my Asus laptop is 100.7 PPI, Apple MacBook Air is 127.7 PPI, Xiaomi Mi Pad 3 is 163 PPI, my mobile phones (Sony Xperia C5, OPPO R11 Plus) are both 122.6 PPI, Apple iPhone 5 is 163 PPI, iPhone 7 is 162 PPI, iPhone X is 151.7 PPI. There are five general display sizes available for tablets, though specific models vary in their exact dimensions. The most accurate way, after i measure the size of virtual ruler by an actual ruler, i found the markings are not very accurate at 30cm, so i adjust the default pixels per inch(PPI) to 100.7, now i have the most accurate virtual ruler on the web.

Below ruler adjuster help us to calibrate accuracy more easily. Use standard objects to compare lengths, check your wallet, use any paper money to be our compare object, then search 'the width of your paper money' online, when you know the width, you can adjust the ruler's PPI setting by it, you can compare to any standard size object at your side, e.g., coin, credit card, CD, paper money, mobile phone, in the office, a A4 size printable paper is a good comparison object, the longer the more accurate.Search 'display by pixel density' online, check if there has your device's brand and model, i am lucky and found my screen has 100 PPI.The screen resolution of your current device is 1366x768 pixels. To have the most accurate ruler online, just set the pixels per inch(PPI), below are some ways to know the pixels per inch to your device.

18 of an inch actual size