VIN check digit validator

What is the VIN check digit?

A vehicle identification number is seventeen characters, and the ninth of them is not information about the car. It is a check digit: a number computed from the other sixteen, whose only job is to catch a mistake made while copying the VIN down. US federal regulation, 49 CFR Part 565, requires it and puts it in position nine.

The arithmetic is small. Every character has an assigned value, every position has a weight, the products are added, and the total is divided by eleven. The remainder is the check digit — and because a remainder can be ten, which will not fit in one digit, ten is written as the letter X. That is the same trick ISBN-10 uses, for the same reason.

The values are worth a look, because they are not what most people assume. They are not A=1 through Z=26: J is 1, S is 2, P is 7. And three letters have no value at all, because they cannot appear in a VIN in the first place.

How to use it

1. Paste the VIN. Seventeen characters, upper or lower case, spaces ignored. The examples cover a valid VIN, a mistyped one, and one containing a letter VINs never use.
2. Read the verdict and the two digits. The panel shows the check digit the arithmetic demands beside the one actually present, along with the weighted total and the remainder, so a mismatch is not just an assertion.
3. Follow the working. The table gives every position its value, weight and product. Position nine is highlighted and weighs zero, so the check digit never contributes to its own sum.

I, O and Q, and what a failed check really tells you

The regulation states the permitted characters as a set, and the letters I, O and Q are missing from it. That is deliberate: on a stamped plate under bad light, I is a 1 and O is a 0, and a number people copy by hand cannot afford characters that are read two ways. So if you have typed an I or an O, it is almost certainly a one or a zero, and the tool says so rather than just refusing.

It is worth knowing what the check digit is actually worth. Because the remainder is taken modulo eleven, people often say a wrong VIN has a one-in-eleven chance of slipping through. Measured, an arbitrary seventeen-character string built from the VIN alphabet passes about one time in thirty-three, not one in eleven — position nine is drawn from thirty-three possible characters and only one of them can be the right answer. The one-in-eleven figure is correct only once you already know position nine holds a legal check character. Either way, what the digit reliably catches is the thing it was designed for: any single mistyped character, and most transpositions.

A failing check digit does not always mean a wrong VIN, and this is the part most validators do not tell you. Part 565 is a United States regulation. A vehicle built for another market carries whatever number its manufacturer assigned, and the same regulation instructs importers to keep that original number rather than issue a conforming one — it even provides a plate reading SUBSTITUTE FOR U.S. VIN for cars brought in by registered importers. A European or Japanese-market car can therefore fail this check with absolutely nothing wrong with it.

What this deliberately does not do

It does not decode the manufacturer, the plant, the engine or the model year, and that is a decision rather than an omission. Part 565 specifies the check digit and the character set; it does not contain the model-year letter table that VIN decoders present as authoritative. That table is industry convention layered on top of the regulation, and the manufacturer directory behind a maker lookup is a list that goes out of date — a bundled copy would eventually be confidently wrong about a car built after it was compiled.

What the regulation does say about time is more interesting than a year code anyway. Its guarantee is that the VINs of any two vehicles manufactured within a thirty-year period shall not be identical. That is a uniqueness rule, not a permanence one: VINs are expected to come round again after thirty years, which is precisely why the model-year character cycles rather than counting.

So this page answers one question completely — does the ninth character agree with the other sixteen — and points you elsewhere for the rest. A check digit that matches means the VIN was probably copied correctly. It does not mean the vehicle exists, that the VIN was ever issued, or anything at all about the car's history.

Why is it free?

It is seventeen multiplications and a division, running in your browser. There is no database to query, no server to pay for and no account to create.

Nothing you type is uploaded, stored or logged. That matters here more than on most pages: a VIN identifies a specific vehicle, and it never leaves the tab.