Tool Overview

This tool checks if a number passes the Luhn algorithm and shows the check digit you need to make a number valid.

Many IDs (e.g. card numbers, IMEI) use a Luhn check digit to catch typing mistakes. Checking or computing it by hand is tedious and easy to get wrong. This tool does the math for you so you can validate numbers or find the right check digit.

It is for developers, testers, and anyone who works with numbers that use Luhn (e.g. credit cards, IMEI). You can use it with basic math; no coding needed.

Background and Concepts

The Luhn algorithm is a simple checksum. It uses the last digit of a number as a check digit. The rest of the digits are processed in a fixed way and added together. If the total ends in zero, the number is valid. If not, the number is invalid or the check digit is wrong.

Luhn is used in credit card numbers, IMEI numbers, and some other IDs. It catches most single-digit errors and many swapped-digit errors. It does not prove the number is real or correct in any other way; it only checks the pattern.

Doing Luhn by hand is slow: you work from right to left, double every second digit, add digits of two-digit results, sum everything, and check the last digit. This tool does that for you and shows each step so you can see how the result is found.

Key Features

• Validate a number. You enter a full number (e.g. with check digit). The tool strips spaces and hyphens, keeps only digits, and runs the Luhn algorithm. It shows Valid Checksum if the sum ends in zero and Invalid Checksum if not. This helps you confirm a number passes Luhn.
• Find the check digit. The tool always shows the check digit that would make the number valid. For a full number, that is the digit the last position should have (so you can see if the current last digit is wrong). For a number without the last digit, that is the digit you should append. So you can either validate or generate the check digit.
• Digits only. You can type spaces or hyphens; the tool removes them and uses only digits. Maximum length is 50 digits. Non-digit characters (except spaces and hyphens) are not allowed. This keeps input simple and avoids invalid input.
• Sample numbers. You can tap sample buttons (e.g. Credit Card, IMEI, Test) to fill the input with example numbers. This helps you try the tool quickly without typing a long number.
• Step-by-step breakdown. You can expand a section that shows each digit, whether it was doubled, and the value added to the sum. It also shows total sum, remainder when divided by 10, and valid or invalid. This helps you learn how Luhn works or debug a number.
• Copy and clear. You can copy the current input to the clipboard or clear the input and start over.
• Optional AI insight. For longer numbers you can request an optional insight (e.g. number type, security note, whether it looks like a test number). This is optional and not required for validation or check digit.

Common Use Cases

Validating card or IMEI numbers. You have a number and want to know if it passes Luhn before sending it to an API or storing it. Enter the number and check Valid or Invalid.

Generating a check digit. You have a number without the last digit (e.g. from a form or system) and need the correct Luhn check digit. Enter the number without the last digit; the tool shows the digit to append.

Testing and development. You need valid or invalid Luhn numbers for tests or mocks. Use the tool to check sample numbers or to get the check digit for a prefix.

Learning the algorithm. You want to see how Luhn works. Enter a number and open the step-by-step breakdown to see each digit and the final sum.

Fixing a typo. If a number fails Luhn, the tool shows what the last digit should be so you can correct it (when the error is only in the last digit).

How to Use This Tool (Step-by-Step)

1. Open the Luhn check digit calculator page.
2. Enter your number in the input box. Use only digits; you can use spaces or hyphens and they will be removed. The number can be full (with check digit) or without the last digit. Maximum length is 50 digits.
3. Look at the result. If you entered a full number, the tool shows Valid Checksum or Invalid Checksum. The check digit shown is the digit that would make the number valid (so for a full number it is what the last digit should be; for a number without the last digit it is the digit to add).
4. Optionally tap a sample (e.g. Credit Card, IMEI, Test) to fill the input with an example and see the result.
5. Optionally expand the step-by-step section to see each digit, doubled or not, and the total sum and remainder. Use this to understand the algorithm or to verify the result.
6. Use copy to copy the input to the clipboard or clear to empty the input and enter a new number.
7. Optionally request an AI insight for the number. This is not required for validation or check digit.

Calculations and Logic

The tool uses the Luhn algorithm only. No other formulas are used.

Input is sanitized: only digits are kept; spaces and hyphens are removed. If the result is empty or too long, the tool may show invalid or truncate. Digits are processed from right to left. The rightmost digit is position 1 and is not doubled. The next digit left is position 2 and is doubled. Then position 3 not doubled, position 4 doubled, and so on. When a digit is doubled, if the result is greater than 9, 9 is subtracted (so 10 becomes 1, 12 becomes 3, etc.). Each digit contributes one value to a running sum. When all digits are processed, the sum is taken. If the sum modulo 10 is zero, the number is valid; otherwise it is invalid. The check digit shown is computed from the number without its last digit: the same Luhn steps are applied to that shorter number, then the remainder of the sum modulo 10 is found; if the remainder is zero the check digit is 0, otherwise the check digit is 10 minus the remainder. That digit, appended to the number without the last digit, makes the full number pass Luhn.

Tips, Limitations and Best Practices

Digits only. The tool ignores everything except digits, spaces, and hyphens. Letters or symbols are stripped. Enter only numeric input for correct results.

Length. Input is limited to 50 characters (before stripping). Very long numbers may be truncated. Most card and IMEI numbers fit within this limit.

Full number vs missing check digit. If you enter a full number, the tool validates it and shows the check digit that the last position should have (so you can see if the current last digit is wrong). If you enter a number without the last digit, the same check digit is the digit you should append. The tool does not ask which case you have; it always computes the check digit for the number formed by dropping the last digit.

Luhn is not security. A valid Luhn number only means the checksum is correct. It does not mean the number is real, issued, or safe to use. Do not rely on Luhn alone for security or identity checks.

Test numbers. Common test numbers (e.g. 79927398713) are provided as samples. Use them to verify the tool; do not use real card or IMEI numbers in shared or untrusted places.

Privacy. Do not enter real card or IMEI numbers if you use the optional AI insight; that data may be sent for processing.