Banner advertising is the display of a static or animated image ad inside a fixed rectangular space on a web page — the format that's been running since the first clickable ad in 1994. If you're building a site's ad inventory or setting up a display campaign, the size of that rectangle matters more than almost any other creative decision: the wrong size gets rejected by exchanges, ignored by ad servers, or simply doesn't fit where you need it to run. This banner ad sizes guide covers the standard formats, where each one belongs on a page, and how to choose without guessing.
The short version: a handful of sizes cover most of the internet's banner inventory, and they're standardized by the IAB (Interactive Advertising Bureau) so that a creative built once can run across thousands of sites and apps without resizing. Knowing which sizes those are, and why they became standard, saves you from designing or buying formats nobody actually serves.
The fundamentals of banner advertising
A banner ad is defined by three things: its pixel dimensions, its file format, and its placement on the page. The dimensions are the part that gets standardized, because ad exchanges, publishers, and advertisers all need to agree on a shared set of shapes before an auction can happen in milliseconds. Without a common size vocabulary, every publisher's ad slot would need a custom-built creative, which doesn't scale.
That's the role the IAB plays. Its Display Advertising Guidelines define a small set of "standard ad units" — sizes that publishers are expected to support and that ad networks default to when serving creative (IAB, Display Advertising Guidelines). A banner built at one of these sizes is portable: it can run on almost any site that sells display inventory, because the slot is already shaped to fit it.
File format matters too. Static banners are usually JPG, PNG, or GIF; animated or interactive ones are HTML5 (the format that replaced Flash industry-wide after browsers dropped support for it). Most ad servers cap file weight — commonly around 150KB for the initial load — specifically because a bloated banner slows the page it's sitting on, which drags down both user experience and the site's own SEO.
How banner ads work in practice
When a page with an ad slot loads, the publisher's ad server (or an exchange, in a real-time bidding setup) checks the slot's declared dimensions and looks for a matching creative. If your banner is 300x250 and the slot is defined as 300x250, it's eligible to serve. If it's 305x255, most systems will reject it outright rather than stretch or crop it — a mismatched size is one of the most common reasons a creative gets disapproved before a campaign even starts.
This is also why publishers rarely custom-build ad slot dimensions. A slot sized to a non-standard shape can only be filled by advertisers who happened to build that exact size, which shrinks the pool of demand bidding on it. Sticking to IAB standard sizes keeps a slot eligible for the widest possible set of advertisers and creative libraries, which in turn tends to mean more competitive bids.
The placement — header, sidebar, in-content, footer — determines which sizes are physically practical, and that's where the standard formats split into families: wide horizontal units for headers, tall units for sidebars, and squarer units that work almost anywhere in the content flow.
The banner ad sizes guide: IAB standard formats
These are the sizes that make up the bulk of served display inventory. Each has a typical placement and a reason it became standard.
| Size (px) | Common name | Typical placement | Notes |
|---|---|---|---|
| 300 x 250 | Medium rectangle | In-content, sidebar | The most widely served size; works almost anywhere |
| 728 x 90 | Leaderboard | Page header, above content | Standard desktop header unit |
| 320 x 50 | Mobile leaderboard | Mobile header/footer | The default mobile banner size |
| 160 x 600 | Wide skyscraper | Sidebar | Tall format for content-adjacent sidebars |
| 300 x 600 | Half page | Sidebar | Larger, high-visibility sidebar unit |
| 336 x 280 | Large rectangle | In-content | Similar to 300x250 but larger; strong in-article performer |
These six account for most of the inventory that exchanges and ad servers support by default (IAB, Display Advertising Guidelines). Google's own guidance on ad sizes, drawn from AdSense serving data, lists 336x280, 300x250, 728x90, 300x600, and 320x100 among the sizes that tend to generate the most revenue for publishers, largely because they're the ones advertisers build creative for most often (Google AdSense Help, ad sizes).
Two things follow from that overlap between "commonly built" and "commonly served": first, a slot in one of these sizes will almost always have more competing bids than an unusual one, because more advertisers have creative ready to fill it. Second, if you're only going to support a limited number of sizes on a new site, these are the ones to prioritize — not because they're mandated, but because demand concentrates there.
Choosing sizes for your goal
The right size depends on what the placement needs to do, not on which one is "best" in the abstract.
