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São Tomé and Príncipe IBAN Check Digits: Calculation & Verification
The check digits of a São Tomé and Príncipe IBAN are the two digits at positions 3 and 4 (after the country code). In the example ST23000200000289355710148, they are 23. They detect keying errors using the MOD 97 algorithm.
What Are IBAN Check Digits?
Check digits (positions 3–4 of the IBAN) validate the integrity of the account number. They are computed per ISO 13616 using the modulo 97-10 algorithm. Any single-character error or adjacent-character transposition will be detected.
ST
CC
23
Check
000200000289355710148
BBAN
MOD 97 Algorithm: Steps
- 1Move the first 4 characters (country code + check digits) to the end of the IBAN.
- 2Replace each letter with its numeric value (A=10, B=11, … Z=35).
- 3Compute the remainder of dividing this large number by 97.
- 4The result must equal 1 for a valid IBAN.
Step-by-Step Example
Example IBAN:
ST23000200000289355710148Step 1 — Rearrange
Moving "ST23" to the end:
000200000289355710148ST23Step 2 — Letter substitution
Replacing letters (country code and alphabetic BBAN chars):
000200000289355710148282923S=28T=29
Step 3 — Modulo 97
Computing the modulo:
00020000028935571014… mod 97 = 1Result
✓ Valid IBAN (remainder = 1)Errors Detected by MOD 97
Detected
- ✓Any single digit/letter error
- ✓Transposition of two adjacent characters
Not detected (generally)
- ~Some twin errors
Why It Matters
The MOD 97 algorithm detects ~99.96% of simple typing errors. For a 25-character São Tomé and Príncipe IBAN, the probability of incorrectly accepting an invalid IBAN is 1/97 ≈ 1%.