Financial Instrument Identifiers Explained

1. Quick reference — all identifiers at a glance

2. ISIN — the global standard

The International Securities Identification Number (ISO 6166) is the closest thing to a universal security identifier. It is accepted by regulators, exchanges, and clearing systems worldwide.

• Format: 2-letter ISO country code + 9-character NSIN + 1 check digit
• Check digit: Luhn algorithm applied to the digit string obtained by expanding each letter A–Z to its two-digit value (A=10, B=11, … Z=35)
• Issued by: National Numbering Agencies (NNAs) — CUSIP Global Services for US, London Stock Exchange for GB, Deutsche Börse for DE, etc.
• Used in: MiFID II reporting, EMIR, T2S settlement, FCA filings, SEC reporting

The NSIN (National Securities Identifying Number) is country-specific. For US securities it is the CUSIP. For UK securities it is 00 + SEDOL. For German securities it is the WKN padded to 9 characters.

API: ISIN validation → · Node.js guide →

3. CUSIP — North America

The Committee on Uniform Security Identification Procedures number is the dominant identifier for securities traded in the United States and Canada. Introduced in 1968, it underpins settlement at DTC, clearing at NSCC, and regulatory reporting at the SEC.

• Format: 6-char issuer number + 2-char issue number + 1 check digit
• Characters: Digits 0–9, letters A–Z, and special chars * @ #
• Check digit: Modified Luhn — same doubling/digit-sum approach as standard Luhn, but with A=10…Z=35, *=36, @=37, #=38
• Issued by: CUSIP Global Services (FactSet, licensed from ABA)
• Used in: DTC settlement, SEC Form 13F, TRACE fixed-income reporting, NSCC clearing

API: CUSIP validation → · Node.js guide → · Python guide →

4. SEDOL — United Kingdom & Ireland

The Stock Exchange Daily Official List code is assigned by LSEG (London Stock Exchange Group). It is the primary identifier for UK and Irish securities, used in CREST settlement and FCA transaction reporting. Despite its UK origins, the SEDOL Masterfile covers over 100 million instruments globally.

• Format: 6 alphanumeric characters + 1 check digit
• Characters: Digits 0–9 and letters. Modern SEDOLs (post-2004) exclude vowels A, E, I, O, U to reduce ambiguity
• Check digit: Weighted sum with weights [1,3,1,7,3,9], modulo 10. Character values: 0–9=0–9, A–Z=10–35
• Issued by: LSEG (London Stock Exchange Group)
• Used in: CREST settlement, FCA MiFIR reporting, UK investment manager data feeds

Unlike ISIN and CUSIP, a SEDOL identifies a specific listing on a specific market — the same company may have multiple SEDOLs for different share classes or exchange venues.

API: SEDOL validation → · Node.js guide → · Python guide →

5. LEI — identifying the issuer, not the instrument

The Legal Entity Identifier (ISO 17442) is fundamentally different from the other identifiers in this guide — it identifies the legal entity that issues or trades securities, not the security itself. Created after the 2008 financial crisis to improve systemic risk monitoring, it is now mandated for OTC derivatives reporting, MiFID II, and EMIR.

• Format: 4-char LOU prefix + 14-char entity code + 2 check digits (20 chars total)
• Check digit: MOD-97-10 (ISO 7064) — same algorithm as IBAN
• Issued by: GLEIF-accredited Local Operating Units (LOUs)
• Used in: EMIR OTC derivatives reporting, MiFID II, SFTR, bank counterparty identification
• Public database: GLEIF publishes a free, downloadable, full LEI dataset updated daily

API: LEI validation →

6. FIGI — Bloomberg's open identifier

The Financial Instrument Global Identifier was developed by Bloomberg and made publicly available in 2013 as an open standard under the Object Management Group. Unlike ISIN and SEDOL, FIGI is exchange-specific — each listing on each exchange gets its own FIGI.

• Format: 12-character alphanumeric string — no embedded structure or check digit
• Types: FIGI (per listing), Composite FIGI (across all listings of the same instrument), Share Class FIGI
• Issued by: Bloomberg (via OpenFIGI API — free with registration)
• Used in: Bloomberg Terminal, buy-side data infrastructure, cross-reference databases

The OpenFIGI API accepts ISIN, CUSIP, SEDOL, and other identifiers as input and returns the corresponding FIGI. This is how isvalid enriches ISIN, CUSIP, and SEDOL responses with live instrument data.

7. Other identifiers: WKN, VALOR, RIC, Bloomberg ticker

8. How identifiers relate to each other

The identifiers are not independent — several are mathematically derived from each other:

// Deriving a US ISIN from a CUSIP (JavaScript)
function cusipToUsIsin(cusip) {
  const prefix = 'US' + cusip; // "US" + 9 chars = 11 chars
  return prefix + luhnCheckDigit(expandToDigits(prefix));
}

// Deriving a GB ISIN from a SEDOL
function sedolToGbIsin(sedol) {
  const prefix = 'GB00' + sedol; // "GB00" + 7 chars = 11 chars
  return prefix + luhnCheckDigit(expandToDigits(prefix));
}

// Both use the same Luhn expansion (A→10, B→11, …, Z→35)
function expandToDigits(s) {
  return [...s].map(ch =>
    ch >= 'A' && ch <= 'Z' ? String(ch.charCodeAt(0) - 55) : ch
  ).join('');
}

9. Which identifier should you use?

10. Validating identifiers via API

The IsValid API provides dedicated endpoints for each identifier. A single call handles format validation, check digit verification, and live instrument enrichment from ESMA FIRDS and OpenFIGI.

// Validate an ISIN and get full instrument details
const res = await fetch('https://api.isvalid.dev/v0/isin?value=GB0007980591', {
  headers: { Authorization: 'Bearer YOUR_API_KEY' },
});
const data = await res.json();
// {
//   valid: true,
//   countryCode: 'GB',
//   nsin: '007980591',
//   found: true,
//   name: 'BP PLC',
//   issuerLei: 'ZZFHHZLM2EP5ZJSUDL95', // → use /v0/lei to get entity details
//   cfiCode: 'ESVUFR',
//   currency: 'GBP',
//   figi: 'BBG000BT4FC2',
//   ...
// }

See also: Check digit algorithms explained · ISIN API docs · CUSIP API docs · SEDOL API docs · LEI API docs