For a publisher adding a new ad slot, start with where the space actually sits on the page. A slot above the fold in a sidebar next to article content is a natural fit for 300x250 or 336x280 — both read as part of the content rather than as a separate banner strip, which tends to help viewability (the share of an impression that's actually seen on screen, the metric that matters more than raw impression count; see the Media Rating Council's viewability standard). A header slot calls for 728x90 on desktop and 320x50 on mobile, since that's the shape the space allows.
For an advertiser building creative, the calculation is about reach versus specificity. Building only a 300x250 gets you eligible for the largest pool of inventory with the least production effort. Building a full set — leaderboard, mobile leaderboard, medium rectangle, half page — costs more up front but means your campaign can compete for placements a single-size creative would simply be ineligible for. Most ad platforms will tell you, before launch, roughly what share of available inventory each size unlocks; it's worth checking rather than guessing.
Mobile deserves separate attention. Desktop skyscrapers and leaderboards don't fit a phone screen, so mobile inventory runs its own set of standard sizes — 320x50 and 300x250 dominate, with 320x100 as a taller mobile-header option. If your traffic (or your target audience) skews mobile, building only desktop sizes leaves a large share of inventory unreachable.
Common mistakes to avoid
Designing a custom size before checking demand. A non-standard dimension might look better in a mockup, but if no exchange serves it by default, the slot sits empty or falls back to a lower-value format. Check what your ad server or network actually supports before finalizing a layout.
Ignoring mobile sizes entirely. A site built desktop-first sometimes only wires up 728x90 and 300x250, then wonders why mobile fill rate is weak. Mobile traffic needs mobile-native sizes, not a shrunk desktop banner.
Over-loading the file weight. An animated HTML5 banner that's visually impressive but heavy will get throttled, rejected, or will simply slow the page enough to hurt both ad viewability and the host page's own load-time metrics. Build within the file-size limits your ad server states, not the maximum your creative tool allows.
Treating "standard" as "only option." Standard sizes cover the bulk of inventory, but native ad formats, in-feed units, and newer IAB formats like the ones in the IAB's New Ad Portfolio exist for good reasons — better content integration, less banner blindness. They're worth testing alongside standard banners, not as a replacement by default.
Assuming bigger always performs better. A 300x600 half page commands attention, but it also takes more screen real estate on a small viewport and can push content down far enough to hurt engagement. Test placement and size together rather than assuming the largest available unit is automatically the right call.
FAQ
What is the most common banner ad size?
300x250 (medium rectangle) is generally the most widely served size, because it fits comfortably in-content and in sidebars on both desktop and mobile-adapted layouts, and it's the size most advertisers build first (Google AdSense Help).
What size banner should I use for mobile?
320x50 is the standard mobile banner size, typically placed at the top or bottom of the screen. 300x250 also performs well on mobile when placed within content rather than as a fixed strip.
Do all ad networks support the same banner sizes?
Most major exchanges and ad servers support the IAB standard sizes by default, but support for less common formats varies by platform. Check your specific ad server's documentation before designing a non-standard size.
What file format should a banner ad be?
Static banners are typically JPG, PNG, or GIF. Animated or interactive banners use HTML5, which replaced Flash as the industry standard after browsers dropped Flash support.
Is a bigger banner ad always better for revenue?
Not necessarily. Larger units like 300x600 can command higher CPMs, but they take up more page space and can hurt user experience or page speed if not placed carefully. Size should match the placement and the content, not just chase maximum dimensions.
Conclusion
Standard banner sizes exist because a shared set of dimensions lets creative run across the widest possible pool of inventory, which is good for advertisers looking for reach and publishers looking for demand. A handful of formats — 300x250, 728x90, 320x50, 160x600, 300x600, and 336x280 — cover most of what gets served, and starting there, rather than with a custom size, is almost always the practical choice.
Key takeaways
- The IAB defines standard banner ad sizes so creative can run across many publishers without custom resizing.
- 300x250, 728x90, and 320x50 are among the most commonly served sizes across desktop and mobile.
- Non-standard sizes reduce eligible demand and can get rejected outright by ad servers.
- Mobile inventory needs mobile-native sizes — a shrunk desktop banner isn't a substitute.
- Bigger isn't automatically better: match the size to the placement and watch page speed and viewability, not just dimensions.
