Reference
How the check digit works
Position 9 is not part of the vehicle description. It is a checksum, computed from the other sixteen characters, that catches most transcription errors.
Where the rule comes from
The check digit is a federal requirement, not a manufacturer habit. Part 565 of Title 49 of the Code of Federal Regulations sets the format of the VIN for vehicles subject to US federal motor vehicle rules: passenger cars, multipurpose vehicles, trucks, buses, trailers, motorcycles and low speed vehicles. Section 565.13(c) says that “a check digit shall be part of each VIN” and that it appears in position 9. Section 565.15(c) gives the method, in four short paragraphs and four tables. Vehicles their manufacturer identifies as model year 2009 or earlier are also covered by a parallel subpart with the same requirement (565.23(c)) and the same method (565.25(c)).
Every character is converted to a number, multiplied by a weight that depends on its position, and the products are added up and divided by 11. The remainder is the check digit.
The method, step by step
1. Transliterate
Digits keep their face value. Letters take the values in the regulation's Table III. I, O and Q have no value because they are not allowed in a VIN at all (565.13(g)), which is exactly why the letters do not follow the alphabet in neat runs of nine:
| Letter | Value |
|---|---|
| A J | 1 |
| B K S | 2 |
| C L T | 3 |
| D M U | 4 |
| E N V | 5 |
| F W | 6 |
| G P X | 7 |
| H Y | 8 |
| R Z | 9 |
2. Apply positional weights
Each position carries a fixed weight from Table IV. Position 9 — the check digit itself — has no weight, which in practice means 0: it cannot contribute to the sum that produces it.
8 7 6 5 4 3 2 10 0 9 8 7 6 5 4 3 2
3. Add and divide by 11
Multiply each value by its weight and add the seventeen products. The regulation says to “divide the total by 11”.
4. Read off the remainder
The regulation's Table V expresses the remainder as a fraction of 11 (4/11, say) with a decimal equivalent rounded to the nearest thousandth (0.364), and maps each one to a check digit. Read as whole numbers it is simpler: a remainder of 0 to 9 is the check digit itself, and a remainder of 10 is written as the letter X.
A worked example
The regulation includes its own example in Table VI, using the string 1G4AH59H?5G118341 with position 9 left blank to be calculated. It is a sample in a rulebook, not a vehicle we are describing, which makes it a good one to learn on: you can check our arithmetic against the government's.
| Position | Character | Value | Weight | Product |
|---|---|---|---|---|
| 1 | 1 | 1 | 8 | 8 |
| 2 | G | 7 | 7 | 49 |
| 3 | 4 | 4 | 6 | 24 |
| 4 | A | 1 | 5 | 5 |
| 5 | H | 8 | 4 | 32 |
| 6 | 5 | 5 | 3 | 15 |
| 7 | 9 | 9 | 2 | 18 |
| 8 | H | 8 | 10 | 80 |
| 9 | ? | — | 0 | 0 |
| 10 | 5 | 5 | 9 | 45 |
| 11 | G | 7 | 8 | 56 |
| 12 | 1 | 1 | 7 | 7 |
| 13 | 1 | 1 | 6 | 6 |
| 14 | 8 | 8 | 5 | 40 |
| 15 | 3 | 3 | 4 | 12 |
| 16 | 4 | 4 | 3 | 12 |
| 17 | 1 | 1 | 2 | 2 |
The products total 411. Eleven goes into 411 thirty-seven times (37 × 11 = 407) with 4 left over. The regulation writes the same result as 37 4/11, or 37.364, and Table V maps a remainder of 4/11 to the check digit 4. The finished string is 1G4AH59H45G118341, and if you paste it into the check digit tool it passes.
Two details are worth noticing. Position 8 carries the heaviest weight, 10, so an error there moves the total the most. And position 9 contributes nothing, which is what makes the calculation possible at all.
Why X stands for 10
Dividing by 11 can leave any remainder from 0 to 10. That is eleven possible results and only one character to hold them, and there is no single digit for ten. The regulation's answer is the letter X: “The check digit, zero through nine (0-9) or the letter ‘X’ shall appear in VIN position nine (9).” Elsewhere in a VIN, X is an ordinary letter with the value 7. Only in position 9 does it mean ten, and it is the only letter that can appear there.
