PNG to ICO: Pad a Rectangle to Keep Its Shape

Three preparations from one synthetic source

We drew a transparent 400 × 200 PNG with a blue rounded block near the left edge, a lime circle near the right edge, and a small white center mark. We then prepared three 256 × 256 PNGs from exactly that same source: a direct stretch, a centered square crop, and an aspect-preserving transparent pad. The full script and output files are provided. The visual difference is intentional and directly inspectable; no subjective quality score was assigned.

The stretch maps 400 horizontal pixels and 200 vertical pixels into equal 256-pixel dimensions, so the subject's proportions change. The crop fills the square by trimming the wide source around its center; parts of the two edge shapes disappear. The pad fits the whole source inside the square and leaves transparent space above and below. In the padded 16 × 16 preview, its alpha content occupies rows 3 through 12, with transparent rows around it. That small preview is a reminder that preserving the whole design can also make it occupy less of a tiny icon.

What the saved ICO files actually contain

We exported each prepared PNG as an ICO with requested sizes 16, 32, 48, and 256 pixels, then parsed the saved ICO directory. All three sample files contain those four square entries. Their exact lengths in this run were 13,600 bytes for stretch, 9,355 for crop, and 9,113 for pad. Different pixel content compresses differently, so these byte counts are observations about our files and should not be used to rank the methods generally.

For this artwork, padding preserves both far-edge shapes but gives them less vertical footprint in the 16-pixel square. A crop can make the remaining center larger while losing both side shapes. A stretch fills the canvas while deforming the circle and rounded block. The best choice is therefore a content decision, not a contest to get the smallest ICO. If the padded 16-pixel view is too small, simplify or redraw that small layer rather than silently cutting off essential details.

A repeatable check for your own icon

Open the source and identify its aspect ratio. Prepare a square PNG using transparent padding if every part must remain. Preview it against both light and dark backgrounds so transparent margins are distinguishable from white pixels. Convert that square PNG to ICO, then inspect the actual directory sizes and the small decoded image. Keep the source PNG, prepared PNG, ICO, and export settings so a surprising result can be reproduced.

Our script uses Pillow's resize, fit, and pad operations and writes measurements.csv with each output's byte count, directory sizes, and alpha bounding boxes. Those bounds describe nonzero alpha in the preview, including antialiasing at edges; they are not a visual-quality grade. Re-run with the pinned Pillow version if exact numbers matter. For a Windows application, Microsoft's icon guidance lists 16, 32, 48, and 256 pixels as a full set, but the target application still determines what you need to inspect.

One-source rectangle preparation test

Synthetic transparent 400 × 200 PNG; Pillow 12.3.0; three 256 × 256 preparations; local files measured on 28 September 2026.

Coordinates are Pillow alpha bounding boxes in the 16 × 16 previews; the right and bottom endpoints are exclusive. Byte lengths and bounds apply to these files only.

• Synthetic 400 × 200 source PNG
• Padded square PNG
• Stretched square PNG
• Center-cropped square PNG
• Padded ICO
• Stretched ICO
• Center-cropped ICO
• Measurements CSV
• Reproduction script

Prepare a rectangular PNG for an ICO

1. 01
   Identify what must stay visible

   Mark the subject and any edge details you cannot lose. In our deliberately wide sample, objects sit near both ends; that makes a center crop easy to distinguish from padding.

2. 02
   Place the image on a transparent square canvas

   Resize proportionally to fit the target square and center it, leaving transparent space on the shorter axis. Verify that the padding is actual alpha transparency rather than white paint.

3. 03
   Export and inspect the ICO

   Convert the prepared square PNG. Inspect the downloaded file's image entries and preview the sizes you need; a size selected in the converter interface is not proof it exists in the saved ICO.