Where the Extra Taps Come From

On a static product page, the "Add to Cart" button lives once, near the top, next to the price. The moment a shopper scrolls to read specifications, reviews, or shipping details — which is most shoppers, most of the time — that button disappears off-screen. Buying then requires a deliberate scroll back up, a small but real interruption that breaks the momentum of an impulse decision. A sticky button pinned to the bottom of the viewport removes that round trip entirely: the purchase action is always exactly one thumb's reach away, regardless of how far down the page the shopper has scrolled. In controlled A/B tests, this single change to button persistence lifts the mobile cart addition rate by about 7-14% relative to a page with a standard, scroll-away button.

Why the Lift Is Concentrated on Mobile

Desktop shoppers have a mouse that can jump to any part of a page instantly and often keep the buy button visible via hover states or shorter viewports. Mobile shoppers are constrained to a single thumb, a narrow viewport, and a scroll gesture that is slower and more deliberate per pixel of content covered. That physical constraint is exactly what a sticky button neutralizes — it converts "scroll back up and tap" into "tap where your thumb already is." This is also why the effect is reported specifically for mobile cart addition rate rather than sitewide conversion: the mechanism is about closing a reach gap that barely exists on larger screens.

What the Button Actually Needs to Do

Not every fixed-position button produces this result. The versions that move the needle share a few traits: they stay visible through scroll and don't reappear only after a delay, they show live state (in stock, price, selected variant) so the shopper never taps a button that doesn't match what they're looking at, and they don't cover content the shopper needs to make the decision, like size selectors or the final price line. A button that simply duplicates the top one without reflecting current selections tends to produce confused taps and returns rather than added carts, which is why the range tops out at 14% rather than higher — implementation quality inside the sticky pattern still matters.

Interactions With Other Page Elements

A persistent bottom bar competes for the same real estate as browser chrome, cookie banners, and any existing bottom navigation. Sites that already run a bottom tab bar or a sticky filter bar on mobile need to reconcile the new element with those, either by combining them into one bar or by making the add-to-cart bar appear only on product pages. Tests that stack multiple fixed elements without resolving this tend to land at the lower end of the 7-14% range, or occasionally show no lift, because the visual clutter offsets the reach benefit.

Reading the Effect Size Correctly

The 7-14% figure is a relative lift in the rate of adding an item to cart, not a lift in completed purchases or revenue — checkout, shipping cost, and payment friction still sit downstream and are untouched by this change. It also assumes a baseline where the button was not already persistent; sites that already pin the button, or that have very short product pages where the original button rarely scrolls out of view, should expect a smaller effect than sites with long, detail-heavy product pages. Treat the range as a starting expectation for a single, isolated A/B test on this one element, not a guaranteed outcome once other page changes are layered on top of it.