1. Paste or upload
Drop the original into the left pane and the revision into the right. Text files, code, JSON, Markdown and CSV all work.
Free diff checker
Paste two things. See exactly what changed.
Redline compares text, code, JSON and documents in under a second. Everything runs in your browser — nothing is uploaded, nothing is stored, and there is nothing to sign up for.
Original
Revised
Everything is compared in your browser. No text or file ever leaves this device.
Paste your original text on the left, the revised version on the right, then press Compare.
How it works
2. Choose the granularity
Line for code and config, word for prose and contracts, character for typos. Ignore whitespace or case when the formatting is noise.
3. Read the marks
Green underlines are additions, red strikeouts are removals. The margin strip on the right maps every change to its position — click a tick to jump there.
What people compare with Redline
Code review
Compare two source files or config variants before you merge, without pushing a branch.
Open tool →JSON and API responses
Find the renamed key or changed value between two payloads or environments.
Open tool →Drafts and contract redlines
See which clauses or sentences moved between two versions of a document.
Open tool →Frequently asked questions
- How do I compare two text files online?
- Paste the first version into the left pane and the second into the right, or upload both files, then press Compare. Redline marks insertions in green and deletions in red, line by line.
- Is there a diff checker for JSON?
- Yes — the JSON diff page is preconfigured for comparing JSON files and API responses key by key.
- Can I compare two Word documents?
- Copy the text out of both documents and paste it into the two panes. Redline compares the words; .docx files themselves are not parsed.
- Is Redline free?
- Yes, and there is no account. The core tool has no sign-up wall and no usage limit.
- Is my data private?
- The comparison runs entirely in your browser with JavaScript. Nothing you paste or upload is transmitted or stored.
- What file types can I upload?
- Plain-text formats: .txt, .md, .json, .csv, .yaml, .xml and common source code extensions, up to 2 MB each.