Image to Base64 Converter — Free Online Tool

The browser's FileReader API provides a readAsDataURL() method that reads any file from the local filesystem and returns a data URI containing the file's content encoded as Base64. This entire process runs in the browser using JavaScript — no server is involved at any stage, and the image file is never uploaded anywhere. The resulting data URI has the format data:[mime-type];base64,[base64data], where the MIME type is determined automatically from the file's binary signature.

The FileReader API reads the raw bytes of the image file and encodes them using the standard Base64 algorithm — every 3 bytes of binary data produces 4 Base64 characters. A 100 KB PNG file produces a data URI of approximately 137 KB. This one-third size overhead is inherent to Base64 encoding and applies to all file types. The raw Base64 string (without the data: prefix) is also available separately for use with APIs that expect raw Base64 input rather than a full data URI.

Data URIs allow images to be embedded directly in HTML, CSS, JavaScript, JSON, and XML without any external file dependency. This is particularly useful for HTML email templates where external images are frequently blocked, offline web applications that must function without network access, CSS sprites and icon systems, and REST API payloads that include image data as part of a JSON request body.

A front-end developer is building an HTML email newsletter. Email clients including Outlook 2019 and many corporate mail servers block external image URLs by default, displaying broken image icons in place of any img tag that loads from an external URL. The designer has provided a 3 KB company logo as a PNG file.

The developer drops the PNG into this tool and immediately receives the raw Base64 string and the complete data URI. She copies the data URI and pastes it as the src attribute of the img tag in the email template. The logo now renders correctly in every email client — including those with external image blocking — because the entire image is part of the email itself. The 4 KB data URI is a small price for guaranteed rendering across clients.

In another workflow, a mobile app developer is testing an image upload endpoint. Rather than setting up a multipart form upload to test the API, she drops a sample photo into this tool, copies the raw Base64 output, and pastes it into her API testing tool (Postman, Insomnia, or HTTPie) as the request body value. The endpoint receives valid image data and the developer can verify the API's behavior in seconds, before writing any upload code in the application.

Base64 increases file size by approximately 33%. A 1 MB image produces roughly 1.33 MB of Base64 text that the browser must parse and decode on every page load. For images larger than 200–300 KB, the size penalty usually outweighs the benefit of embedding. For images used on multiple pages, a standard file URL is always more efficient — the browser caches the file independently and reuses it without re-downloading or re-decoding.

Data URIs are not cached separately from the HTML document. A standard image file loaded via URL is cached by the browser and reused across pages and sessions. An image embedded as a data URI is re-parsed every time the HTML document is loaded, because the image data is part of the document itself. This matters most for large data URIs on high-traffic pages.

Blob URLs are an alternative to data URIs for large files. Created with URL.createObjectURL(file), a blob URL is a temporary in-memory reference to the file's raw binary data — no Base64 encoding required, no 33% overhead. Blob URLs are ideal for image previews in file upload interfaces where you want to show the selected image before submitting the form. They are revoked when the page unloads or when explicitly released with URL.revokeObjectURL().