The short answer
If you run a commercial motor vehicle in interstate commerce, both sides of the power unit have to show your legal name and your USDOT number, and both have to be readable from fifty feet in daylight while the truck is standing still.
That is the whole federal requirement. Everything else on this page is detail, exception, or a correction to something you have probably been told.
Nobody can tell you how tall the letters must be
This is the part the industry gets wrong, so it is worth being blunt.
Search for dot number size requirements and you will be told, repeatedly and confidently, that federal law sets a two-inch minimum. It does not. No letter height appears anywhere in the section. The operative paragraph is quoted at the top of this page so you can check that rather than take our word for it, and the link goes to the eCFR text itself.
What the rule sets instead is a performance standard: readable at fifty feet, in daylight, stationary. Whether your lettering clears it depends on the typeface, the stroke weight, the contrast against your door, and how clean the panel is. A tall thin font can fail at a height a shorter bold one passes comfortably.
So why does everyone say two inches?
Because a performance standard is uncomfortable. You cannot check it with a tape measure, and people want a number. Two inches is the figure the trade invented to fill the hole the regulation left, and it has been repeated so often that it now comes back from search engines as though it were law. It is a sound working figure. It is not a legal minimum, and a seller who tells you otherwise has not read the section they are citing.
We cut two inches by default because on a truck door, at normal stroke weights, it clears fifty feet reliably. That is a recommendation from people who cut vinyl, not a rule from Washington. If your door geometry or your colour pairing makes it marginal, go bigger. Nothing in the rule sets a ceiling.
Your name has to match your FMCSA record exactly
This is where the real trouble is, and it costs nothing to get right.
The name on the door is not your company name in the way you think of it. It is the legal name or single trade name as it appears on your FMCSA registration — not the name on your invoices, not the trading name you use with shippers, not whatever was on the truck when you bought it.
The mismatches that catch people are small ones:
- An LLC or Inc. that is on the filing but got dropped on the door
- A DBA on the door where the filing carries the legal entity
- A name changed with the state and never updated with FMCSA
- A unit still wearing the previous carrier's name after a sale
And one that is purely typographic: the number has to be preceded by the letters USDOT, not DOT. Operators do get pulled up on it. It is the cheapest possible violation to avoid and we add the prefix ourselves, correctly spaced, so you cannot get it wrong on our watch.
If you are not certain what your record says, look it up before you order. We would rather you spent five minutes checking than paid us to cut something that has to come off again.
If someone else's name is on the truck
If any name other than the operating carrier's appears on the vehicle — a leased unit, an owner-operator running under another carrier's authority, a truck still carrying its old livery — then the operating carrier's name and USDOT number must appear as well, introduced by the words operated by.
This is one of the most common shapes of marking violation, and it is structurally different from an ordinary door marking because it needs two names rather than one. We do not stock it as a standard product. Ask us and we will cut it.
Vinyl, paint, or magnetic
All three are allowed. The rule permits the marking to be painted on, or to consist of a removable device, provided that device meets the legibility and contrast requirements and is kept in a condition that continues to meet them.
So a magnetic panel is not a compliance problem in itself. The practical question is durability rather than legality — a magnet that lifts on the highway, slides down the door, or gets left in a yard is a marking you are no longer displaying. Cut vinyl stays where you put it.
The one thing worth understanding about cut vinyl is that your door is the background. The letters are cut to shape with nothing behind them, so contrast is measured against your paint rather than against a white rectangle. That makes colour a compliance decision rather than a styling one, and it is why white lettering on a white cab is not really a marking at all.
A faded marking is a violation
Markings have to be kept and maintained so that they stay legible. Applying them correctly is not the end of the obligation.
Peeling, lifting, fading, dirt, a decal half-hidden behind a bracket or streaked with road grime — all of it puts you out of compliance no matter how good the vinyl was on day one. This is the failure that actually catches working trucks, because it arrives gradually and nobody notices the morning it crosses the line.
Adhesion is mostly surface preparation. Degrease the panel properly, apply it somewhere above freezing, and leave it a couple of days before you take a pressure washer to it.
Do you need an MC number on the door
Probably not, and this is the second thing the category gets wrong.
The federal marking rule asks for your legal name and your USDOT number. It does not ask for an MC number. Plenty of carriers still display one, some brokers and shippers still expect one, and some contracts require it — but that is a commercial requirement, not this one.
We sell MC number decals for the people who genuinely need them, and we say the same thing on that product page, because selling a marking as federally required when it is not is how a compliance brand stops being one.
Frequently asked questions
Who is exempt from a DOT number
The federal marking rule reaches commercial motor vehicles in interstate commerce. If you operate entirely inside one state the federal rule may not reach you at all — but your state will have its own, and several states require a USDOT number from intrastate carriers regardless. The state pages below answer it jurisdiction by jurisdiction.
Does the city and state have to be on the door
No. It was once common practice and plenty of trucks still carry it, which is why people assume it is required. The rule asks for the legal name and the USDOT number. Anything else you choose to display is permitted, so long as it does not contradict the required information.
Can I use a magnetic sign instead of a decal
Yes. Removable devices are expressly allowed, provided they meet the same legibility and contrast requirements and are maintained. The risk is practical rather than legal — a magnet that departs at seventy miles an hour is a marking you are no longer displaying.
What size lettering should I actually order
For a standard truck door, two inches is a sound working minimum and it is what we cut by default. Go larger if the panel allows it, if your contrast is marginal, or if you would rather it were read comfortably than merely legally.
Does the marking go on the trailer too
The rule covers the power unit. Trailer markings are a separate question, usually driven by state rules, permit conditions, or a carrier's own fleet numbering rather than by this section.
How do I check what my FMCSA record says
FMCSA publishes carrier records, and the name on that record is the name that belongs on the door. Check it before you order rather than after — particularly if you have changed entity type, trade under a DBA, or bought a truck that came with somebody else's lettering already on it.
Where this comes from
Everything above is drawn from the section itself, read at eCFR on the date shown in the rule card at the top of this page and reviewed by a person before publication. FMCSA's own guidance on who needs a USDOT number is worth reading alongside it.
We publish the retrieval date and the reviewer because a regulatory page without either is asking you to trust an anonymous claim about the law. If the section is amended, this page changes and that date changes with it.