Which Icon Sizes a Linux Theme Actually Needs, and Which Nobody Ever Sees

Desktop icon lookups in modern Linux environments still rely heavily on fixed-size directories even in the era of scalable vector graphics. Understanding which sizes to export can mean the difference between a crisp, complete theme and one that falls back to generic placeholders or ugly pixels.

The freedesktop.org Icon Theme Specification, the primary standard describing Linux icon theme structure and lookup, says icon availability is checked by size and context. A theme can include both fixed-size subdirectories and a scalable SVG directory for availability at any requested size. But in practice, supplying a small set of per-category fixed sizes alongside scalables provides more consistent and high-quality results across the majority of Linux desktops.

Fixed Sizes Still Matter

Even in the era of scalable SVGs[1], fixed-size directories still matter. Common icon theme directories cluster around a select set of raster resolutions.

The ArchWiki "Icons" guide lists a base set of theme subdirectories: 16x16, 22x22, 24x24, 32x32, 48x48, 64x64, 72x72, 96x96, 128x128, 192x192, 256x256, and 512x512. Topped with @2 subdirectories for high-DPI support, this forms the optional tree for a complete fixed-size theme to supply. These match the set frequently cited in icon authoring community resources and FAQs, including 8x8, 8x8@2, 16x16@2, and others up through doubled variants of the largest sizes[3,4].

The xdg-icon-resource utility behind gtk-update-icon-cache lists commonly requested icon sizes in the "Apps" context as 16, 22, 32, 48, 64, and 128, with a similar in-use set for desktop's "MimeTypes" icons[5,6]. Missing any of those key sizes means the desktop must assume either the closest available raster size, or degrade to a lower-quality scalable fallback.

Which Sizes Do Desktops Actually Ask For?

Specific desktops expose sets of icon sizes for different use cases:

In KDE, defaults in kdeglobals and theme settings show 48x48 as the panel default, 64x64 for dialogs, 22x22 for toolbars, and other category-specific sizes [7,8]. A sample KDE icon theme indexes desktop defaults of 48x48, toolbar 22x22, panel 48x48, and dialog 32x32, with a full size fall-back list for each category to enforce some guarantees.

The GTK 4 IconTheme API reference says a size of -1 means the icon is available in a scalable format.

HiDPI and Scale Factor 2

Icon theme tree structures show an awareness of differing display densities. ArchWiki's HiDPI page shows GNOME’s scaling-factor 2 setting and notes that icon size can be chosen separately for each category in KDE system settings. The GTK 4 IconTheme API reference says a size of -1 means the icon is available in a scalable format. @2 subdirectories or higher-density asset supplements are common in icon theme manifestations of this Density Independent Pixel (DIP) treatment.

But scale factor 2 is not a guarantee of a universal 2x fixed size requirement. It's more specific to the compositor and window manager that sets the scaling and how scaling impacts the perceived size of the icons[11,12].

What Happens When a Size is Missing?

When a requested icon size is unavailable, a theme… must assume one of several fallback paths. The exact fallback logic, while a prominent concern in desktop usability, is not precisely documented in the sources.

What to Ship, Practically?

If themes must include fixed-format icons, the current lookups suggest a simple fixed set that matches the most frequently requested resolutions. As the ArchWiki and Linux.org guides show, a complete raster lists includes items in the "common optional sizes" category, from 8x8@2 through 96x96 (the latter is frequently the U:PPI). Smaller sizes around these are commonly used and checked in desktops[2,3,4,5,9,11].

In summary, effective Linux icon themes ship fixed instances around common sizes, alongside SVG scalable icons, while adapting to HDPI with double-density directories or assets.

For icon authoring, understanding the standard iconR theme specification details create clear expectations around default, optional, and Easter egg folder capabilities. With theme authors adapting to desktop and HiDPI trends, the most important index.theme capability is a responsive rendering function that supports a range of per-category sizes—even when a specific request is missing.