Base64URL to Base64 Converter

What does the Base64URL to Base64 Converter do?

The Base64URL to Base64 Converter transforms URL-safe Base64URL strings (RFC 7515, commonly used in JWTs, OAuth tokens, and WebAuthn credentials) into standard RFC 4648 Base64 strings. It replaces - with +, _ with /, and computes and appends the exact trailing = padding needed for strict backend decoders.

Core Concepts

Understanding Base64URL to Base64 conversion mechanics:

• Alphabet Character Swapping: Maps URL-safe characters (- and _) back to standard Base64 characters (+ and /).
• Modulo-4 Padding Calculation: Base64 strings must have a length divisible by 4. If length modulo 4 is 2, two = signs are appended; if 3, one = sign is appended.
• Input Validation: Strict error detection flags input strings containing invalid characters (such as standard + or / characters in Base64URL input) or malformed lengths (length % 4 == 1).

How to use the tool?

1. Paste Base64URL String: Enter or paste your Base64URL string into the editor or click Sample.
2. Live Auto-Map & Padding: The tool automatically maps characters and computes trailing = padding in real time.
3. Copy Output: Click Copy or Download to export the standard Base64 result.

Related Developer Utilities

If you work with Base64 encoding, JWT tokens, and URL safety, explore these complementary tools:

• Base64 to Base64URL Converter: Convert standard Base64 into URL-safe Base64URL.
• Base64 Decoder: Decode Base64 payloads into UTF-8 text or JSON.
• Base64 Encoder: Encode plain text into Base64 or URL-safe Base64.
• JWT Decoder: Parse and inspect JSON Web Tokens.

REST API Integration

blueutils.com provides a free REST API endpoint (POST https://blueutils.com/api/base64/base64url-to-base64) to programmatically convert URL-safe Base64URL strings into standard Base64 strings with exact padding calculations.

API Request Parameters

API Request Payload Examples

cURL

curl -X POST https://blueutils.com/api/base64/base64url-to-base64 \
  -H "Content-Type: application/json" \
  -d '{
    "rawText": "eyJzdWIiOiIxMjM0In0-w_1"
  }'

Python

import requests

url = "https://blueutils.com/api/base64/base64url-to-base64"
payload = {"rawText": "eyJzdWIiOiIxMjM0In0-w_1"}
headers = {"Content-Type": "application/json"}
response = requests.post(url, json=payload, headers=headers)
print(response.json())

Java

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class Main {
    public static void main(String[] args) throws Exception {
        String jsonPayload = """
            {
                "rawText": "eyJzdWIiOiIxMjM0In0-w_1"
            }
            """;

        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://blueutils.com/api/base64/base64url-to-base64"))
            .header("Content-Type", "application/json")
            .POST(HttpRequest.BodyPublishers.ofString(jsonPayload))
            .build();

        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println(response.body());
    }
}

API Response Parameters

API Response Payload Examples

Success Response (HTTP 200 OK)

{
  "isValid": true,
  "inputLength": 24,
  "outputLength": 28,
  "paddingAdded": 2,
  "converted": "eyJzdWIiOiIxMjM0In0+w/1=="
}

Validation Failure Response (HTTP 400 Bad Request)

{
  "isValid": false,
  "error": "Invalid Base64URL: String contains standard Base64 characters (\"+\" or \"/\")."
}

Rate Limit Exceeded Response (HTTP 429 Too Many Requests)

{
  "error": "API rate limit exceeded. Please wait or contact support@blueutils.com."
}

Why use an API to convert Base64URL to Base64?

Integrating the Base64URL to Base64 API into authentication microservices, token validation layers, or AI agent tool calling provides key benefits:

• Rapid Script Validation: Converts unpadded URL-safe tokens from OAuth identity providers into standard Base64 payloads compatible with strict legacy crypto libraries.
• Optimized Token Efficiency for AI Agents: LLMs frequently miscount modulo-4 padding characters. Invoking the API normalizes strings deterministically without hallucination.
• Deterministic Accuracy Without Hallucinations: Ensures 100% accurate character swapping and RFC 4648 padding syntax.

Native Usage

How to convert Base64URL to standard Base64 locally in terminal environments or scripts:

Windows (CMD / PowerShell)

# Convert Base64URL to standard Base64 in PowerShell
$b64url = "eyJzdWIiOiIxMjM0In0-w_1"
$b64 = $b64url.Replace('-', '+').Replace('_', '/')
$pad = (4 - ($b64.Length % 4)) % 4
$b64 = $b64 + ('=' * $pad)
Write-Output "Standard Base64: $b64"

Linux / Unix (Bash)

# Convert Base64URL to standard Base64 in Linux
b64url="eyJzdWIiOiIxMjM0In0-w_1"
b64=$(echo -n "$b64url" | tr '_-' '/+')
pad=$(( (4 - ${#b64} % 4) % 4 ))
printf '%s%*s\n' "$b64" $pad '' | tr ' ' '='

Python

Using Python:

def base64url_to_base64(b64url_str):
    b64 = b64url_str.replace('-', '+').replace('_', '/')
    pad = (4 - len(b64) % 4) % 4
    return b64 + ('=' * pad)

token = "eyJzdWIiOiIxMjM0In0-w_1"
print("Standard Base64:", base64url_to_base64(token))

Java

Using Java:

public class Base64UrlToBase64Example {
    public static String toBase64(String base64Url) {
        String base64 = base64Url.replace('-', '+').replace('_', '/');
        int remainder = base64.length() % 4;
        if (remainder == 2) base64 += "==";
        else if (remainder == 3) base64 += "=";
        return base64;
    }

    public static void main(String[] args) {
        String urlSafe = "eyJzdWIiOiIxMjM0In0-w_1";
        System.out.println(toBase64(urlSafe));
    }
}