Does the tool work with Base64 strings without a data URI prefix?

Yes. Paste the raw Base64 string and the tool detects the image format from the binary signature (the first decoded bytes). It then constructs the data URI internally for the preview and download. Strings with a data:image/...;base64, prefix are also accepted and the MIME type is read from the prefix directly.

What image formats are supported?

JPEG, PNG, GIF, WebP, BMP, ICO, and SVG are detected and previewed. Other binary formats (PDF, ZIP) may decode successfully but may not render in the preview panel — the Download button still lets you save the decoded file for inspection in a native application.

How is the file size estimated?

Base64 encodes 3 bytes as 4 characters. The decoded byte count is therefore approximately (base64Length × 3) / 4, adjusted for padding. The displayed estimate is accurate to within a few bytes of the actual decoded file size.

Developer Utilities

Base64 Image Decoder

The problem: Base64-encoded images in API responses, CSS or HTML are unreadable blobs — you can't tell what they are without decoding.

ASG Privacy VerifiedVerified

100% In-Browser Execution. Zero server uploads. Your data never leaves this tab — disconnect your internet and the tool keeps working.

ASG Privacy VerifiedVerified

100% In-Browser Execution. Zero server uploads. Your data never leaves this tab — disconnect your internet and the tool keeps working.

Base64 String (with or without data URI prefix)

Base64 Image Decoder: the complete guide

A Base64 image decoder translates the compact, text-safe encoding that APIs, CSS data URIs, HTML embeds and email attachments use for images back into viewable, downloadable image files. When a backend API returns a profile photo as a multi-kilobyte alphanumeric blob or a designer delivers a logo embedded as a data:image/png;base64,... string, this tool decodes it instantly — detecting the image format from the binary signature, rendering a live preview, reporting the estimated file size, and offering a one-click download. Everything runs inside your browser using native JavaScript; the bytes are never uploaded anywhere.

How Base64 encoding represents binary images

Base64 encodes every three bytes of binary data into four ASCII characters from a 64-character alphabet (A–Z, a–z, 0–9, + and /), producing output about 33% larger than the original. The result is safe to embed in JSON, HTML attributes, CSS background properties, and XML without escaping binary control characters. Images are commonly Base64-encoded for email attachments (MIME), API responses that avoid serving separate static files, CSS sprite sheets where one HTTP request is preferred, and HTML canvas toDataURL() exports. A data URI wraps the encoding in a standard prefix — data:image/png;base64, — which browsers understand natively, allowing the img element's src attribute to contain the entire image inline.

Format detection from binary signature

Different image formats start with recognizable byte sequences: JPEG files begin with 0xFF 0xD8, PNG files with the 8-byte signature 0x89 PNG\r\n\x1a\n, WebP with RIFF....WEBP, GIF with GIF87a or GIF89a. When a Base64 string arrives without a data URI prefix, the decoder checks the first decoded bytes against these signatures to infer the MIME type. In Base64 form, the PNG signature always starts with iVBORw0K, JPEG with /9j/, WebP with UklGR, and GIF with R0lGOD. This fingerprinting means you do not need to know the format before decoding — paste any blob and the tool tells you what it is.

Privacy: why offline decoding matters

Most online Base64 decoders send your input to a server-side decode endpoint before displaying the result. For images embedded in API responses — which often contain private user photos, medical scans, proprietary designs or confidential documents — that upload is unacceptable. This tool decodes using the browser's own atob() function and constructs the result as a data URI entirely in memory, then hands it to an img element to render. Open the Network tab in DevTools while decoding: after the initial page load, no request is made. The image data never leaves your device.

Step by step: how to use Base64 Decoder

1
Copy the Base64 string from your API response, CSS, HTML source, or any other context.
2
Paste it into the input area — either as a raw Base64 string or with a data:image/...;base64, prefix.
3
Click Decode Image. The tool validates the Base64 alphabet, detects the format, and renders a preview.
4
Check the detected format, MIME type, and estimated file size in the metadata panel.
5
Click Download Image to save the decoded file with the correct extension.
6
Use Copy Data URI to get the full data:image/...;base64,... string for use in CSS or HTML.

Security & privacy

Your Base64 string is decoded entirely inside this browser tab using the native JavaScript atob() function and a data URI passed directly to an img element. No network request is made after the page loads — you can verify this in DevTools Network tab, or disconnect from internet and decode offline. The image data never exists on any server. For medical images, private user photos, or proprietary graphics embedded in API payloads, this is the privacy guarantee that makes the tool safe for real work.