How to encode a large image to Base64 on a Mac

A forty-megapixel photograph, a scanned contract, a multi-page press kit. At these sizes the result runs to several million characters, which is more than any text view should be asked to lay out at once.

This guide covers what B64 does instead.

What happens with a large source

• The result is shown in pages. A result too long to display comfortably is paged, with an indicator telling you so. Scrolling stays smooth because the window is never asked to lay out the whole string.
• Copy and Export are unaffected. They always take the complete result, whichever page is on screen. Paging is a display choice, never a truncation.
• Above a threshold, you are offered a direct write. B64 can encode straight to a file rather than building the whole string in memory first.

The route to take

1. Load the file

   Drag it in or press ⌘O. Loading is not where the time goes.

2. Set the format before you do anything else

   Switching format rewrites the result. On a very large file, decide first.

3. Export rather than copy

   ⌘E writes the complete result to a .txt file. For anything above a few megabytes this beats pushing the string through the clipboard.

4. Check the numbers

   The Inspector (⌥⌘I) gives you the character count and the overhead. That is what you compare against whatever limit you are working to.

Ask whether it should be embedded at all

It works. The question is whether a ten-megabyte payload is the right shape for its destination. An embedded asset is downloaded with every copy of the document that carries it and cannot be cached separately.

When the payload has to exist regardless — an API that takes a document as a string, an archive format, a transfer — none of that applies and the size is simply the size.

Reducing it, if you can

There is no compression setting in Base64. Every lever is on the source file: crop it, resize it, re-compress it, or pick a format that suits the content. See how to see how much bigger Base64 makes a file.

Many large files

Batch (⌘3) streams results to disk rather than accumulating them, which is what lets a folder of large sources go through in one pass. See how to batch convert thousands of images.

Troubleshooting

I can only see part of the result

It is paged. Check the character count in the Inspector against what you expect, and remember Copy and Export take all of it.

Copying is slow

Several megabytes through the clipboard is slow everywhere. Use Export… instead.

The destination rejected the payload

Compare the character count with its limit. If it is over, the source file has to get smaller — there is nothing in the encoding to adjust.

The file was refused

Size is not the reason. A file is refused when its bytes are not an image or a PDF, whatever the extension says.