How to use it
- Paste the ISIN from the contract note, the factsheet or the broker's page. Spaces and hyphens do not matter.
- Read Result: green means the layout is right and the check digit matches the 12th character.
- If it is red, read the reason and compare the character it names with the original document.
- Use Country prefix and National number to see the parts, and How the check digit was worked out to follow the sum.
How an ISIN is built
An ISIN identifies a security or another financial instrument with 12 characters. The first two are the country prefix, an alpha-2 country code. The next nine are the national numbering code, filled with leading zeros when it is short. The last character is the check digit.
ANNA's ISIN guidelines give this layout and name the check digit method, modulus 10 Double-Add-Double, as defined in ISO 6166 Annex C. ESMA's reporting instructions give the same method in steps, and the page follows those steps.
letters become two digits (A = 10 ... Z = 35), for the first 11 characters
double every second digit, counting from the rightmost
total = the digits of the doubled values + the other digits
check digit = (10 − total mod 10) mod 10- If the total is a multiple of 10, the check digit is 0, never 10.
- The page checks that the prefix is two letters. It does not test the prefix against the list of ISO country codes, so a prefix the browser does not name is shown as typed.
What a valid check digit does not show
- A valid ISIN is well formed. It does not show that the security exists, is listed or is traded.
- Some typing errors leave the check digit correct, so a green result does not prove that you have the right ISIN.
- Whether the security is the right one for a trade is for the broker, the issuer or a securities database to confirm.
Worked example: US0378331005
The first 11 characters are US037833100. The letters U and S become 30 and 28, so the digits are 3028037833100.
Counting from the rightmost digit, the doubled values are 6, 4, 0, 14, 6, 2, 0. Their digits add up to 23 and the other digits to 22, a total of 45. Then 45 mod 10 = 5 and 10 − 5 = 5, so the check digit is 5, as the last character of US0378331005 says.
| Step | Result |
|---|---|
| Letters converted | 3028037833100 |
| Doubled values | 6, 4, 0, 14, 6, 2, 0 |
| Digits of the doubled values | 23 |
| Other digits | 22 |
| Total | 45 |
| Check digit | 5 |
| Result | Valid |
Frequently asked questions
What is an ISIN?
It is the international code for a security, such as a share, a bond or a fund unit. The format is set by ISO 6166, and ANNA publishes the guidelines that apply it.
Why can the check digit come out as 0?
When the total is a multiple of 10, the subtraction gives 10, which cannot be one digit. ESMA's final step makes that 0, so the check digit is always a single digit.
Is a valid ISIN the same as a security that exists?
No. The check only confirms that the 12 characters are well formed. Whether the security exists, is listed or can be traded is for the broker, the issuer or a securities database to confirm.
Why does the page not name the country for every prefix?
The page looks the two letters up as a country name in the browser's own list. When the list has no name for them, the code is shown as typed. The check digit does not depend on the name.
Where can I read the official method?
ISO 6166 Annex C is the normative text and is not free to read. ESMA's reporting instructions and ANNA's guidelines give the method in steps, and the page's example is the one ESMA gives.
Does the page keep the ISIN I type?
No. The check runs in your browser, and nothing you type is stored or sent to us.
Sources
- ANNA: ISIN Uniform Guidelines (26 June 2026), section 7: the 12-character structure and the check digit method
- ESMA: FIRDS reference data reporting instructions v2.1 (17 September 2020), INS-114 to INS-116: the check digit steps and the example US037833100
Figures and text last checked on Thursday, November 12, 2026.