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Posted on Feb 12 • Originally published at datacheck.dev

IBAN Validation: Complete Developer Guide (2026)

#javascript #webdev #finance #tutorial

If your app handles international payments, money transfers, or banking integrations, you'll need to validate IBANs. The International Bank Account Number is used across 80+ countries to identify bank accounts for cross-border transfers.

Getting IBAN validation wrong means failed transfers, returned payments, and unhappy users. This guide shows you exactly how the validation works, with code you can drop into any project.

What's Inside an IBAN?

Every IBAN follows the same structure:

DE  89  3704 0044  0532 0130 00
──  ──  ─────────  ─────────────
 │   │      │            │
 │   │      │            └── Account number
 │   │      └── Bank code (BLZ)
 │   └── Check digits (MOD-97)
 └── Country code (ISO 3166-1)

Country code (2 letters) — DE = Germany, GB = UK, FR = France
Check digits (2 digits) — computed using MOD-97 to catch typos
BBAN (Basic Bank Account Number) — the rest, format varies by country

The total length depends on the country. Germany is 22 characters, the UK is 22, France is 27, and Norway is just 15.

IBAN Lengths by Country

Country	Code	Length	Example
Germany	DE	22	DE89 3704 0044 0532 0130 00
United Kingdom	GB	22	GB29 NWBK 6016 1331 9268 19
France	FR	27	FR76 3000 6000 0112 3456 7890 189
Spain	ES	24	ES91 2100 0418 4502 0005 1332
Netherlands	NL	18	NL91 ABNA 0417 1643 00
Italy	IT	27	IT60 X054 2811 1010 0000 0123 456
Switzerland	CH	21	CH93 0076 2011 6238 5295 7
Norway	NO	15	NO93 8601 1117 947
Poland	PL	28	PL61 1090 1014 0000 0712 1981 2874
Belgium	BE	16	BE68 5390 0754 7034

The MOD-97 Checksum Algorithm

IBAN validation uses the MOD-97 algorithm defined in ISO 7064:

Move the first 4 characters to the end: DE89... becomes ...DE89
Convert letters to numbers: A=10, B=11, ..., Z=35. So D=13, E=14
Calculate the remainder when divided by 97
If the remainder is 1, the IBAN is valid

Walk-Through: DE89370400440532013000

Step 1: Move first 4 to end
  370400440532013000DE89
  → 370400440532013000131489  (D=13, E=14)

Step 2: MOD-97
  370400440532013000131489 % 97 = 1

Result: 1 → Valid ✓

The challenge: this is a 24+ digit number, which exceeds JavaScript's Number.MAX_SAFE_INTEGER (2^53). You need to process it in chunks.

JavaScript Implementation

function validateIBAN(input) {
  // 1. Clean and uppercase
  const iban = input.replace(/\s/g, '').toUpperCase();

  // 2. Basic format check
  if (!/^[A-Z]{2}\d{2}[A-Z0-9]{4,30}$/.test(iban)) {
    return { valid: false, reason: 'Invalid IBAN format' };
  }

  // 3. Check country-specific length
  const lengths = {
    DE: 22, GB: 22, FR: 27, ES: 24, IT: 27, NL: 18,
    BE: 16, AT: 20, CH: 21, SE: 24, NO: 15, DK: 18,
    FI: 18, PL: 28, PT: 25, IE: 22, LU: 20, CZ: 24,
  };

  const country = iban.substring(0, 2);
  if (lengths[country] && iban.length !== lengths[country]) {
    return { valid: false, reason: `${country} IBAN must be ${lengths[country]} characters` };
  }

  // 4. MOD-97 check
  // Move first 4 chars to end
  const rearranged = iban.substring(4) + iban.substring(0, 4);

  // Convert letters to numbers
  let numeric = '';
  for (const char of rearranged) {
    if (char >= 'A' && char <= 'Z') {
      numeric += (char.charCodeAt(0) - 55).toString();
    } else {
      numeric += char;
    }
  }

