Banner Ad Space: Sizes and Placements

Banner ad space is a section of a website, with a set size and position, where an advertiser's creative is shown. To choose the right slot, you need to look at three things together: standard banner ad sizes, the position on the page and how the slot appears on mobile.

In this guide we summarise the common formats in a table, explain how header, sidebar and in-content positions differ, and show why viewability matters as much as price. The aim is to help you judge any slot a publisher offers you.

What Banner Ad Space Is

A banner slot is a fixed box reserved for advertising in the site's template. The advertiser's static image, animated image or HTML5 creative goes into this box, usually linking to the advertiser's landing page. The slot might appear on every page, only in certain categories or on a single page.

Banners are the most familiar form of display advertising, and their purpose is visibility and traffic. They should not be confused with buying link value for SEO. Google's guidance on paid links asks that advertising links be marked with rel="sponsored" or nofollow, and most publishers already serve banner links that way or route them through an ad server.

When you evaluate a banner slot, ask the publisher for:

• The slot's size in pixels and the accepted file formats.
• Which pages the slot appears on and where it sits on each page.
• How the same slot looks on mobile, or which size it switches to.
• Whether the slot is exclusive to one advertiser or rotated between several.

Common Banner Ad Sizes

The industry uses a set of widely accepted standard banner sizes. These standards make it easier for advertisers to reuse the same creative across different sites. The table below is a sample template summarising frequent formats and where they typically appear; individual sites may use different dimensions in their own templates.

The table only lists formats; it does not rank which size gets more clicks. Clicks and engagement vary by industry, audience and placement. Where a size lands on the page and what surrounds it usually matter more than the dimensions alone.

When preparing creative, design each size separately. A wide, horizontal leaderboard and a tall half page carry the same message in very different layouts, and shrinking or stretching one image makes the text hard to read.

On-Page Positions: Header, Sidebar, In-Content

Ad placement decides how many visitors actually get the banner on their screen. Two slots of the same size behave very differently if one sits at the top of the page and the other at the very bottom. Each of the three main positions deserves a separate look.

Header

Header slots sit at the top of the page, around the logo and navigation. They are likely to be visible the moment a visitor opens the page. Once the reader starts scrolling into the content, though, they leave the screen quickly, so time on screen can be short.

Sidebar

On desktop, sidebar slots run vertically beside the content. Some sites keep the sidebar ad fixed while the reader scrolls, which keeps it visible for longer. On mobile, the sidebar usually drops below the content and is seen far less often.

In-content

In-content slots are placed between paragraphs of an article. Because they sit where the reader is concentrating, their visibility is often strong. They should be limited in number so they do not break the reading flow, and they should be clearly labelled as advertising.

Imagine a hypothetical case: a cycling accessories brand is choosing between two slots on a cycling blog. The first is a sidebar slot on every page; the second is an in-content slot only on maintenance guides. On a site with few desktop visitors, the second option may look smaller but reach the intended audience more directly.

How Banners Display on Mobile

On many sites, a large share of visitors arrive on mobile devices. Judging a slot only from its desktop view leaves out a big part of the picture. Mobile banners have to deliver a shorter message in a much smaller space.

When reviewing mobile banners, check the following:

• Is the desktop slot hidden on mobile, or does it switch to a different size?
• If the banner is pinned to the bottom of the screen, does it have a close button and does it cover content?
• Is the text in your creative large enough to read on a small screen?
• Does the landing page load quickly and work smoothly on mobile?

Mobile ads also affect page experience. If space is not reserved for the slot, the content jumps down when the banner loads. This hurts visual stability (CLS), one of the Core Web Vitals, and annoys readers. It is worth asking the publisher whether the slot's height is fixed in the template.

Viewability and Time on Screen

A banner being served does not mean it was seen. A slot at the bottom of a page never reaches the screen if the reader leaves before scrolling that far. Viewability is the concept used to measure this gap.

Under the widely used industry definition, a display ad counts as viewable when at least half of its pixels stay on screen for at least one second. This is usually measured with an ad server or a verification tool. Smaller publishers cannot always provide the figure; in that case, asking about the slot's position and typical scrolling behaviour is a good start.

Time on screen matters too. A slot that is visible only briefly may not be enough for brand awareness. Sticky sidebar or in-content slots tend to stay visible for longer. We cover how to judge placements across sites in our publisher quality checklist.

Once you have picked a slot, you can move on to duration and price. Our guide to renting ad space for a fixed period explains time-based deals, and the factors behind ad space pricing shows how position and size affect cost. For the full process, start with our step-by-step guide to buying ad space.

Frequently Asked Questions

What are the most common banner sizes?

On desktop, the 728 x 90 leaderboard, 300 x 250 medium rectangle and 300 x 600 half page are widely used; on mobile, 320 x 50 and 320 x 100 are common. Each site may still offer its own dimensions, so confirm the size with the publisher.

Do sidebar banners still work?

On sites with many desktop visitors, sidebar slots can stay visible, especially when they remain fixed while scrolling. On mobile-heavy sites the sidebar usually drops below the content, so ask what the slot becomes on mobile.

Which banner size should I use on mobile?

The sizes used most often on mobile are 320 x 50, 320 x 100 and 300 x 250. Choose based on where the slot sits on the page and how readable your creative is on a small screen.

Do sticky banners annoy users?

Sticky banners that do not cover content, include a close button and use a small part of the screen usually offer an acceptable experience. Slots that overlap content or cannot be closed can frustrate readers and damage how they see the advertiser.

How is banner viewability measured?

Viewability is usually measured by an ad server or verification tool, based on the share of pixels on screen and how long they stay there. If the publisher cannot provide this, combine the slot's position, the page's scrolling behaviour and your own click data.

Conclusion and Next Step

Choosing banner ad space is about more than picking a size. When you weigh the standard format, page position, mobile display and viewability together, your budget goes into a slot people actually see. Ask the publisher for real screenshots, design creative for each size and make sure the banner link is marked in line with paid link rules. To compare slots of different sizes and positions, you can browse the ad space listings.