According to the FMCSA website, a USDOT Number is required if you have:
For intrastate carriers, number requirements vary from state to state. If you do not know if your state requires intrastate motor carriers to have a USDOT Number, contact the Office of Motor Carrier Safety field office in your state. For a listing of State offices, go to the FMCSA website and click on Field Offices. These offices are listed by region with telephone numbers to answer your questions and to provide you with guidance concerning the Federal Motor Carrier Safety Regulations.
There is no application fee, but you must complete the MCS-150 (Motor Carrier Identification Report) to obtain a number. The form can be found at the FMCSA website. Click on online registration. There is a step by step registration guide to assist you.
If you request a number on the Internet you will receive your number immediately and a confirmation letter will be mailed by the FMCSA.
Generally, if you are operating as a “for-hire” motor carrier of regulated commodities or passengers in interstate commerce, you must also obtain interstate Operating Authority (MC Number) There are some exemptions to this requirement, which is explained in detail on the FMCSA website.
For detailed information, guidance through the registration process, and to obtain a US DOT or Operating Authority (MC) number, visit the FMCSA website.