How to preview a Base64 image on a Mac
You can do this with B64 for free, without writing a file first. Here is how to look at a payload properly before you do anything else with it.
Often the only question is what is this? — before you save it, before you send it on, before you decide whether it is the asset you were looking for. You want to look at it, at full size, without committing anything to disk.
There is no purchase and no account for any of this, and no time limit on it. Encoding and Batch are the paid features; turning Base64 back into a picture is not.
Preview a payload
Paste it into Decode
Press ⌘2 and paste. The picture appears beside the input as you type.
Read the summary
Format, dimensions and size sit next to the picture. That is often the whole answer — a payload that turns out to be 48 × 48 is a favicon, not the hero image somebody thought it was.
Quick Look it
Click Quick Look, or press space, to see it at full resolution in the standard macOS preview.
Why full size matters
The in-window preview is fitted to the pane, which is enough to recognise a picture and not enough to judge it. At full resolution you can see things the thumbnail hides: that a screenshot was captured at half the resolution you wanted, that an icon is blurry because it was upscaled, that a photograph is the compressed version rather than the original.
The dimensions line beside the picture usually answers the question faster than looking does. A 3729 × 3022 payload and a 640 × 519 one look identical in a small pane.
What the preview tells you
| Shown | Where | Tells you |
|---|---|---|
| Format | Beside the picture | What the bytes really are |
| Dimensions | Beside the picture | Whether it is the resolution you wanted |
| Size | Beside the picture | How heavy the original file is |
| Declared vs actual type | Inspector (⌥⌘I) | Whether a data URI was mislabelled |
| SHA-256 | Inspector | Whether it matches a file you already have |
Previewing without saving anything
Nothing is written to disk when you decode or Quick Look. The picture exists in the app and nowhere else until you choose Save…. For an unknown payload from an unknown source, that is the right order: look first, decide second.
Previewing a lot of payloads
Drop a folder of Base64 text files into Batch (⌘3) and every one is decoded and shown with a thumbnail. Double-click a row to open that one properly; select it and press space to Quick Look it. Nothing is written until you save.
Troubleshooting
The preview is blank
Either nothing decoded — check the inline note beside the input — or the payload decoded to something that is not an image.
Quick Look does not open
There has to be a decoded image in front of you. If the input is invalid or truncated there is nothing to preview.
The picture looks soft
Check the dimensions. A small source displayed large is soft because it is small — the encoding has nothing to do with it and cannot lose detail.
Frequently asked questions
Do I have to save the file to see it properly?
No. Quick Look shows the decoded image at full resolution without writing anything to disk. Nothing is saved until you choose Save.
Is previewing free?
Yes. Decoding, previewing, copying and saving a decoded image are all free forever.
How do I tell whether a payload is the high-resolution version?
Read the dimensions beside the picture. That answers it faster than looking at a fitted preview.
Can I preview a whole folder of payloads?
Yes — drop it into Batch and every one is decoded and shown with a thumbnail, with nothing written until you save.