Quick answer
An ICO directory describes each embedded bitmap, including dimensions, color depth, byte size, and offset. Modern ICO files commonly store PNG-compressed frames.
- Last verified
- July 31, 2026
- Evidence
- Primary documentation plus transparent editorial analysis
- Primary sources
- 2
What the container stores
An ICO directory describes each embedded bitmap, including dimensions, color depth, byte size, and offset. Modern ICO files commonly store PNG-compressed frames.
Connected next stepSVG Favicons: Crisp Icons, Dark Mode, and Safe Markup for the closest prerequisite or comparison.
Choose the frames
Include 16×16 and 32×32 for browser UI, plus 48×48 when broader desktop compatibility matters. More frames increase file weight and are not automatically better.
Connected next stepFavicon API Automation: Deterministic Generation in CI/CD for the next connected implementation decision.
Quality control
Inspect each small frame rather than trusting automated downscaling. A carefully simplified 16×16 frame can outperform a mechanically reduced 512×512 logo.
Connected next stepInspect every embedded ICO frame and verify the decision with a working output.
Read the directory before decoding frames
The ICO header reports the image count, followed by entries containing dimensions, color information, byte length, and offsets. A 0 width or height byte represents 256 pixels, which is easy to misread in a hand-built parser.
Validate that every offset and length stays within the file and that each payload decodes. One working thumbnail does not prove the remaining entries are safe.
Choose frame content deliberately
Include strong 16×16 and 32×32 versions, a 48×48 fallback, and larger frames only when the supported clients use them. PNG-compressed 256-pixel frames can reduce weight while retaining alpha.
Inspect transparency and small-size detail in every frame. Automatically downsampling one large image is convenient, but a simplified tiny frame can outperform it for complex marks.
Apply this guide with the relevant tools
Use the smallest tool that verifies the specific decision described above. Each destination keeps the source format, deployment evidence, or platform purpose explicit.
Review the headings, sources, implementation artifact, and update record for this guide as structured JSON.
Download guide evidence ↓- Guide
- Favicon ICO Format: Inside Multi-Resolution Icon Files
- Coverage
- Client behavior can change by browser, operating system, platform version, cache state and deployment configuration. Unperformed manual observations are not claimed.
Questions, answered
Is ICO an image or archive?+
It is an icon container format with an internal image directory.
Can Pillow create multi-size ICO?+
Yes. Provide an RGBA master and a sizes list when saving.