Email A/B Testing: The Complete Guide to Better Results

Email A/B testing transforms opinions into data. Instead of guessing what works, you let your audience tell you through their actions.

This guide covers everything from basic split testing to advanced multivariate strategies. By the end, you'll know how to run tests that drive real improvements.

What Is Email A/B Testing?

A/B testing (split testing) sends two versions of an email to see which performs better. You change one element, measure results, and apply the winner.

Simple example:

• Version A: "50% off everything today"
• Version B: "Half off everything today"
• Send each to 50% of your list
• Measure opens
• Winner becomes your template

What to A/B Test (Priority Order)

High Impact: Test These First

Subject Lines

• Biggest impact on opens
• Easiest to test
• Quick results
• Compounds over time

Send Time

• When does your audience engage?
• Test days and times
• Consider time zones
• Platform-specific optimization

From Name

• Company name vs. person
• Different team members
• Descriptive vs. simple
• Major impact on trust

Medium Impact

Email Content

• Short vs. long
• Formal vs. casual
• Story vs. direct
• Educational vs. promotional

CTA (Call to Action)

• Button text
• Button color
• Placement (top vs. bottom)
• Single vs. multiple CTAs

Images

• With images vs. text-only
• Product vs. lifestyle photos
• Image placement
• Number of images

Lower Impact (But Worth Testing)

• Preview text
• Personalization depth
• Social proof placement
• Footer content
• Sender email address

How to Run Valid A/B Tests

Step 1: Test One Variable Only

Change only one element per test. If you test subject line AND send time, you won't know which caused the difference.

Good test: Subject A vs. Subject B (same everything else) Bad test: Subject A at 9am vs. Subject B at 2pm

Step 2: Determine Sample Size

Statistical significance requires adequate recipients:

Total List Size	Per Variant	Confidence Level
1,000	300+	Moderate
5,000	500+	Good
10,000+	1,000+	High

Rule of thumb: 1,000+ per variant for reliable results.

Step 3: Define Your Success Metric

What defines "winning"?

Test Type	Primary Metric
Subject line	Open rate
Content	Click rate
CTA	Click rate or conversion
Send time	Open rate + click rate

Step 4: Run the Test

• Send variants simultaneously (controls for time)
• Wait for adequate data (2-4 hours minimum for opens, 24 hours for clicks)
• Don't peek and call early

Step 5: Analyze Results

Is the difference statistically significant?

Quick rule: If winner is 5%+ better with 1,000+ recipients per variant, it's likely significant.

For precise analysis, use a statistical significance calculator.

Step 6: Apply and Document

• Use the winner going forward
• Document what you learned
• Plan the next test
• Build a testing knowledge base

Subject Line A/B Test Ideas

Test Ideas with Examples

1. Personalization

• A: "Your weekly update"
• B: "[Name], your weekly update"

2. Curiosity vs. Clarity

• A: "The #1 mistake marketers make"
• B: "Avoid this common email mistake"

3. Numbers

• A: "Tips to improve your emails"
• B: "5 tips to improve your emails"

4. Emoji

• A: "New products just dropped"
• B: "🎉 New products just dropped"

5. Length

• A: "Sale"
• B: "Our biggest sale of the year starts now"

6. Question vs. Statement

• A: "Ready for better email results?"
• B: "Get better email results today"

CTA A/B Test Ideas

Button Text Tests

Version A	Version B
"Buy Now"	"Get Yours"
"Learn More"	"See How It Works"
"Start Free Trial"	"Try Free for 14 Days"
"Download"	"Get Your Free Copy"

Button Placement Tests

• Above the fold only
• Multiple buttons (top and bottom)
• Single button at bottom
• Inline link vs. button

Button Design Tests

• Brand color vs. contrasting color
• Large vs. standard size
• With arrow icon vs. without

Content A/B Test Ideas

Length Tests

• Short (100 words) vs. long (500+ words)
• Often depends on offer complexity
• B2B may prefer longer; B2C shorter

Format Tests

• Single column vs. multi-column
• Text-heavy vs. image-heavy
• Bullet points vs. paragraphs
• Numbered steps vs. prose

Tone Tests

• Formal vs. conversational
• First person vs. second person
• Emotional vs. logical

Common A/B Testing Mistakes

1. Testing Too Many Variables

One change at a time. Multiple changes = meaningless results.

2. Ending Tests Too Early

At least 2-4 hours for opens, 24 hours for clicks. B2B may need 48 hours.

3. Declaring Winners Prematurely

Statistical significance requires sample size. Small lists mean less certainty.

4. Not Documenting Results

Create a testing log. Patterns emerge over time.

5. Testing Trivial Things

Focus on high-impact elements first. Button shade differences rarely matter.

6. Never Testing at All

Some testing beats no testing. Start somewhere.

Building a Testing Calendar

Weekly Tests

• Subject line variations
• Send time optimization

Monthly Tests

• CTA optimization
• Content format

Quarterly Tests

• Major design changes
• From name/sender
• Segment-specific messaging

Measuring Test Impact

Track improvements over time:

Metric	Baseline	After 3 Months	Improvement
Open rate	20%	25%	+25%
Click rate	2%	3%	+50%
Conversion	1%	1.5%	+50%

Small improvements compound. 10% better opens × 10% better clicks = 21% more conversions.

Advanced Testing Strategies

Multivariate Testing

Test multiple variables simultaneously:

• Subject line × Send time
• CTA text × CTA color

Requirements: Large lists and statistical software.

Holdout Testing

Keep a percentage that never gets optimization. Compare long-term performance to measure cumulative impact.

Sequential Testing

Build improvements incrementally:

• Week 1: Optimize subject line → Winner
• Week 2: Optimize CTA → Winner
• Week 3: Optimize content → Winner

A/B Testing With AI

Brew makes testing easier:

• AI generates subject line variants automatically
• Quick iteration on content options
• Consistent quality across variants

Instead of spending hours writing test variants, describe what you want and AI creates options.

Getting Started

1. Pick your first test — Subject line is easiest
2. Set up in your platform — Most ESPs support A/B testing
3. Run the test — Send to equal groups
4. Analyze results — Statistical significance check
5. Apply and repeat — Use winners, test new elements

Ready to optimize your emails? Try Brew free and create test variants in seconds with AI.

Start testing smarter →