Does PNG to ICO keep transparency? A measured alpha test at 16, 32 and 48 px

banggui — Sun, 01 Nov 2026 20:13:07 +0000

Does PNG to ICO keep transparency? A measured alpha test at 16, 32 and 48 px

Yes. A PNG with an alpha channel, converted to ICO, keeps alpha = 0 in every transparent pixel at all three sizes tested. Below is the pixel-by-pixel proof.

The question

When you save an image as JPEG, the alpha channel is gone — JPEG has no transparency. If your icon source was a JPG, renaming the file extension to .ico does not bring alpha back; the pixel data simply doesn't contain it.

But if your source is a PNG with an alpha channel, does the ICO conversion preserve it? I ran a measurement to find out.

Source image

A test PNG with a gradient alpha channel: at 16×16, every row steps through alpha values 0, 64, 128, 255 — four columns of each. The image has 64 fully transparent pixels (alpha = 0), 128 partially transparent pixels (alpha 64 and 128), and 64 fully opaque pixels (alpha = 255). At 32×32 the pattern scales up to 256/512/256. At 48×48 the anti-aliasing produces a richer distribution but the transparent count is 528.

The ICO file produced from this PNG is 625 bytes total, containing three frames: 16×16 (138 bytes), 32×32 (151 bytes), 48×48 (282 bytes). Pillow independently decoded each PNG entry inside the ICO to verify the pixel data.

Method

Open the PNG in the converter at icoforge.com/png-to-ico-transparency-test.
Convert to ICO and download the output file.
Decode each frame inside the ICO using Pillow's Image.open().
Count pixels at each alpha value: 0 (transparent), 1–254 (partially transparent), 255 (opaque).

Results

Size (px)	Frame bytes	Alpha 0 (transparent)	Alpha 1–254 (partial)	Alpha 255 (opaque)
16	138	64	128	64
32	151	256	512	256
48	282	528	1,248	528

ICO total: 625 bytes. Source SHA-256: a6286835afc613468120d32d780b3d954bfb5b85e983cc0ea845617b8eb04e7a. Output ICO SHA-256: 33d026fca2ea106d6e46b51c944f20531f16105dd40797b155295b1e56715903.

Every number in this table comes from the measurements at icoforge.com/png-to-ico-transparency-test, where the source PNG and output ICO are also available for download.

Why alpha = 0 at the transparent pixels means transparency survived

The source PNG has 64 pixels at alpha = 0 in the 16×16 frame. If the ICO conversion had flattened the image — filling transparent areas with white or black — those pixels would show alpha = 255. Instead, the ICO frame contains exactly 64 pixels at alpha = 0, matching the source. The same holds at 32×32 (256 pixels at alpha 0) and 48×48 (528 pixels at alpha 0). The transparent regions are preserved.

The partially transparent pixel counts also match: 128 at 16×16, 512 at 32×32, 1,248 at 48×48. These are the pixels where the source alpha is 64 or 128 — and the ICO preserves those intermediate values exactly.

What about JPEG sources?

JPEG does not store an alpha channel. A JPG file contains only RGB data. If you convert a JPG to ICO, the result has no transparency — not because the converter dropped it, but because the source never had it. Changing a .jpg filename to .ico doesn't change the pixel data inside.

Data and sources

All measurements were taken on 2026-10-26 using Chrome's canvas rendering pipeline and verified with Pillow. The raw measurements JSON is available at:

/samples/png-alpha-test/measurements.json — frame byte counts, alpha value distributions, and SHA-256 hashes for source and output.

The source PNG and output ICO files are downloadable from the PNG to ICO transparency test page.

License: CC0 (public domain).

DEV Community: banggui

Does PNG to ICO keep transparency? A measured alpha test at 16, 32 and 48 px

Does PNG to ICO keep transparency? A measured alpha test at 16, 32 and 48 px

The question

Source image

Method

Results

Why alpha = 0 at the transparent pixels means transparency survived

What about JPEG sources?

Data and sources