Zerdly

Text Diff Checker

Compares two blocks of text and highlights added, removed and changed lines.

100% FreeRuns in your browserBasic
How to use this calculator
Compare by

Differences

Press Compare.

Runs entirely in your browser. Your input is not uploaded or stored.

How the method works

One line replaced → one removal and one addition
Illustrative method
One line replaced → one removal and one additionLeft: original lines a, b, c. Right: changed lines a, x, c. Below, the unified listing marks b as removed with a minus sign and x as added with a plus sign; a and c are unchanged.OriginalChanged asame−bremoved csame asame+xadded csameUnified view asame−bremoved+xadded csame
Original a / b / c versus changed a / x / c: line “b” is removed (−) and line “x” is added (+); “a” and “c” are the same. Result: 1 removed, 1 added. Every change is labelled in text, not only by colour.

Ignore-case and ignore-whitespace comparisons are reports; only an exact line comparison can produce a faithful .diff patch. Fixed example values for illustration — not your result.

How to use this calculator

  1. Paste the original and the changed text, choose line, word or character comparison, and any ignore options.
  2. Press Compare. Added and removed parts are marked with + / − and screen-reader labels, not only colour. Choose unified or side-by-side view, wrap on or off, and jump between changes with Previous / Next (Alt+↓ / Alt+↑); side-by-side panes scroll together unless you switch that off.
  3. Download a .diff patch (line mode, nothing ignored) or a comparison report. Swap sides to reverse the comparison.

How it works

A Myers diff (linear-space bisection) runs in a background worker, so memory grows with the text rather than with lines × lines, and a cap stops hopelessly different inputs with a clear message — never a false "identical".

Ignore case, ignore whitespace and CRLF/LF options change only the comparison; the original text is kept, and the counts and view use it.

A patch is offered only when it can recreate the exact text, so ignore options switch it off.

Long line comparisons (over 2,000 rows, or with wrapping off) are drawn through a scrolling window of fixed-height rows, so every row stays reachable without rendering them all at once.

Worked example

One line changed. a / b / c against a / x / c is one removal (b) and one addition (x). CRLF against LF differs unless you treat them as the same.

Common mistakes

  • Comparing with ignore-whitespace on and then expecting an exact patch.
  • Missing a CRLF-only difference — turn on Show tabs, trailing spaces and CR.
  • Comparing two huge unrelated files instead of the sections that matter.

Tips

  • Runs entirely in your browser. Your input is not uploaded or stored.

Frequently asked questions

Line or character diff?

Either — line for documents and code, word or character for small edits within a line.

Privacy?

Runs entirely in your browser. Your input is not uploaded or stored.

Related tools

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Cite this tool

Zerdly. (2026). Text Diff Checker. Retrieved from https://www.zerdly.com/tools/text-diff-checker/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.