The VIN check digit: how position 9 is calculated

What the check digit is for

Federal regulation defines the check digit as a single number or the letter X used to verify the accuracy of the transcription of the VIN, and it requires one in position 9 of every VIN on a vehicle built for the US market. It does not describe the vehicle. It is arithmetic done over the other sixteen characters, so that a typing or reading mistake usually makes the arithmetic fail.

The calculation in four steps

Section 565.15(c) specifies the method. First give every character a value: digits keep their own value and letters take the value from Table III, which gives A to H the values 1 to 8, then J, K, L, M, N, P and R the values 1, 2, 3, 4, 5, 7 and 9, and S to Z the values 2 to 9 in turn.

Second, multiply each value by the weight of its position. The weights for positions 1 to 17 are 8, 7, 6, 5, 4, 3, 2, 10, 0, 9, 8, 7, 6, 5, 4, 3, 2, so the check digit itself, in position 9, counts for nothing. Third, add the products and divide the sum by 11. Fourth, the remainder is the check digit: 0 to 9 stand for themselves and a remainder of 10 is written as X.

The regulation’s own worked example

The text of § 565.15 includes a sample, Table VI. Its VIN with the check digit filled in is 1G4AH59H45G118341. The values of positions 1 to 17, leaving position 9 out, are 1, 7, 4, 1, 8, 5, 9, 8, (skipped), 5, 7, 1, 1, 8, 3, 4, 1, and the products with the weights are:

• 8 + 49 + 24 + 5 + 32 + 15 + 18 + 80 for positions 1 to 8
• 0 for position 9
• 45 + 56 + 7 + 6 + 40 + 12 + 12 + 2 for positions 10 to 17
• The total is 411. Dividing by 11 gives 37 with a remainder of 4, so the check digit is 4, which is what the sample VIN carries.

What a mismatch tells you

When the digit in position 9 differs from the result, one or more characters were probably copied wrongly. PlateVIN shows both numbers, the digit on your VIN and the one the calculation gives, and NHTSA vPIC reports the same finding as its own message: a VIN decoded with a wrong ninth character comes back with the note that the check digit in the ninth position does not calculate properly.

Try the usual confusions first: 0 and O are not interchangeable (O is never valid), 1 and I likewise, and 5 and S, 8 and B, 2 and Z are easy to swap when a number is read off a worn plate.

Why a passing check digit proves little

There are only eleven possible outcomes, so a string of random characters has about a one in eleven chance of passing. The check also cannot separate letters that share a value: A and J are both 1, B, K and S are 2, C, L and T are 3, D, M and U are 4, E, N and V are 5, F and W are 6, G, P and X are 7, H and Y are 8, and R and Z are 9. Swapping two such letters leaves the sum unchanged.

So a valid check digit means the VIN is consistent, not that the vehicle exists, is registered or has a clean history. Only a decoder that finds the manufacturer in NHTSA’s records, and ideally an inspection of the car, says more.

Finally, the rule applies to vehicles built for North America. Many vehicles made for other markets carry a 17-character VIN without a valid check digit, so a mismatch on an imported car is not by itself a warning sign.

Quellen

• 49 CFR § 565.15 Content requirements (eCFR)
• 49 CFR § 565.13 General requirements (eCFR)
• NHTSA vPIC API documentation