No. It is read with the browser's File API and hashed locally. Nothing is transmitted, which is why this works with the network disconnected.

The checksum does not match.

The file differs from the one that produced that value — an incomplete download, a different version, or tampering. Download it again before assuming the worst.

Which algorithm should I use?

Whichever the publisher used, since you can only compare like with like. Where you have a choice, SHA-256.

File Checksum

Hash a local file in chunks and compare it to an expected value.

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About this tool

Computes a checksum for a file entirely on your device, reading it in chunks so large files do not have to fit in memory at once. Comparing the result against a published value tells you the download is byte-identical to what the publisher released.

Limitations

The file is read locally and never uploaded.
A matching checksum proves the bytes match the value you were given. It says nothing about whether that value came from a trustworthy source.

Questions

Is my file uploaded?: No. It is read with the browser's File API and hashed locally. Nothing is transmitted, which is why this works with the network disconnected.
The checksum does not match.: The file differs from the one that produced that value — an incomplete download, a different version, or tampering. Download it again before assuming the worst.
Which algorithm should I use?: Whichever the publisher used, since you can only compare like with like. Where you have a choice, SHA-256.

Privacy

This page does its work in your browser. What you paste is processed by JavaScript running on your own device and is never sent to a DevToolery server — there is no server that could receive it. The page itself is a static file delivered by a CDN. See exactly what is stored.