You may already noticed that most of ruler apps are actually wrong.
Accurate Ruler renders according to screen width spec of the device that your are currently using. So It is accurate.
Already tested in following devices including:
iPhone 6 plus
iPhone 6s plus
iPhone 7 plus
iPad air 2
iPad min 2
iPad min 3
iPad mini 4
iPod Touch 5/6
If your device is one of them, we insure it is accurate.