Free Text Diff Checker

Paste the original text on the left and the changed version on the right, then click Compare to see every added and removed line highlighted. Works for code, configs, SQL, JSON, prose — any line-based text. Everything runs in your browser; nothing is uploaded.

About this Text Diff Checker

"What changed?" is one of the most common questions in software work — between two config files, two API responses, two deploy manifests, two drafts of a document. This free Text Diff Checker answers it instantly: paste both versions and it highlights every line that was added (green, prefixed with +) or removed (red, prefixed with −), with unchanged lines shown in place so you keep the surrounding context.

The comparison runs entirely in your browser using a classic LCS (longest common subsequence) line diff — the same family of algorithm behind git diff and most code review tools. There is no API call, no upload, and no storage, which makes it safe to compare configs and internal files without them ever leaving your machine.

How line diffing works

A line diff treats each text as a sequence of lines and finds the longest subsequence of lines the two texts have in common. Everything in that common subsequence is "same"; lines in the original that fall outside it are "removed", and lines in the changed text outside it are "added". A few consequences worth knowing:

• A modified line shows as remove + add. Line diffs have no concept of "edited in place" — changing one character on a line produces a red removed line followed by a green added line.
• Whitespace counts. Two lines that differ only by a trailing space are different lines. Enable Ignore trailing whitespace above to filter out that noise (common when editors auto-strip whitespace on save).
• Moved blocks show as remove + add. If a block of lines moves from the top of a file to the bottom, a line diff reports it as removed in one place and added in another — it does not track movement.

If you want to understand the algorithm itself, our post on how diff algorithms work (LCS explained) walks through the dynamic-programming table step by step.

Common uses for a diff checker

• Config drift — compare the config running in staging against the one in production to find the setting that explains a behavioral difference.
• Before/after refactors — sanity-check that a generated file, SQL export, or build artifact changed only where you expected.
• API responses — paste two JSON or XML responses to spot the field that appeared, disappeared, or changed. (For structure-aware JSON comparison, see comparing two JSON objects — pretty-printing both sides with our JSON Formatter first makes line diffs much more useful.)
• Documents and prose — compare two drafts of a README, policy, or email to see exactly which sentences changed.
• Log excerpts — diff a healthy request log against a failing one to find the first line where the paths diverge.

Reading the output

The result is a unified view: lines flow in order, with removals shown where they used to be and additions where they now are. The summary line above the diff counts added and removed lines. This is the same mental model as git diff output — if you are new to reading diffs, our beginner guide to git diff explains hunks, prefixes, and context lines in more depth.

Limits

Each side is capped at 5,000 lines. The LCS algorithm builds a table proportional to the product of the two line counts, so unbounded inputs could lock up the browser tab. Within the limit, comparisons are effectively instant. For multi-megabyte files, a local tool (git diff --no-index a b, diff -u, or your editor's compare mode) is the right choice.

FAQ: Text Diff Checker