  // MOD-97 on large number (process in chunks)
  let remainder = 0;
  for (let i = 0; i < numeric.length; i++) {
    remainder = (remainder * 10 + parseInt(numeric[i])) % 97;
  }

  if (remainder !== 1) {
    return { valid: false, reason: 'Checksum failed (MOD-97)' };
  }

  return {
    valid: true,
    country,
    check_digits: iban.substring(2, 4),
    bban: iban.substring(4),
    formatted: iban.replace(/(.{4})/g, '$1 ').trim(),
  };
}

validateIBAN('DE89 3704 0044 0532 0130 00');
// { valid: true, country: "DE", check_digits: "89",
//   bban: "370400440532013000",
//   formatted: "DE89 3704 0044 0532 0130 00" }

The key trick is the chunk-based MOD-97 on lines 33-36. Instead of converting the entire string to a BigInt, we process digit by digit, taking the modulus at each step. This keeps the number small and works in any JavaScript environment.

Extracting the Bank Code

The BBAN contains the bank code, but its position varies by country:

Country	Bank Code Position	Example
Germany (DE)	Characters 5-12 (8 digits)	37040044
UK (GB)	Characters 5-8 (4 letters)	NWBK
France (FR)	Characters 5-9 (5 digits)	30006
Netherlands (NL)	Characters 5-8 (4 letters)	ABNA
Spain (ES)	Characters 5-8 (4 digits)	2100

function extractBankCode(iban, country) {
  const bankCodeLengths = {
    DE: 8, GB: 4, FR: 5, NL: 4, ES: 4,
    IT: 5, BE: 3, AT: 5, CH: 5, SE: 3,
  };

  const len = bankCodeLengths[country];
  if (!len) return null;

  return iban.substring(4, 4 + len);
}

Common Mistakes

1. Not handling spaces
IBANs are often displayed with spaces (DE89 3704 0044...) but stored without them. Always strip spaces before validation.

2. Case sensitivity
IBANs can be entered in lowercase. Always convert to uppercase before processing — the letter-to-number conversion depends on it.

3. Skipping the length check
An IBAN that passes MOD-97 but has the wrong length for its country is still invalid. A German IBAN must be exactly 22 characters — no more, no less.

4. Using parseInt on the full number
parseInt("370400440532013000131489") loses precision because it exceeds 2^53. Always use the chunk method or BigInt.

Using the DataCheck API

If you'd rather not maintain country-specific length tables and bank code extraction logic:

const res = await fetch(
  'https://datacheck.dev/api/validate?input=DE89370400440532013000&type=iban'
);
const data = await res.json();

// {
//   valid: true,
//   formatted: "DE89 3704 0044 0532 0130 00",
//   country: "DE",
//   details: {
//     country: "Germany",
//     check_digits: "89",
//     bban: "370400440532013000",
//     bank_code: "37040044"
//   }
// }

Or with the npm package:

npm install datacheck-api

import { validateIBAN } from "datacheck-api";
const result = await validateIBAN("DE89370400440532013000");

Test IBANs for Development

Country	Valid Test IBAN
Germany	DE89 3704 0044 0532 0130 00
UK	GB29 NWBK 6016 1331 9268 19
France	FR76 3000 6000 0112 3456 7890 189
Spain	ES91 2100 0418 4502 0005 1332
Netherlands	NL91 ABNA 0417 1643 00
Switzerland	CH93 0076 2011 6238 5295 7
Belgium	BE68 5390 0754 7034
Norway	NO93 8601 1117 947

Wrapping Up

IBAN validation comes down to three checks: format (letters + digits), country-specific length, and MOD-97 checksum. The tricky part is the big-number arithmetic, which the chunk-based approach solves cleanly.

For apps that just need to verify IBANs and extract bank codes, the 40-line implementation above works perfectly. For production use with 45+ countries and always-current formats, consider using an API like DataCheck.

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