When your home license works in the US, what an International Driving Permit really is, and when to switch.
There is no single US rule for foreign driver's licenses. Each state decides who may drive with a foreign license and for how long. This differs by state; see In your state.
The general picture
Visitors (tourists, people on short trips) can usually drive with a valid license from their home country for some time.
People who become residents of a state (they live and work or study there) are usually expected to get that state's license within a set number of days. Each state defines "resident" and the number of days.
Car rental companies have their own rules. Most accept a valid foreign license, and some also ask for an International Driving Permit.
What an IDP is
An International Driving Permit (IDP) is a booklet that translates your license into several languages. It is not a license. It only works together with your real license from home. Always carry both.
You must get an IDP in the same country that issued your license, before you travel. A US organization cannot issue an IDP for a foreign license.
An IDP is especially useful if your license is not in English or does not use the Latin alphabet. It helps a police officer or rental company read it.
Websites that sell an "international driver's license" online are often scams. The FTC warns about them. They are not valid.
Steps
1
Before you leave home, get an IDP from your country's official issuing body (often the national automobile club).
2
Bring your original license. Keep it valid: if it expires, the IDP also stops being useful.
3
When you arrive, check your state's rules on the DMV website: who counts as a resident, and how many days you may drive on a foreign license.
4
Buy car insurance before you drive your own car. Tell the insurer you have a foreign license.
5
If you will live in the state, start the US license process early. Wait times for tests can be long.
Common mistakes
Driving on only an IDP. It is not a license on its own.
Driving on a foreign license for years after becoming a resident. In many states that counts as driving without a valid license.
Assuming another state's rule applies. If you move to a new state, check again.