Why 11 and not 10? Because 11 is prime and none of the weights is a multiple of it. Change any single transliterated value and the total changes by the weight times the difference, and with a prime divisor that product can never be a whole multiple of 11. Dividing by 10 would not have that property: position 8's weight of 10 would make that position invisible to the check, and any even weight multiplied by a difference of 5 would vanish too.
What it catches
The check was designed for honest mistakes, and it catches the common ones reliably. We tested each of the statements below against the same code that runs on /validate.
- Any single wrong character whose value differs from the right one. A 3 typed as an 8, a 0 read as a D, a 2 read as a Z: all fail.
- Any wrong check digit. Only one character can be correct in position 9, so a mistake there always fails.
- Swapping two neighbors, the classic copying slip, as long as the two characters have different values. Swapping positions 15 and 16 of the example, for instance, gives 1G4AH59H45G118431, which fails.
- I, O and Q. These are rejected before any arithmetic, because the format does not allow them. Our tool reads them as 1 and 0 and tells you it has done so.
What it misses
Arithmetic on seventeen characters has blind spots, and they are predictable.
- Characters that share a value. Because A, J and 1 are all worth 1, swapping one for another changes nothing. JG4AH59H45G118341 passes just as the original does. The same goes for 5 and V, or 2 and S.
- Swaps between positions with the same weight. Positions 1 and 11 both weigh 8, 2 and 12 both weigh 7, and so on down to 7 and 17. Exchange the characters in such a pair and the total is unchanged. Swapping positions 6 and 16 of the example gives 1G4AH49H45G118351, which passes.
- Swapping positions 8 and 9 when both are digits. Weight 10 behaves like −1 when you are working in elevens, so this particular swap always balances. 1G4AH59485G118341 and 1G4AH59845G118341 both pass.
- Several errors at once. Two mistakes can cancel. A badly garbled string has roughly a one-in-eleven chance of passing by luck.
- Deliberate recomputation. Anyone who changes a VIN on purpose can recompute position 9 in seconds. The check offers no protection against that, and was never meant to.
Why a genuine VIN can fail
The usual reason is still a typo, so read the number again from the vehicle itself, ideally from a photograph of the windshield plate or the door jamb label.
The other reason is legitimate. The requirement lives in a US regulation, so it binds vehicles built to US rules. A vehicle originally built for another market may carry a perfectly genuine VIN whose position 9 is not a check digit at all. Part 565 tells importers to keep the original manufacturer's VIN (565.14(a)), and makes separate provision for some vehicles brought in by registered importers that lack a conforming one (565.14(b)). That is why our tool treats a failed check digit as a warning rather than a verdict. See why a VIN won't decode for the other causes.
What a passing check digit does not prove
This is arithmetic performed on seventeen characters. It establishes that those characters are internally consistent. It establishes nothing whatsoever about the vehicle in front of you.
The formula is public — it is printed above, and it is in the Code of Federal Regulations. Anyone putting a false VIN on a vehicle can compute a check digit that passes. That is why a cloned VIN decodes perfectly: the number was copied from a real car, so of course it validates.
What the check digit catches is honest error: the mistyped character, the transposed pair, the digit misread off a sun-faded windshield plate. That is genuinely most of what goes wrong, and it is worth catching. It is just not a fraud check. See VIN cloning and fraud for what is.
Quick answers
What is the 9th digit of a VIN?
The check digit. Federal rules in 49 CFR Part 565 require a check digit in position 9 of every VIN, and it is a digit from 0 to 9 or the letter X.
How is the VIN check digit calculated?
Each character is converted to a number, multiplied by a weight that depends on its position, and the products are added up and divided by 11. The remainder is the check digit, and a remainder of 10 is written as the letter X.
Does a valid check digit mean a VIN is genuine?
No. It only shows that the seventeen characters are internally consistent. The formula is public, so anyone putting a false VIN on a vehicle can compute a check digit that passes, and a cloned VIN validates because it was copied from a real car.
Why would a real VIN fail the check digit?
Usually because of a typo, so read the number again from the vehicle itself. A vehicle originally built for another market may also carry a genuine VIN whose position 9 is not a check digit at all, because the requirement is a US regulation.
Sources
- 49 CFR 565.15, Content requirements (eCFR), including Tables III to VI
- 49 CFR 565.13, General requirements (eCFR)
- 49 CFR Part 565, Vehicle Identification Number requirements (eCFR), for 565.2, 565.14, 565.23 and 565.25