Calculate two alphabetic check characters.
Initialising algorithm engine for ISO 7064 MOD 661-26 Calculator...
How ISO 7064 MOD 661-26 Calculator Works?
The ISO 7064 MOD 661-26 Calculator works by treating each letter as a value 0–25, running the pure-system recurrence with radix 26 and modulus 661, and splitting the 0–660 check value into 2 letters.
- 1Convert letters A–Z to values 0–25.
- 2Set p = 0 and update p = ((p + value) × 26) mod 661 for each letter.
- 3Multiply by 26 once more and reduce modulo 661 to reserve 2 check positions.
- 4Compute c = (662 − p) mod 661.
- 5Write the first check letter as c ÷ 26 and the second as c mod 26.
ISO 7064 MOD 661-26 Calculator Formula
The ISO 7064 MOD 661-26 formula is c = (662 − p) mod 661, split into ⌊c / 26⌋ and c mod 26.
pᵢ = ((pᵢ₋₁ + v(aᵢ)) × 26) mod 661 p = (pₙ × 26) mod 661 c = (662 − p) mod 661 Check = letter(⌊c / 26⌋) + letter(c mod 26)
v(a)is the letter's zero-based alphabet index (A = 0, Z = 25).661is the prime modulus M, close to 26² = 676.
(0 + 0) × 26 mod 661 = 0 (0 + 1) × 26 mod 661 = 26 (26 + 2) × 26 mod 661 = 67 (67 + 3) × 26 mod 661 = 498 (498 + 4) × 26 mod 661 = 493 (493 + 5) × 26 mod 661 = 389 (389 + 6) × 26 mod 661 = 355 (355 + 7) × 26 mod 661 = 158 (158 + 8) × 26 mod 661 = 350 (350 + 9) × 26 mod 661 = 80 Extra step for 2 check characters: 80 × 26 mod 661 = 97 Check value = (661 + 1 − 97) mod 661 = 565 Check letters = VT
ISO 7064 MOD 661-26 Calculator Conversion Table
The ISO 7064 MOD 661-26 conversion table maps letters to values and shows sample words with 2-letter checks.
| Letters | Values | Sample payload | Check letters | Complete string |
|---|---|---|---|---|
| A–E | 0–4 | ABCDEFGHIJ | VT | ABCDEFGHIJVT |
| F–J | 5–9 | BAISDLAFK | BM | BAISDLAFKBM |
| K–O | 10–14 | ISOSTANDARD | FC | ISOSTANDARDFC |
| P–T | 15–19 | PUBLISHER | KM | PUBLISHERKM |
| U–Z | 20–25 | ZZZZ | XJ | ZZZZXJ |
What is ISO 7064 MOD 661-26 Calculator?
An ISO 7064 MOD 661-26 Calculator is a tool that computes 2 alphabetic check characters for letter-only strings. The pair detects all single substitutions, all adjacent transpositions, and almost all double errors.
MOD 661-26 suits identifiers built from A–Z only, such as reference words and alphabetic codes read over the phone.
What are the uses of ISO 7064 MOD 661-26 Calculator?
There are 3 main uses of the ISO 7064 MOD 661-26 Calculator, including alphabetic reference codes, booking codes, and conformance testing.
- Alphabetic reference codes: Organisations protect letter-only codes that must stay free of digits.
- Booking codes: Travel and event systems check alphabetic locator codes.
- Conformance testing: Developers confirm ISO 7064 implementations for the A–Z alphabet.