters

Not every send to every segment is worth making. This formula clarifies whether a campaign, automation, or re-engagement attempt is financially justified before you commit the spend.

Combining Formulas: The Complete Performance Picture

Each formula answers one question. Together they form a complete analytical framework.

Question	Formula to Use
Did my email reach people?	Delivery Rate
Did they open it?	Open Rate
Did they click?	CTR
Was my content compelling?	CTOR
Did I lose subscribers?	Unsubscribe Rate
Was it worth sending financially?	ROI / Profitability Threshold
What does each subscriber generate?	Revenue Per Subscriber
Is my list growing in value?	List Growth Rate × SLV
Can I afford to grow my list faster?	SLV vs. Acquisition Cost
How does email compare to paid channels?	Cost Per Click comparison

Common Mistakes When Using Email Formulas

1. Using total opens instead of unique opens Most formulas should use unique opens and unique clicks — not totals. One person opening three times counts as one unique open.

2. Dividing by sent instead of delivered Open rate and CTR both use delivered (sent minus bounced) as the denominator. Using sent instead produces artificially low numbers.

3. Attributing all revenue to email Use UTM parameters and attribution models. Last-click attribution overstates email's contribution in multi-touch journeys.

4. Measuring one campaign in isolation A single campaign can be a statistical outlier. Trends across 20+ campaigns reveal the real patterns.

5. Ignoring Apple MPP inflation Open rates since late 2021 may be inflated by Apple Mail Privacy Protection pre-loading image pixels. Lean on CTOR, CTR, and conversion data as more reliable signals.

How Email Calculator Helps

Calculating these metrics manually — across every campaign, for multiple clients, from different platforms — is time-consuming and error-prone.

Email Calculator automatically applies every formula the moment you enter or sync your campaign data:

Open rate, CTR, CTOR, delivery rate, bounce rate, unsubscribe rate — calculated instantly
Quality Score — a composite performance grade across all key metrics
Industry benchmarks — compare your results against relevant sector benchmarks
AI insights — understand what drove performance and what to improve next
Export-ready reports — PDF, Word, PowerPoint, Excel, HTML
API integrations — auto-sync from Mailchimp, Klaviyo, HubSpot, and 50+ platforms

No manual calculation. No spreadsheets. Just answers.

Summary

Email marketing formulas turn raw campaign data into business intelligence.

The 18 formulas in this guide cover the full spectrum:

Delivery & engagement: delivery rate, open rate, CTR, CTOR, bounce rate, unsubscribe rate, spam complaint rate, conversion rate
Cost & revenue: email ROI, RPE, RPS, list value, SLV, cost per open, cost per click
Strategic planning: list growth rate, breakeven send frequency, profitability threshold

Use all of them together for a complete picture — not just "what happened" but what it means for your business.

Email marketing success isn't luck — it's the result of understanding and acting on the right numbers. Start tracking these formulas and you'll have a measurable, improvable system instead of just a channel.