Photo is an essential part of your visa or passport application and it usually has certain size and format requirements. "Millimeter - screen ruler" is a handy tool to ensure your photo meets all the size requirements. With this app you can check not only width and height of the photo but also measure exact size of the head and other parts of the photo that are crucial for the acceptance.
Tip: if you put brightness of the display to max you will be able to see measurement lines underneath your photo for better alignment and accurate results (see picture).
The app is available for Android and iOS on Google Play and AppStore: