Lenovo T61, how to check if the monitor is 14 inches or 15.4

Really, I tried to google help but can't. Well, I need to check whether the notebook monitor is 14 inches or 15.4. The notebook is not a square (T61) but a rectangle (widescreen?) And I don't know how to count the screen size. The seller said it was 15.4 inches, but Everest says the display area is 14 inches.
Of course, I can measure the screen with a ruler, but I don't know whether I should measure the matrix itself (the area where the image is displayed) or also the plastic housing. As far as I remember, 15.4 inches means that the screen size is 2.54 * 15.4 = 39.1 cm?
case dimensions: 34 x 24 cm
matrix dimensions: 31 cm x 20 cm
housing diagonal (matrix and frames all around it): 40 cm (about 15.4 inches)
matrix diagonal (only the part that displays the image): 36 cm (about 14 inches)
And the question is whether I am guessing correctly that if I would like to replace the bottom cover, it will be this: 15.4 inches widescreen (I can't give the auction number on Ebay on this forum).

But I need to buy a case and I need to know if I have 14 inches or 15.4 inches. And my maximum resolution is 1280 x 800.

I can answer my question myself. When assessing the size of the matrix, only the size of the element showing the image is assessed, without the housing. You take a ruler and measure the length of the diagonal either in inches or converted into inches (2.54 cm is roughly one inch). And this tells about the size of the matrix.
The second parameter is the matrix's proportions, but I just meant its size.
Thank you for your help, I close the topic.