What Mobile Viewport Means for a Landing Page
The mobile viewport is the visible area of the browser on a phone screen. Unlike desktop, where the entire page is usually visible at a glance, mobile visitors only see a narrow slice of the page at a time, typically 375px wide on the most common paid ad devices.
375px is not an arbitrary number. It corresponds to older iPhone models and represents the smallest common screen used by ad-clicking audiences. Designing for 390px or 414px means roughly 20% of your paid traffic sees a broken layout. Designing for 375px means your page works everywhere.
The fold, the point where the page is cut off without scrolling, lands at roughly 667px tall on a 375px viewport. Everything above that line must do the work: headline, subhead, and the primary CTA button.
Why Mobile Matters More for Paid Traffic Than Organic
Organic search traffic skews desktop. Paid ad traffic skews mobile. The difference matters because the same page can perform well on desktop and fail completely on a phone.
More than 60% of ad clicks from Google and Meta land on a mobile device. If you are running a paid campaign and your landing page has a mobile layout problem, you are paying full cost-per-click for visitors who bounce before they can convert.
Organic visitors found your page through a search query; they have moderate intent and enough curiosity to scroll. Paid visitors clicked an ad promise. If the page does not immediately match that promise with a visible headline and a reachable CTA, they leave. Organic traffic forgives friction. Paid traffic does not.
The Specific Failures That Kill Mobile Conversions
Three layout failures account for most mobile conversion losses on paid campaigns:
CTA below the fold on mobile
A hero image that looks clean on desktop pushes the button below the fold on a 375px screen. The visitor sees the headline and a large image, but the button requires scrolling. Most do not scroll.
Form pushed down by the hero
Lead capture forms placed below a full-height hero section are invisible on mobile first load. If the form is the conversion action, it needs to be above the fold or reached by a single scroll.
Text too small to read without zooming
Body text under 15px and headline text under 24px forces visitors to pinch-zoom. Zooming breaks the layout and signals that the page was not built for the device. Most visitors exit instead of zooming.
How to Test Your Page at 375px
No plugins required. This takes about 60 seconds in any Chromium browser:
- Open your landing page in Chrome or Edge.
- Press F12 (or right-click anywhere and select Inspect) to open DevTools.
- Click the device toolbar icon in the top-left of the DevTools panel (two overlapping rectangles).
- Set the width to 375 and height to 667 in the dimension fields at the top.
- Reload the page so assets render at the correct size.
- Check: is the primary CTA button visible without scrolling? Is the headline readable without zooming? Does anything overflow horizontally?
Previous Nebula benchmark figures are withdrawn after a corpus eligibility review. No comparable failure rate or conversion effect is established.
What a Passing Mobile Layout Looks Like
A page that passes the 375px check has three things in place:
- One dominant CTA button visible above the fold, with enough contrast to identify it immediately as a button.
- A headline that reads at 24px or larger with no horizontal overflow or text cut off at the edge.
- No horizontal scroll: the entire page fits within the 375px width without requiring side-scrolling.
The rest of the page can scroll. Long-form copy, testimonials, and feature lists all belong below the fold. The fold itself must show one clear action.
Nebula checks mobile CTA visibility automatically.
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