SHA-1 hash
Compute a SHA-1 digest of any file, on your device.
Runs on your device. The file is never uploaded.
SHA-1 hash turns a file into a 160 bit digest, 40 lowercase hex characters wide, the width the RIPEMD-160 page prints from an unrelated algorithm. The row carries no options, so twenty files dropped give twenty digests. Public collision attacks let two crafted files share one SHA-1, which leaves this a compatibility format.
Questions
Is SHA-1 still safe?
No, not against an attacker. Public collision attacks mean two different inputs can be crafted to share one SHA-1 digest, so a matching SHA-1 does not prove a file is the one you expected. Treat it as a legacy compatibility format. For integrity that has to hold up, use SHA-256 hash or SHA-512 hash.
Why is SHA-1 still everywhere then?
Because formats outlive their algorithms. Git names objects with SHA-1, plenty of vendors still publish SHA-1 checksums beside their downloads, and older signing and certificate formats used it. That is what this tool is for: reproducing a value some other system already committed to, not choosing SHA-1 for something new.
How long is the digest?
160 bits, printed as 40 lowercase hex characters. The output is two lines: the algorithm name, two spaces and your file name, then the digest on its own. There is nothing to configure on this page, so the same file always gives the same value, and case does not matter when you compare against a checksum somebody published in uppercase.
Is this the same SHA-1 as the File checksum tool?
Yes, the same algorithm and the same digest. They differ in plumbing and in how many files they take. This tool uses hash-wasm and accepts as many files as you drop, returning one digest each. File checksum uses your browser WebCrypto and takes a single file per run.
Does this check a signature or a certificate?
No. It computes a digest of the bytes you give it and stops there. To read what is inside a certificate, use the X.509 certificate parser, which reports subject, issuer, validity, public key and extensions, and which is equally clear that it does not verify a chain or a signature.