Subject Lines and How to Measure their Success

by The Giles Agency

In our recent article on writing subject lines that drive engagement, we looked at a few things to consider when working on those all-important first few words. Now let’s look at exactly what we mean by engagement so you can measure it and accurately gage the success of your subject line.

Open rates

The primary aim of a subject line is to persuade your busy and distracted reader to open your email. If that doesn’t happen then all else is lost. So looking at your eDM’s open rate is the simplest measure of success.

What is a good rate? It varies from industry to industry and also according to the type of email you’re sending. However, the average sits at 21%. So since we’re assuming you don’t want to be average, 30% would be a good number to aim for. Push that number up to 40% and you can give yourself a big pat on the back.

Now, diving a bit deeper, it’s worth remembering that open rates are just one data point. Yes, your reader opened the email. But the next question is, did the right reader – the one who’s interested and motivated – open your email and did they find what they expected? I.e., did your subject line lead them to the right decision?

A more in-depth exploration of success, then, must examine whether the reader took action after they opened the email. Did they spend a little time finding out more about your offering? Did they sign up for a newsletter, book a call or actually make a purchase? Ultimately, the way you measure success depends on the specific aim of your EDM and what action you want the reader to take.

Email marketing


This is the number of readers who clicked on at least one link in your eDM. This could be a click on your offer or it could just be readers exploring your website, reading your blog or downloading more information. This number tells you if your email piqued your reader’s interest and made them want more. Lower than open rates but higher than conversions, this rate should be between 5 – 10%.


A conversion is when the reader takes the next desired step. This is the big win and if your reader converts we can safely say your eDM was a success! In many cases, the next desired step is a sale but it could also be making an appointment, filling in an interest form or signing up for a newsletter. Of course, you should expect your conversion rate to be much lower than your open rate, and again, rates vary hugely depending on the industry and the offer. However, for guidance, a good conversion rate in ecommerce is between 2 and 5%.

Now, there are times when thinking more long-term would be beneficial. For example, if you’re a brand that sells consumer goods that are only purchased a few times in a lifetime – white goods for example – then the chances of your entire database being in the market for a new item as they receive your promotional email are pretty low. Your aim then is to stay front of mind so that when your potential customers are looking for a new fridge, your brand is the first that comes to mind.

Email Subject line success

Brand recognition

In fact, every email you send should be building brand reputation and brand recognition. Of course, we all love a quick sale, but any serious business should also be thinking beyond that, putting in the work now to build relationships and nurture the paying customers of the future.

Brand recognition is not going to be built in a single day with a single eDM. To be truly effective, a multi-pronged approach is needed, reaching your audience on multiple channels at different times of the day. Brand recognition is also notoriously difficult to measure and large corporations invest in brand recall studies to gage levels.

Bringing it back to your eDM, one way to assess the success of your brand awareness emails is to throw in a promotional email and see what the results are. Were conversions higher among the readers who received your awareness emails compared to those who only got the promotion?

What if your subject line bombed?

With so many moving parts, crafting the perfect subject line is a tricky business. The good news is, subject lines are ideal for split testing. Divide your recipients into two (or more if you have a large database) and send subject line A to the first half and subject line B to the second half. Perhaps A could focus on a benefit and B could introduce a sense of urgency, for example. Which subject line got more engagement? The answer will give you important insights into your readers preferences. Make note of the lessons and apply them in your next eDM. Keep testing and tweaking until you’re getting the results you want.

So now you know how to craft your subject line, how to assess it and make adjustments, the fun can begin! Happy writing!