How the VIN check digit is calculated, worked through

Turn each of the 17 characters into a number (digits as themselves, letters by a fixed table), multiply each by the weight for its position, add the 17 products, and divide by 11. The remainder is the check digit, written as X when it is 10. A VIN whose ninth character does not match was mistyped, or is not a valid North American VIN. The rule is set out in the United States’ 49 CFR Part 565 and is used in Canada too; the international standard, ISO 3779, does not require a check digit, so a European or Japanese VIN may fail the check and be perfectly genuine. CHECKDIGIT checks a VIN, or a pasted column of them, in the browser tab.

The letter values

I, O and Q are never used in a VIN, so they cannot be confused with 1 and 0. The other letters take these values:

It looks irregular because it follows the old punched-card codes for letters, which numbered A to I, J to R and S to Z each from the start, with gaps where I, O and Q would be: there is no 9 among A to H because I is gone, and no 6 or 8 among J to R because O and Q are.

The weights

Position by position, from the first character to the seventeenth:

8 7 6 5 4 3 2 10 0 9 8 7 6 5 4 3 2

Position 9, the check digit itself, has a weight of 0, so it takes no part in its own calculation.

A worked example

Take 1M8GDM9AXKP042788, the example VIN used in many references.

The products add up to 351. 351 ÷ 11 is 31 remainder 10, and a remainder of 10 is written X, which is the ninth character. The VIN passes.

What the check catches, and what it does not

Dividing by 11, a prime, with weights that differ from position to position means that changing any single character to a different value changes the remainder, so any one mistyped character is caught, with one exception: two characters that share a value, such as A and J (both 1) or E and 5, can be swapped for each other without changing the sum. Most transpositions of two neighbouring characters are caught too, but not all.

Passing the check says the VIN was copied correctly, or was made up by someone who knew the arithmetic. It says nothing about whether the vehicle exists, whether it is the car in front of you, or whether it has been stolen or written off. A cloned car usually carries a real, valid VIN copied from another vehicle, and passes the check perfectly. For history, use the official services: in the US, NHTSA’s VIN decoder and recall lookup and an NMVTIS report; in the UK, the DVLA’s vehicle enquiry service and a private history check.

When the VIN is not North American

ISO 3779, the international VIN standard, fixes the structure (a world manufacturer identifier, a descriptor section, an indicator section) but leaves the check digit optional. Vehicles built for North America carry one; so do vehicles for China, which adopted the same rule. Many European and Japanese vehicles have a ninth character that is not a check digit. A failed check on a non-North American VIN is not evidence of anything.

Checking one in the browser

CHECKDIGIT checks a VIN against position 9, and says what the digit should have been when it fails, which usually points straight at the mistyped character. Paste a column of VINs from a spreadsheet to check a fleet list at once and save the results as CSV. It also checks card numbers, IBANs, ISBNs, barcodes, UK VAT numbers, US routing numbers, ISSNs and IMEIs by their own rules. Nothing is looked up and nothing typed leaves the tab. Checking is free; saving the results is part of OBSCURA Pro.