Luhn Check Digit Calculator
Validate a number by the Luhn algorithm and get the check digit to append.
Validate and complete Luhn numbers
The Luhn algorithm is the checksum behind credit card numbers and many identifiers. It catches most single-digit typos and transpositions, and this tool both validates a number against it and works out the missing check digit.
For the payload 7992739871 the check digit is 3, making 79927398713 a valid Luhn number. Enter a full number to see whether it already passes, or a payload to get the digit that completes it.
Exact and private
The doubling-and-summing is done in your browser with plain arithmetic, so it is instant and nothing you enter leaves your device.
Frequently Asked Questions
What is the Luhn algorithm?
A simple checksum used by credit cards, IMEIs, and many ID numbers. It doubles every second digit from the right, sums everything, and requires the total to be a multiple of 10.
What does the tool return?
Whether your number already passes Luhn, and the single check digit you would append to make it pass.
Is my number uploaded?
No. The check runs entirely in your browser and nothing you type is stored or sent anywhere.
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