How to URL Encode and Decode Strings for APIs and Query Parameters

Q: How do I spot double encoding?

A % is normally encoded to %25 when it's part of a value. If the decoded result contains %25, or it doesn't return to the original URL after one decode, you likely have a double-encoded value—an already-encoded string that was encoded again. Decoding once and checking whether it returns to the expected URL helps isolate the cause.

Learn what percent encoding means, when to encode URLs, and how to inspect encoded query strings safely during API and tracking-link debugging.

Tools mentioned in this article

Open the browser-based tool while you read and try the workflow immediately.

URL Encode / Decode Encode or decode URL strings to avoid garbled characters. Safe, fast conversion entirely in your browser. URL Parameters to JSON Convert URL query strings into JSON to inspect keys and values quickly in your browser.

Why URLs contain strings like `%20` and `%2F`

URLs can only use certain characters safely. Spaces, non-English text, slashes inside values, symbols, and reserved characters often need to be percent-encoded before they are placed into a URL.

How to URL Encode and Decode Strings for APIs and Query Parameters

DevToolKits’ URL Encoder and Decoder helps you convert readable text into URL-safe strings and decode existing values back into text.

Example: Putting a search keyword into a URL

Say you want to put the search keyword Tokyo Station cafe into a URL. Spaces can break display or copying in some environments.

Tokyo Station cafe

URL-encoded, it becomes:

Tokyo%20Station%20cafe

Placed in a query parameter, it produces a shareable URL:

https://example.com/search?q=Tokyo%20Station%20cafe

Confirming that it decodes back to the original string helps you catch mangled search conditions or character corruption.

When encoding matters

Encoding is especially important when building search URLs, callback URLs, API query parameters, redirect targets, and tracking links. If a value is not encoded correctly, the browser or server may split it at the wrong character and interpret it as a different parameter.

For example, a space may appear as %20 or + depending on the context. A slash inside a query value may need different handling than a slash in a path.

Inspect query strings separately

When a URL has many parameters, use the URL Parameters to JSON Converter to break the query string into keys and values. This makes tracking links and API requests much easier to review before you share or deploy them.

Summary

URL encoding is a small detail that prevents broken links, malformed API requests, and tracking errors. Decode what you receive, encode what you send, and review long query strings as structured data whenever possible.

Frequently Asked Questions

Are `%20` and `+` both spaces?

Both represent a space, but the context differs. In URL paths and percent-encoding, a space is %20. In query strings (application/x-www-form-urlencoded), a space may be represented as +. Check the format your API or form expects, and verify the actual string you send with the URL Encoder and Decoder.

How do I spot double encoding?

A % is normally encoded to %25 when it’s part of a value. If the decoded result contains %25, or it doesn’t return to the original URL after one decode, you likely have a double-encoded value—an already-encoded string that was encoded again. Decoding once and checking whether it returns to the expected URL helps isolate the cause.

Do non-English characters in a URL always need encoding?

The address bar may show them directly, but when sharing as a link or passing to an API, encode them to avoid environment-dependent corruption. For example, 東京 is encoded as its UTF-8 byte sequence, %E6%9D%B1%E4%BA%AC.