Do you need a USDOT number, a medical card or a CDL?

The federal rules hang on two numbers: 10,001 pounds, where a truck becomes a commercial motor vehicle, and 26,001 pounds, where a driver needs a CDL. A pickup that is under the first line on its own is over it the moment it tows a loaded dump trailer.

Take the ratings off the door-jamb sticker and the trailer’s plate. If you have ever put the loaded truck on a scale, enter that too: the rule uses the rating or the actual weight, whichever is greater.

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How it’s worked out

The weight is the truck’s rating or actual weight, whichever is greater. Towing, it is the greater of the combination rating on the truck’s label or the truck and trailer added together (49 CFR 390.5).
10,001 lb or more makes it a commercial motor vehicle. In interstate commerce that means a USDOT number, a medical card for the driver, a driver qualification file, the hours-of-service rules and daily inspections.
Interstate commerce is not only crossing a state line: an in-state leg of a shipment that started or will end in another state counts too.
A CDL is needed for a single vehicle of 26,001 lb or more (Group B), or a combination of 26,001 lb or more towing a unit over 10,000 lb (Group A). Placarded hazardous materials need at least Group C at any weight (49 CFR 383.5). Drug and alcohol testing follows the CDL.
In-state only, the federal rule does not require a USDOT number, but most states require one for in-state commercial vehicles, with weight thresholds and exemptions each state sets. Your state decides.

Questions

My truck never leaves the state. Am I exempt?

Not from your state’s rules. Most states require their own in-state commercial vehicles to register for a USDOT number, and many adopt the federal inspection and driver rules for in-state work. FMCSA keeps the list of states that require one; check your state’s motor carrier authority for its threshold.

Does an empty trailer count?

Yes. The rule uses the trailer’s rating or its actual weight, whichever is greater, so a trailer rated at 7,000 lb counts as 7,000 lb whether it is loaded or not.

Is this legal advice?

No. It applies the federal definitions to the numbers you enter. Exemptions exist (farm, emergency, military and others) that it does not cover, and states add their own rules. Confirm with FMCSA or your state before relying on it.