U.S. Citizen Services
Passport Services
Photo Requirements
Passport photographs must be 5x5 centimeters, full face, on a plain light background; the length of the head should be not less than 2.5 cm and not more than 3.5 cm, measured from the top of the head to the tip of the chin. We recommend that you print the sample and show it to the photo place when you go to have your photographs taken.
The Department of State's checklist for photographers gives more details on the requirements for U.S. passport photographs.