View Full Version : No quite PLC talk: Re:Formula for LCD Screen Size?

Anibal Hernandez
March 28th, 2005, 03:31 PM

No quite PLC talk: Re:Formula for LCD Screen Size?

Does anyone knows the formula for determining the width and the height dimensions of an LCD Flat screen computer monitor provided you know the Diagonal and the Aspect Ratio dimensions?


21.3 LCD Screen

With an Aspect Ratio of 16:9

What are the width and the height dimensions?

Thanks all

March 28th, 2005, 04:37 PM
Calculate the side of a right triangle. It's been so long since sixth grade. <grin> See:


March 28th, 2005, 04:58 PM
Like the link above will lead you to, a right triangle with a hypotenuse of 21.3" will have a opposite and adjacent side with a 90 deg. angle at the intersection of the opposite and adjacent sides. There are two remaining angles in the shape and one can be 60 and the other 30 or both be 45 degrees and still add up to the required 180 degrees for the total angles. The 60/30/90 triangle produces a rectangle when doubled and the 45/45/90 produce a square when the area is doubled. Now, most monitors have rectangle shape so the rest is just a matter of algebra. Easiest way to explain aspect ratio is to say that it means the ratio between the width of the picture and the height of the picture.

Anibal Hernandez
March 28th, 2005, 05:05 PM

Do you know by any chance the formula? I need to put it in an excel spreadsheet.



Steve Bailey
March 28th, 2005, 05:19 PM
The aspect ratio of 16:9 (1.78) is equal to the tangent of the angle opposite the longer side. The inverse of the aspect ratio (9:16 = .5625) is equal to the tangent of the angle opposite the shorter side.

If you know the tangent of an angle, you can calculate the angle. Once you know the angle, you can calculate its sine or its cosine. The length of the side opposite an angle is equal to the hypotenuse times the sine. The length of the side adjacent to an an angle is equal to the hypotenuse times the cosine.

March 28th, 2005, 05:21 PM
You should look at this link (http://www.math.com/tables/algebra/functions/trig/overview.htm), Andy. Give it a go and then see where that takes you.

Fred Floggle
March 28th, 2005, 05:23 PM
SOHCAHTOA - I resesrve my comments about engineers .
Either didn't go to school in PR or were to smashed for simple maths. Either way - WAKE UP.

March 28th, 2005, 08:18 PM

Dont know if I am an engineer BUT the Navy taught me
S suzy
C can
T tell

O oscar
H has
A a
H ha**
O o*
A always

Does yours have a ditty that everyone ALWAYS remembers ??

Dan Bentler

March 29th, 2005, 10:00 AM
Ah - but this is also easily solvable without resorting to trigonometry with all the SOHCAHTOA stuff. The diagonal and aspect ratio is enough. Want me to show the math?

March 29th, 2005, 10:13 AM
Let us see you shine Bernie. I like to learn new things!

Rick Densing
March 29th, 2005, 10:20 AM
Pythagoras and some algebra.


I hope you can do the rest.

Peter Nachtwey
March 29th, 2005, 10:25 AM
21.3^2 = ( 16* x ) ^2 + ( 9*x ) ^2
453.69 = 256*x^2 + 81*x^2
453.69 = 337*x^2
1.345 = x^2
1.116 = x
9*x = 10.44
16*x = 18.56

March 29th, 2005, 10:35 AM
Pythagoras does indeed rule

16 * X = Long Side
9 * X = Short Side
21.3 = Hypotenuse

Square Root( Long Side ^2 + Short Side ^2) = Hypotenuse

Square Root( (16 * X)^2 + (9 * X)^2)) = 21.3
Square Root( (256 * X^2) + (81 * X^2)) = 21.3
Square Root( (337 * X^2) ) = 21.3
Square Root(337) * X = 21.3
18.3575 * X = 21.3
X = 21.3 / 18.3575
X = 1.16028

(Oops, should have checked again before I posted. Peter beat me.)

16 * 1.16028 = 18.5646 = Long Side
9 * 1.16028 = 10.4426 = Short Side

March 29th, 2005, 11:01 AM
You guys are good. Just goes to show the depth of knowledge we are fortunate to have access to here on PLCs.net. :site:

Anibal Hernandez
March 29th, 2005, 11:39 AM
How would you enter the formula for the aspect ratio 4/3 and 16/9 into an excel cell?

March 29th, 2005, 01:27 PM
This is how I set up a spreadsheet in column A. Enter values in A1, A2, A3
21.3 (The value of the diagonal)

16 (The width of the aspect ratio)

9 (The height of the aspect ratio)


= A2*A4 (Width)

=A3*A4 (Height)