# How do I calculate the correct size of HDTV to buy if . . . **URL:** \ **Category:** Factual Questions\ **Created:** [December 12, 2008, 3:40pm UTC](https://boards.straightdope.com/t/how-do-i-calculate-the-correct-size-of-hdtv-to-buy-if/476885 "2008-12-12T15:40:24Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![sciurophobic](https://avatars.discourse-cdn.com/v4/letter/s/cab0a1/32.png) [@sciurophobic](https://boards.straightdope.com/u/sciurophobic)\ **Post date:** [December 12, 2008, 3:40pm UTC](https://boards.straightdope.com/t/how-do-i-calculate-the-correct-size-of-hdtv-to-buy-if/476885/1 "2008-12-12T15:40:24Z")
I want to be able to watch DVDs or shows that are supposed to be in 4:3 at the same image size as my old tv? Is there a standard formula, or does it depend on the size of the TV (mine is 27")? ---
**Author:** ![muldoonthief](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muldoonthief/32/3188_2.png) [@muldoonthief](https://boards.straightdope.com/u/muldoonthief)\ **Post date:** [December 12, 2008, 4:09pm UTC](https://boards.straightdope.com/t/how-do-i-calculate-the-correct-size-of-hdtv-to-buy-if/476885/2 "2008-12-12T16:09:16Z")
If I understand the question correctly - you want to buy an HDTV sized so that, if you play a DVD in 4:3 mode on it (with black bars to the left and right), the picture will be the same size as your old SDTV. Call D the diagonal size of the HDTV. Since HD sets are 16:9, the height of the screen can be found: D^2 = h^2 + (16/9 \* h)^2 Solving for h: h^2 = D^2 / (1 + (16/9)^2) h = D / sqrt(1 + (16/9)^2) h = D / 2.0397 Since a 4:3 picture on the 16:9 set will have the same height, you can do the same thing to solve for h for a 4:3 set, using S as the diagonal size: S^2 = h^2 + (4/3 \* h)^2 h^2 = S^2 / (1 + (4/3)^2) h = S / sqrt (1 + (4/3)^2) h = S / 1.66667 So D / 2.0397 = S / 1.66667 D = 1.224 S So if your old TV is 27", the new one will have to be D = 1.224\* 27 = 33" to preserve the size of a 4:3 image. ---
**Author:** ![NoCoolUserName](https://avatars.discourse-cdn.com/v4/letter/n/5fc32e/32.png) [@NoCoolUserName](https://boards.straightdope.com/u/NoCoolUserName)\ **Post date:** [December 12, 2008, 4:44pm UTC](https://boards.straightdope.com/t/how-do-i-calculate-the-correct-size-of-hdtv-to-buy-if/476885/3 "2008-12-12T16:44:08Z")
Nice work! I have a related question, if I may piggyback: How far do I sit from an HDTV to appreciate the clarity of the picture? Or, how far away should I not sit, since I won’t be able to see the difference? Related to the size of the screen and the resolution, of course, so we’ll need another formula. ---
**Author:** ![sciurophobic](https://avatars.discourse-cdn.com/v4/letter/s/cab0a1/32.png) [@sciurophobic](https://boards.straightdope.com/u/sciurophobic)\ **Post date:** [December 12, 2008, 4:47pm UTC](https://boards.straightdope.com/t/how-do-i-calculate-the-correct-size-of-hdtv-to-buy-if/476885/4 "2008-12-12T16:47:45Z")
Thanks. That’s exactly what I was looking for. ---
**Author:** ![bouv](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@bouv](https://boards.straightdope.com/u/bouv)\ **Post date:** [December 12, 2008, 5:21pm UTC](https://boards.straightdope.com/t/how-do-i-calculate-the-correct-size-of-hdtv-to-buy-if/476885/5 "2008-12-12T17:21:01Z")
> [@NoCoolUserName](#): > > How far do I sit from an HDTV to appreciate the clarity of the picture? Or, how far away should I not sit, since I won’t be able to see the difference? Related to the size of the screen and the resolution, of course, so we’ll need another formula. [Here’s a little article with some numbers.](http://www.cnet.com/hdtv-viewing-distance/) It tells you the min. and max. viewing distance for both 4:3 and 16:9 TVs. [Here](http://myhometheater.homestead.com/viewingdistancecalculator.html) is a nice website with a calculator.