The percentage of visitors who buy. Conversion rate is the multiplier on every dollar of traffic you pay for: lifting it from 2% to 4% doubles revenue on the same visitors at no extra ad cost, while quietly halving your effective CAC. That is why fixing the website beats buying more ads. Measure it by device, not just sitewide - mobile carries most of the traffic and converts well below desktop, so the blended number hides where the money is. And treat it as never done: your site should not look the same week to week, because optimisation is a continuous process, not a project.
Benchmark. On Shopify the median store converts about 1.4% of sessions; 2.7%+ is top 20% and 3.6%+ top 10% (Littledata, 2026). Category matters more than any single number: beauty runs about 2.9%, fashion 1.1%, home and furniture 1.0%. Watch your own mobile-desktop gap - Shopify's medians are now level, so a big gap on your store is usually the biggest available win, and it starts at checkout.