Most of us have a ruler app we have never checked against a real ruler. Phones don't tell apps how big their screens physically are, so every screen ruler starts with a guess - on a recent iPhone Pro, an unchecked 10 cm can stretch to 10.6 cm. Simple Ruler fixes that in seconds with any credit card, then measures length, gaps, width x height and angles. Lay a screw or a SIM tray on the screen and read it in millimeters. Free with ads, offline, no account.