The diagonal screen is 15.6"(inches) of my laptop computer, resolution is 1366x768 pixels. I google the PPI reference and found 100 PPI to my screen, after i measure the size of virtual ruler by a actual ruler, i found the markings are not very accurate at 30cm, so i set the default pixels per inch(PPI) is 100.7 for myself.
There are two types of scales commonly used on rulers; Fractional and Decimal. Fractional Rulers have graduations or marks based on fractions, for example 1/2", 1/4" 1/8", 1/16", etc.. Decimal Rulers have graduations or marks which are based on the decimal system such as 0.5, 0.25, 0.1, 0.05, etc.. Most Fractional Rulers are based on the English measuring system where scales are graduated in units of one inch and fractions of an inch.
A centimeter is 10 millimeters, or approximately the width of a fingernail. Another way to imagine the size of a centimeter is in relation to inches. One centimeter is approximately three times smaller than an inch.
It's a good idea to check the products size, before we buying it from internet, right ? try this online scale ruler
|Inch to CM,MM | Real Size Ruler|
|Ounces to Grams | KG to Pounds|
|Color Matching | Promotional Products|