Banner Ad Sizes Guide (2026): The IAB Sizes That Sell on Small and Medium Sites

The banner sizes that sell best on small and medium sites are the familiar IAB ones: 300×250 (medium rectangle), 728×90 (leaderboard) and 320×50 (smartphone banner), with 300×600 (half page) and 970×250 (billboard) as premium options if your layout has room. Offer two or three of them, reserve their exact dimensions in your layout, and ask advertisers for images at twice the pixel size so they stay sharp on high-density screens.

Key takeaways

• Three sizes cover most small-site needs: 300×250, 728×90 and 320×50.
• The IAB's New Ad Portfolio favors flexible, aspect-ratio-based ads but still publishes a fixed-size list that includes all three.
• 300×250 is the most versatile size because it fits sidebars, article bodies and phone screens.
• Keep files within the IAB's maximum initial load, such as 150 KB for a 300×250 and 50 KB for a 320×50.
• Ask for @2x images, display them at the slot size with explicit width and height, and reserve the space so the page doesn't shift.

What the IAB standard says today

The IAB Tech Lab's New Ad Portfolio, released in 2017, replaced the older display guidelines. Its main idea is that new ad units are defined by aspect ratio and size range rather than fixed pixel sizes, so they can adapt to different screens.

Fixed sizes didn't disappear, though. The IAB's fixed-size ad specifications recommend a short list of units, chosen based on a study of which units contribute the majority of revenue and work across screen sizes:

The list also includes a small 120×60 unit and tiny feature-phone banners, which few sites need today.

One size you'll still see everywhere isn't on that fixed list: the 300×600 half page. The IAB's sizing grid uses it as an example of a previous standard size, and Google's AdSense guide to ad sizes still lists it among its top-performing sizes, alongside 300×250, 336×280, 728×90 and the 320×100 large mobile banner.

The sizes worth offering on a small or medium site

If you start with one slot, make it a 300×250. It works on both desktop and phones without changes, and advertisers rarely need to design something new for it.

Resist offering every size. Each extra size means another creative the advertiser has to produce and another slot you have to fill. Two or three well-placed slots sell better than ten scattered ones.

Desktop vs. mobile

Phone screens are far narrower than a 728-pixel leaderboard, so you have two options:

1. Separate slots per device. Show a 728×90 on desktop and a 320×50 on phones. Sell them separately, with separate impression numbers, because they reach different visitors.
2. One size that fits both. Use a 300×250 that appears on every device. This is simpler to sell and simpler to report on.

Avoid shrinking a desktop banner to fit a phone. A 728×90 squeezed to phone width loses roughly half its width, and the advertiser's text becomes hard to read. Also avoid mobile placements that cover content or push it around while people are reading.

File weight and formats

Heavy ads slow your pages down, and readers notice. Use the IAB's maximum initial load figures above as hard limits in your specs, and ask for less when you can.

• JPG suits photos and gradients.
• PNG suits flat graphics, logos and sharp text.
• GIF suits simple animation, though file sizes grow quickly.
• WebP usually produces smaller files at the same quality, if your setup accepts it.

The IAB guidelines also ask that ad content be clearly distinguishable from the publisher's own content, for example with a clearly defined border. A small "Advertisement" or "Sponsored" label near the slot does the same job and builds trust with readers.

@2x images for retina screens

High-density ("retina") screens use more than one device pixel for every CSS pixel—often two or three. A 300×250 image shown on such a screen is stretched to fit and looks soft, especially around text. The fix is to request images at twice the slot's dimensions and display them at the slot size. The IAB itself works at double-density (2x) resolution when it sets file-weight limits for fixed-size units.

In the HTML, keep the width and height attributes at the slot size, and optionally offer both versions with srcset:

<a href="https://advertiser.example/" rel="sponsored">
  <img src="banner-300x250.jpg"
       srcset="banner-300x250.jpg 1x, banner-600x500.jpg 2x"
       width="300" height="250" alt="Advertiser name: short message">
</a>

An @2x image has four times the pixels, so compress it well to stay inside the weight limit. For the full embed walkthrough, see how to add a banner ad to your website with HTML.

Reserve the space to avoid layout shifts

When an image loads without reserved space, everything below it jumps. That hurts readers and your Cumulative Layout Shift score, which should be 0.1 or less for a good experience. web.dev's guide to optimizing CLS recommends always including width and height attributes on images, or reserving the space with CSS aspect-ratio or min-height. Fixed-size slots make this easy: you know the exact dimensions in advance. Our guide to ads without hurting Core Web Vitals covers the rest.

A simple spec sheet for advertisers

Publish these requirements on your advertise page:

• Exact size, plus an @2x version
• Accepted formats and maximum file weight
• Animation rules, such as maximum length or none at all
• A working landing page that matches the ad's message
• Short alt text describing the ad
• No fake buttons, fake close icons or misleading claims

Then check every submission against the same criteria; our ad creative approval checklist gives you a ready-made list.

Offering sizes with a booking tool

In Spreadiom, you choose a slot's size when you create it, for example 300×250, and advertisers' uploads are cropped to the slot size automatically, so every creative matches the dimensions you reserved. The embed is a plain <a><img></a> with no JavaScript and no cookies, and when no campaign is running the slot shows an "Advertise here" banner that links to its booking page. It's free to start, with a 5% fee on completed bookings—see pricing.

FAQ

Which banner ad size should I offer first?

Start with a 300×250 medium rectangle. It's on the IAB's fixed-size list, most advertisers already have creatives in that size, and it fits sidebars, article bodies and phone screens without changes. Once it sells reliably, add a 728×90 leaderboard for desktop or a 320×50 smartphone banner, depending on where your visitors come from.

What size should a mobile banner ad be?

The IAB's smartphone banner sizes are 320×50 and 300×50, with a 50 KB maximum initial load. A 300×250 medium rectangle also fits phone screens and gives advertisers more room. Google additionally offers a 320×100 large mobile banner. Whichever you choose, place it where it doesn't cover content, and ask for @2x images so it stays sharp on high-density phone screens.

What does @2x mean for banner ads?

@2x means an image with twice the width and height of the slot, such as 600×500 pixels for a 300×250 slot. High-density screens use multiple device pixels per CSS pixel, so a standard image looks blurry on them. Display the @2x file at the slot size with width and height attributes, and compress it so it stays within your file-weight limit.

Are IAB ad sizes still the standard?

Yes. The IAB Tech Lab's New Ad Portfolio from 2017 shifted new formats toward flexible, aspect-ratio-based units, but it still publishes fixed-size specifications for the most-used banners, including 300×250, 728×90, 320×50, 160×600, 970×250 and 970×90. Because they're published standards that advertisers and designers already know, they're the safest choice for direct-sold slots.

How large can a banner ad file be?

The IAB's fixed-size specifications set a maximum initial load for each unit: 150 KB for 300×250, 728×90 and 160×600, 50 KB for 320×50 and 300×50, 200 KB for 970×90 and 250 KB for 970×250 and 300×1050. Treat these as upper limits rather than targets. Smaller files load faster and protect your page speed.