Check if a number's check digit is right

The job

You are testing a checkout form or building fixture data. Paste a number to check whether its check digit is consistent, or enter a prefix to compute the digit that completes it.

Spaces and dashes are ignored, so formatted and raw strings both work.

What Luhn proves

The algorithm validates internal consistency: it catches single-digit typos and most transpositions. That is all — it says nothing about whether the number belongs to a real account.

Use only published test numbers (like 4242 4242 4242 4242) or synthetic ones you generate. Real card numbers are sensitive data — do not paste them into any web tool, this one included.

Generating valid test numbers

Enter a prefix like 4242 4242 4242 424 and the tool returns the completing digit and the full valid string — handy for fixtures and edge-case testing.