You measure the glass area only from one corner to the other diagonally.
Manufacturer's will often lie about screen sizes in their laptops for marketing purposes. For example, Sony have a range of machines they call the VPC series that have 15.6" screens fitted, however Sony market these as 15.5" models. Upon opening the machine we find a screen manufactured by a generic LCD maker with 15.6 inches in size.
Often the laptop makers will abbreviate the screen size, so a 14.1" screen size maybe advertised as 14 inch. Obviously the first thing that you need to do when searching for a replacement screen for your computer is determine the real size of the screen.
What sizes are screens made in?
Laptops and tablets generally use one of the following standard sizes, regardless of the laptop makers marketing efforts to appear unique or special.
7 inch - mostly used for tablet devices but was also used by some early netbook machines such as the ASUS EEEPC.
7.9 inches - used in tablets and the Apple ipad
8.0 inches - used in tablet devices
8.9 inches - used in netbook machines (older models usually) and some tablet devices
9.7 inches - used mainly by Apple in the older iPads but also used by some tablet devices made by Samsung
10.0 inches - this is an obsolete standard
10.1 inches - this was the most popular size for netbook computers, most low end netbooks use this size
10.2 inches - this is an older size standard that was replaced by 10.1 inch machines. Used in older ASUS models
11.1 inches - Used by Sony, these are now 10 years old and some of the best panels ever made - obsolete now
11.5 inches - Sony use 11.6 inch screens in their VPC netbooks but call them 11.5 inches to be special :)
11.6 inches - the new standard in netbook screen size, this is my personal favourite compromise between portability and good desktop real estate size.
12.0 inches - the new Macbook Retina from 2015 uses this size, its taller than a normal widescreen and goes back to the 4:3 style layout offering less width and more height
12.1 inches - now pretty much obsolete this was a popular size that's been superseded mostly by 13.3 inch models
13.1 inches - a handful of high end screens were made in this size, mostly for Sony machines - they are hard to find now
13.3 inches - the most common 13 inch size, most 13 inch laptops use this size
14.0 inches - the most common 14 inch size which is still in use today, comes in HD, HD+ or FULL HD mainly (UHD coming!)
14.1 inches - this was replaced by 14.0 inch sizes, older WXGA machines use this size.
14.4 inches - a rare size used in some Toshiba models, not available anymore
15.0 inches - this size is still being used but very rarely, it's an old size standard from around 15 years back and was replaced by widescreen models
15.2 inches - a short lived standard now obsolete, used in some Apple computers
15.4 inches - this is an old now obsolete standard and was the most popular screen size ever made
15.6 inches - this replaced the 15.4 inch size and offered a more widescreen layout and HD resolutions
16.0 inches - used by Acer mainly this size went out of production quickly and was never reproduced
16.4 inches - used by Sony, specialist size and screen type often with non standard electronics and connections, now obsolete
17.0/17.1 inches - These are the same thing, although many manufacturers would argue over the real size. Some would say it was 17.1 inches and some 17.0 - they two were interchangeable and this size is now mostly unavailable and obsolete.
17.3 inches - the new 17.1 inch standard offering wider screen and HD resolution
18.4 inches - these giant screens were aimed at gamers but never took off and since became obsolete
Although you find desktop machines and all in one devices with larger screens it's very rare to find any laptop computer with a screen larger than 18.4 inches. The most popular size is 15.6 inch which replaced the previously most popular 15.4 inch size.
How do you measure your screen accurately.
The most common mistake when measuring your screen size is to include the bezel area or the border around the screen. It doesn't matter if you've removed the LCD screen panel from the lid of the computer or if it is still in place, you measure only the glass area of the display without any border.
The 2nd most common mistake is to measure the screen from side to side horizontally. You measure the glass area ONLY from corner to corner diagonally. For example from the bottom left corner to the top right corner, inside the frame area.
As you can see from the image above, we have measured the glass area of the screen only from corner to corner diagonally and have not included the border around the screen. If you have removed the screen you will notice there is a thin metal frame around the glass, this should also not be included if you are measuring a screen that has already been removed from the lid of the machine.