How to price a conversion leak in €/month

A conversion finding without a price is an opinion. Here’s the arithmetic that turns one into a decision — using a worked example with hypothetical numbers.

The worked example

Say a store’s mobile product pages get 12,000 sessions a month. Desktop visitors add to cart at 6%, mobile at 3.4%. Session recordings show why: the variant selector misfires on a meaningful share of mobile taps.

The sizing question is: if the selector worked, how much of that gap closes?

Not all of it — mobile always converts below desktop. Be conservative and attribute a third of the gap to the broken selector: 0.9 percentage points. That’s roughly 108 extra add-to-carts a month (12,000 × 0.9%). Apply the store’s cart-to-order rate (say 40%) and average order value (say €70):

108 × 0.40 × €70 ≈ €3,000/month — at stake. A fix that recovers even a third of that is worth ~€1,000/month.

Now “fix the variant selector” can be ranked against everything else on the roadmap — and it probably wins.

The honesty rules

Sizing goes wrong in predictable ways. Four rules keep it trustworthy:

1. Show the arithmetic. A number without its derivation is marketing. Anyone should be able to challenge each step — that’s a feature, not a risk.

2. Attribute conservatively. Never assign the whole gap between two segments to one cause. A third is often defensible; the whole gap almost never is.

3. Respect small samples. If a page gets 300 sessions a month, a week of data proves nothing. Say “we don’t have enough data to size this yet” and rank it below what you can size.

4. Split before you size. Store-wide averages hide the finding. Split by device first, then by traffic source.

Why this matters more than precision

The point of €/month sizing isn’t to be exactly right — it’s to make conversion work comparable to everything else competing for developer time. An estimate that’s honest about its assumptions and off by 30% still ranks the roadmap correctly. A recommendation with no number at all doesn’t get ranked — it gets postponed. This is how every insight in HI Engine arrives: an estimated €/month impact, the arithmetic behind it, and a confidence level. Steal the method even if you never use the product.