The metric most brands get wrong with sign-up forms
When it comes to pop-up forms, most ecommerce brands focus on the wrong metric entirely. They look strictly at the subscription rate and assume that whatever form gets the highest percentage of sign-ups is the winner.
That is a mistake.
A pop-up form that pulls in 500 low-intent subscribers who never buy is useless compared to one that brings in 300 buyers who spend real money. If you want to build a profitable email programme, you have to look beyond list growth and analyse the entire funnel down to actual revenue, subscription-to-customer conversion rate, and average order value (AOV).
A side-by-side comparison: Dollar-off vs percentage-off
To see how this works in practice, let us look at a store getting 10,000 unique monthly visitors. We will compare two standard introductory offers running on a sign-up form: $10 off versus 10% off.
On the surface, the numbers look straightforward at the top of the funnel:
- $10 off offer: Generates a 4% subscription rate, yielding roughly 350 to 400 new subscribers.
- 10% off offer: Generates a 3% subscription rate, yielding 300 new subscribers.
If you stop looking at the data right here, you would conclude that the $10 off discount is superior because it generated more email addresses. Here is the thing: building an email list is about revenue, not vanity metrics.
The subscription-to-customer conversion rate
The next metric you must track is your subscription-to-customer conversion rate. This tells you what percentage of people who typed in their email address actually went on to place a first order.
In our scenario, the conversion rates break down like this:
- $10 off offer: Converts at 22.5%, producing roughly 78 paying customers from the subscriber pool.
- 10% off offer: Converts at 30%, producing 90 paying customers from the 300 subscribers.
Even though the 10% discount pulled in fewer total email addresses, it generated more paying customers. A percentage-off incentive often attracts shoppers with clearer intent to make a substantial purchase right away, rather than bargain hunters simply claiming an easy ten-dollar voucher.
Why percentage discounts protect average order value
The differences compound further when you look at average order value. In my experience, percentage-based discounts consistently outperform flat dollar discounts when it comes to basket size.
A fixed $10 discount gives customers the exact same cash saving whether they spend $40 or $200. There is no built-in incentive to add another item to the cart.
A 10% discount works on a spend-more-save-more principle. Shoppers naturally add higher-priced items or extra units to their basket to maximise their total discount. In our modelled store, the first-order AOV on the $10 off offer settles at $90, while the 10% off offer lifts first-order AOV to $105. You give up slightly more margin on larger orders, but the higher order value more than compensates for it.
The bottom-line revenue difference
When you multiply the total number of new customers by the first-order AOV, the real winner becomes obvious:
- $10 off offer: 78.75 customers multiplied by a $90 AOV equals $7,087.50 in revenue.
- 10% off offer: 90 customers multiplied by a $105 AOV equals $9,450.00 in revenue.
The 10% off offer generated $2,362.50 more revenue from the exact same 10,000 store visitors, despite bringing in 50 fewer raw email sign-ups. One offer grew the list faster, but the other grew the business faster.
The reporting gap in Klaviyo and how to solve it
Here is one real limitation you will run into: native Klaviyo forms do not track first-time customer conversion rates and order values directly within the pop-up analytics dashboard. Klaviyo shows you views, submits, and a standard submit rate, but stops short of linking that form split-test directly to downstream basket size.
To solve this, we often turn to specialised third-party tools such as FormToro. Tools like this sit on your site and track first-time customer revenue, exact subscription-to-conversion rates (such as 24.8% on live tests), and first-order AOV directly tied to the form variant.
If you are serious about treating email as a major revenue engine, you need tools and reporting that show you commercial return, not just raw opt-in counts.
Final Thoughts
Never pick your sign-up form incentives based on list growth alone. Factor in your subscription-to-customer conversion rate and the resulting average order value to find the offer that drives the most bottom-line cash. List growth is a milestone, but revenue is the goal.
If you want help designing, building, and optimising high-converting forms that drive profitable first orders, explore our sign-up forms service.
