Base64 Encoder and Decoder

What This Tool Does

Base64 Encoder/Decoder converts arbitrary binary or text data into a safe, printable ASCII representation — and converts it back. It runs entirely in your browser using the native btoa() / atob() APIs, so no data is transmitted to any server.

Two modes are available: Encode takes raw text or a binary file and outputs a Base64 string. Decode takes a Base64 string and outputs the original text or lets you download the reconstructed binary file.

How Does It Work?

Base64 maps every 3 bytes of input to 4 printable characters drawn from a 64-character alphabet (A–Z, a–z, 0–9, +, /):

Input bytes:  [xxxxxxxx] [yyyyyyyy] [zzzzzzzz]
Group bits:   [xxxxxx] [xxyyyy] [yyyyzz] [zzzzzz]
Map each group to: A-Z (0–25), a-z (26–51), 0–9 (52–61), + (62), / (63)

When the input length is not a multiple of 3, padding (=) is added:

1 remaining byte  → 2 Base64 chars + "=="
2 remaining bytes → 3 Base64 chars + "="

URL-safe variant replaces + → - and / → _ and omits padding.

What Characters Are Supported?

Variant	Characters	Padding	Common Use
Standard	A–Z a–z 0–9 + /	=	JSON, MIME, HTML
URL-safe	A–Z a–z 0–9 - _	omitted	JWT, URL params
MIME	Standard + line breaks every 76 chars	=	Email attachments

What Are Common Use Cases?

Embedding images in HTML/CSS. Instead of an external request, small icons and logos can be inlined as data:image/png;base64,... data URLs. Useful for single-file HTML exports and CSS background images.

API debugging. Many APIs (Stripe webhooks, GitHub Actions secrets, AWS SDK credential files) transmit binary payloads as Base64. Paste the encoded value here to inspect the raw content.

JWT inspection. JSON Web Tokens are three Base64url-encoded segments. Decode the header and payload segments to read the claims without needing a dedicated JWT library.

Email attachment headers. MIME email encodes attachment content in Base64. If you’re hand-crafting a raw email or debugging a mail server, you need to encode the attachment binary here first.

Local storage workarounds. localStorage only stores strings. Encoding a small binary blob (a user-generated canvas export, for example) as Base64 lets you persist it in localStorage until a backend is available.

Frequently Asked Questions

What is Base64 used for? Base64 allows binary data to travel safely through systems designed to handle text. JSON doesn’t support raw binary, so API responses often embed images or file data as Base64 strings. Email (MIME) uses it for attachments. HTML uses it for data URLs. It’s a universal text-safe envelope for binary content.

Does Base64 make data secure? No. Encoding is not encryption. Anyone who sees the Base64 string can decode it in seconds with any standard tool. If you need to protect the data, encrypt it (AES-256, for example) before encoding it to Base64.

What is URL-safe Base64? Standard Base64 uses + and /, which are interpreted as special characters in URLs. URL-safe Base64 (RFC 4648 §5) replaces them with - and _, making the string safe to include directly in a URL query string or path segment without percent-encoding. JWTs use URL-safe Base64 and also strip the trailing = padding.

Why does Base64 output end with = signs? Base64 processes input in 3-byte groups and outputs 4-character chunks. If the input has 1 or 2 leftover bytes, the encoder adds = padding characters to keep the output length a multiple of 4. This is purely a formatting convention — decoders strip the padding before processing.

How much does Base64 increase file size? Every 3 bytes become 4 characters, so the overhead is exactly 4/3 − 1 ≈ 33.3%. A 1 MB binary becomes approximately 1.37 MB as Base64. For small embedded assets (icons under 2 KB) this is usually acceptable. For larger files, use a proper binary transfer mechanism instead.