Base64 Encode & Decode in Dart & Flutter

Encode or decode below — it runs locally in your browser — then grab the canonical Dart code. Dart's built-in dart:convert library works on bytes, so you utf8.encode() a string before encoding and utf8.decode() the result after. No package needed.

Encode and decode a string

The key pattern is String → utf8.encode() → base64Encode() → base64Decode() → utf8.decode() → String:

import 'dart:convert';

// Encode
final encoded = base64Encode(utf8.encode("Hello, world!"));
print(encoded); // SGVsbG8sIHdvcmxkIQ==

// Decode
final decoded = utf8.decode(base64Decode("SGVsbG8sIHdvcmxkIQ=="));
print(decoded); // Hello, world!

URL-safe Base64

For JWTs, URLs, and filenames, use the URL-safe alphabet (- and _ instead of + and /) via the base64Url codec from dart:convert:

import 'dart:convert';

final bytes = utf8.encode("subjects?_d=qux");

// URL-safe encode
final urlSafe = base64Url.encode(bytes);

// URL-safe decode
final decoded = base64Url.decode(urlSafe);

Encode an image (Flutter)

In Flutter, images arrive as a Uint8List of bytes. Since Uint8List is a List<int>, pass it straight to base64Encode:

import 'dart:convert';
import 'dart:io';

// From a file on disk
final encoded = base64Encode(File(path).readAsBytesSync());

// Or from a Uint8List you already hold
final dataUri = "data:image/png;base64,$encoded";

Frequently asked questions

How do I Base64 encode a string in Dart?

Encode the string to bytes, then Base64-encode: base64Encode(utf8.encode("Hello, world!")) returns "SGVsbG8sIHdvcmxkIQ==".

How do I Base64 decode in Dart?

base64Decode("SGVsbG8sIHdvcmxkIQ==") returns the bytes; add utf8.decode(...) for a string.

What is the difference between base64 and base64Url in Dart?

base64 uses + and /; base64Url uses the URL-safe - and _. Both are in dart:convert with no extra package.

How do I encode an image to Base64 in Flutter?

Read the image as bytes and pass it to base64Encode: base64Encode(File(path).readAsBytesSync()), then build a data:image/png;base64,... URI.