How to A/B Test Your Email Subject Lines in Flodesk
As of summer 2026, Flodesk have launched their a/b subject line testing feature!
Honestly if you’ve been using Flodesk’s email marketing software for as long as I have, you’ll know this feature has been a long time coming. It’s something the community has been asking for for a while, and they have delivered!
I’ve been trying this new feature out and wanted to share some best practices here.
Table of Contents
*Disclaimer: This post contains affiliate links where I get a small commission if you make a purchase, so thank you in advance for your support! Please know I would never recommend anything that I don’t personally LOVE myself.
Watch the video below for the tutorial or keep scrolling to read:
Erm, hold up… what is Flodesk?
Flodesk* is my email marketing tool of choice, and one I recommend to all my clients. Why? Because it’s the most intuitive, beautifully designed email software on the market, created with small and creative business owners in mind.
I’ve been a Flodesk partner since they launched ~2018, and am also a Flodesk instructor - so I’m always getting exciting when they launch new features.
Learn more about Flodesk 👇
Read my ultimate guide to Flodesk here.
Read my comparison between Flodesk and Squarespace email campaigns here.
Read my comparison between Flodesk and Mailerlite here.
What does A/B testing mean?
A/B testing literally just means testing two different options (ie. Option A and Option B) and seeing which one is the winner!
So when we talk about A/B testing email subject lines, we are seeing which of two options (Option A and Option B) our audience engages with more and makes them more likely to open our email.
How A/B testing works:
You have two different subject lines for the same email, eg:
Option A: “Do you still enjoy what you do?”
Option B: “I fell out of love with my business”
^ Both are relevant to the email content you’re sending, but have slightly different angles or are phrased differently, and you want to see which one your audience is most likely to open!
Your email marketing tool (ie. Flodesk) will take a small random % of the whole email list you’re sending to (eg. if you are sending to 2000 people, it might take 400 of those people), and send Option A to half of them (200 people) and Option B to the other half (200 people).
After a certain amount of time (eg 24 hours), it will then see which Option had the most ‘opens’ (and therefore was the more engaging option for your audience), and send the winning subject line email to the remaining 1600 people on your list!
Why is A/B testing your email subject lines important?
Stop overthinking and get real data
Rather than get stuck between two subject lines you’re unsure of, get testing straight away and receive REAL data from your audience, rather than relying on your own vibes.
learn your own audience’s preferences
The internet is full of suggestions for writing email subject lines that get opened, BUT everyone’s audiences are totally unique! What might work for someone else’s list might not actually be the right fit, and the only way to figure that out is by doing your own testing. A/B testing helps speed this up!
small improvements compound over time
One A/B test may help improve that email’s open rate by a small %, but over time as you learn what your audience’s preferences are and you get more data, these improvements can compound over time!
How to A/B test in Flodesk
step 1. create your email and then go ‘next’ to set your subject lines
The email content you’re sending stays exactly the same, so create that as you normally would, and then when you go ‘next’ you’ll notice you now have the option to set up two different subject line options.
step 2. Choose your subject lines & send time
If you have your subject line options added, Flodesk will then tell you that it’s going to send the Option A and Option B tests to a small segment of your list first, and wait for 24 hours to decide a winner before sending to the rest.
step 3. Wait while flodesk sends your ‘test’
Your test email will be sent (50% will receive Option A email subject line, same email content, and 50% will receive Option B subject line, same email content), and then it’s a waiting game to see which subject line gets opened the most!
step 4. discover the ‘winning’ subject line as it gets sent to the rest!
After 24 hours, Flodesk will automatically check which subject line got the most opens, and send the email with the winning subject line to your remaining subscribers! You don’t have to do anything - it all happens for you and you get an email confirming which was the winner as well.
Tips for effective A/B subject line testing
you don’t have to do it for every email
In fact if you have a very time sensitive email that doesn’t make sense if some people receive it a day early (the test segment), then you probably shouldn’t do an A/B test.
Save it for your weekly email newsletters, and build up data over time!
test curiosity vs. clarity
Curiosity subject lines are ones that only subtly HINT at the content of the email, and create a sense of mystery or piqued interest that makes your audience NEED to open the email to find out what you’re talking about.
Meanwhile clarity subject lines are those that give a clear idea of what someone can expect from the contents of the email.
eg. ‘Do you still love what you do?’ (curiosity)
vs ‘How I fell out of love with my business’ (clarity)
Some audiences prefer curiosity while some prefer clarity! Test a few times to figure which your audience responds best to.
Note: It’s important that your curiosity subject lines don’t border on ‘clickbait’. This is where the subject line is greatly exaggerated and the email content doesn’t live up to it, so you lose trust with your audience. This can lead to more opens, but also more unsubscribes!
test ‘i’ vs ‘you’
Perhaps your audience prefer to hear from your personal experience vs being talked directly to them… or the other way around!
eg. ‘Do I actually still love what I do?’
vs ‘Do you actually still love what you do?’
watch your unsubscribes too
Just because one email subject line option got more OPENS (and therefore became the A/B test winner), doesn’t mean it was the best option for your business. As I mentioned above, if your subject line came across as too ‘clickbaity’ and the email content didn’t feel relevant or live up to expectations, you might see your unsubscribes spike too.
Best to always check your unsubscribes and see if they are lower or higher than average for your typical numbers on your account.
test on broadcast emails, apply to automations
You can easily test what you’ve learned in your one-off newsletter emails, and then apply your discoveries to your automations and workflows, so that their open rates can start improving too!
How to get 25% off your first year of Flodesk
Flodesk is honestly such a joy to use, in a world of clunky and overwhelming email software. As an affiliate partner for Flodesk, I can get my audience a discount on their plans too.
If you sign up using my link (or start your paid plan using my code below), you can get 25% off your first year 🎉
Or use my code (00G2QH) at the checkout when you subscribe to a paid plan for the first time.
