Luhn Check Digit Calculator
Calculate the Luhn (mod-10) check digit for any number, or append it to make a valid sequence — the algorithm behind card numbers, IMEIs and more.
About Luhn Check Digit Calculator
Luhn Check Digit Calculator computes the mod-10 check digit for any number and appends it to form a valid sequence — the algorithm that guards credit-card numbers, IMEIs, NPIs and Canadian SINs against typos.
How to use Luhn Check Digit Calculator
- 1Paste or type your input into the box above — a sample is pre-filled so you can try it instantly.
- 2Set any options on offer (mode, key, format) to match what you need.
- 3The result appears the moment you type — no button to press, no waiting.
- 4Click Copy to grab the output — Luhn Check Digit Calculator keeps everything on your device.
Why use Luhn Check Digit Calculator?
- ✓Generating a valid test card number for development and QA
- ✓Adding a check digit to a custom ID scheme so typos are caught
- ✓Understanding how the last digit of a card number is derived
- ✓100% free with no sign-up, no watermark and no usage limits
- ✓Runs entirely in your browser — your text and keys are never uploaded, stored or logged
- ✓Works offline once loaded, with instant results and one-click copy
Frequently asked questions
Does passing the Luhn check mean a card is real?+
No. Luhn only confirms the number is internally consistent — it catches mistyped digits. It says nothing about whether the card exists, is active or has funds. It is an error-detection code, not a validity or fraud check.
What errors does the Luhn algorithm catch?+
It detects every single-digit error and almost all adjacent transpositions (swapping two neighbouring digits). The one transposition it misses is 09 ↔ 90. For full transposition coverage, use the Verhoeff or Damm algorithms.
Is it free to use?+
Yes — completely free with no sign-up, no account and no usage limits. There is no paywall, so you can encode, decode and convert as much as you like.
Is my data private and secure?+
Yes. This tool runs entirely in your browser using client-side JavaScript — your input is never uploaded to a server, stored or logged. Once the page has loaded you can use it offline, which makes it safe for tokens, keys and confidential data.
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