How does ABA routing number validation work?

ABA routing numbers use a weighted checksum algorithm. Each of the 9 digits is multiplied by a weight (3, 7, 1, repeating), and the sum must be divisible by 10. This catches common transcription errors like swapped or mistyped digits.

What is the Luhn algorithm for credit cards?

The Luhn algorithm (also called mod-10) is a checksum formula used to validate credit card numbers. Starting from the rightmost digit, every second digit is doubled. If doubling produces a number greater than 9, subtract 9. The total sum of all digits must be divisible by 10.

Does this tool store my numbers?

No. All validation is performed entirely in your browser using JavaScript. No numbers are sent to any server or stored anywhere. You can verify this by checking the network tab in your browser's developer tools.

Number Validator — Verify Routing Numbers, Credit Cards & IBANs

Q: How are IBANs validated?

IBAN validation uses the ISO 13616 mod-97 algorithm. The first four characters (country code + check digits) are moved to the end, letters are converted to numbers (A=10, B=11, etc.), and the resulting number must have a remainder of 1 when divided by 97.

Try an Example

Routing Numbers

Chase NY 021000021
BofA CA 121000358
Wells Fargo 121000248
Invalid 021000022

Credit Cards

Visa 4111 1111 1111 1111
Mastercard 5500 0000 0000 0004
Amex 3782 822463 10005
Discover 6011 1111 1111 1117

IBANs

UK GB29 NWBK ...
Germany DE89 3704 ...
France FR76 3000 ...
Invalid GB00 NWBK ...

How Number Validation Works

ABA Routing Numbers (9 digits)

Each digit is multiplied by a repeating weight of 3, 7, 1. The weighted sum must be divisible by 10. This catches transposition and single-digit errors that are common when manually entering routing numbers.

Credit Cards (13–19 digits) — Luhn Algorithm

Starting from the right, every second digit is doubled. If the result exceeds 9, subtract 9. The total of all digits must be divisible by 10. The first digits also identify the card network: 4 = Visa, 51–55 = Mastercard, 34/37 = Amex, 6011/65 = Discover.

IBANs (15–34 characters) — ISO 13616

The first four characters (country code + check digits) are moved to the end. Letters are converted to numbers (A=10, B=11, etc.). The resulting number must have a remainder of 1 when divided by 97.

Number Validator FAQs

Yes, it’s completely secure. All validation runs entirely in your browser using JavaScript. No numbers are ever sent to a server, stored, or logged. You can verify this by opening your browser’s developer tools and checking the Network tab — no requests are made when you type.

Not necessarily. Checksum validation confirms the number follows the correct mathematical format, which catches typos and transcription errors. However, a valid checksum doesn’t guarantee the number is assigned to an active account. For routing numbers, you can cross-reference with our Routing Number Lookup tool.

The validator detects Visa (starts with 4), Mastercard (starts with 51–55 or 22–27), American Express (starts with 34 or 37), and Discover (starts with 6011 or 65). All card numbers are validated using the Luhn/mod-10 algorithm regardless of network.

The IBAN validator supports all countries that use the ISO 13616 standard, including the UK, Germany, France, Spain, Italy, Netherlands, Switzerland, and 70+ other countries. The mod-97 checksum algorithm is universal across all IBAN-issuing countries.

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Number Validator