How to check whether two files are identical

Two copies of a document, one from email and one from a shared drive. Are they the same file, or has one been edited?

This is a question people usually answer badly — by comparing file sizes, or modification dates, or by opening both and scrolling. Size is weak evidence, because two different files very often share one. Modification dates say when a copy was written, not what is in it.

Comparing digests answers it properly. If two files have the same SHA-256, they are the same file.

Step-by-step

1. Add the first file. Drag it into the left slot.
2. Add the second. Right slot.
3. Read the result. Sizes appear immediately; the digests follow.

Why size is checked first

Because it is free and it settles most cases. Files of different sizes cannot be identical, so there is no reason to read both of them through a hash function to find that out. When the sizes differ the tool says so at once and tells you by how much.

When the sizes match, that proves nothing on its own, and the digests decide it.

What "identical" means here

Byte-for-byte identical. This is stricter than "the same document" in the way people usually mean it. Two PDFs that look the same on screen can differ in their embedded timestamp, their producer string, or the order of internal objects. Two photographs of the same scene, saved twice by the same editor, will often differ in metadata alone.

So a "different" verdict does not always mean the content changed. It means something in the file changed. If you want to know whether the visible text differs, a text comparison is the better tool.

Frequently asked questions

Are the files uploaded?

No. Both are read in your browser and hashed there. Nothing is sent anywhere.

The files look the same but the tool says they differ. Why?

Something in the bytes differs even though the visible content does not — commonly an embedded timestamp, metadata, or the internal ordering a program used when saving. Byte comparison is stricter than visual comparison.

Can I compare more than two files?

The tool compares a pair at a time. To check several against one reference, hash them individually with the hash calculator and compare the digests.