# How do I do this conversion -- SDTV to HDTV?

**URL:** <https://boards.straightdope.com/t/how-do-i-do-this-conversion-sdtv-to-hdtv/385213>\
**Category:** Factual Questions\
**Created:** [December 22, 2006, 1:00pm UTC](https://boards.straightdope.com/t/how-do-i-do-this-conversion-sdtv-to-hdtv/385213 "2006-12-22T13:00:18Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![crazyjoe](https://avatars.discourse-cdn.com/v4/letter/c/f14d63/32.png) [@crazyjoe](https://boards.straightdope.com/u/crazyjoe)\
**Post date:** [December 22, 2006, 1:00pm UTC](https://boards.straightdope.com/t/how-do-i-do-this-conversion-sdtv-to-hdtv/385213/1 "2006-12-22T13:00:18Z")

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I have a 25 inch TV at home (I know, mock me…) and I am wondering what size I would need to buy in order to get the same size picture when watching SDTV on a 16:9 HDTV.

(of course I want a really big one, but I want to know what the minimum is to get the same size picture).

So, how do I do the math?

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**Author:** ![Ximenean](https://avatars.discourse-cdn.com/v4/letter/x/aca169/32.png) [@Ximenean](https://boards.straightdope.com/u/Ximenean)\
**Post date:** [December 22, 2006, 1:34pm UTC](https://boards.straightdope.com/t/how-do-i-do-this-conversion-sdtv-to-hdtv/385213/2 "2006-12-22T13:34:00Z")

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25" is the diagonal of your 4:3 TV, so by Pythagoras its width is 20" and its height 15". You need a 16:9 widescreen TV with a height of 15". The diagonal of a 16:9 rectangle is 18.36", scale it up in the ratio 9:15 and you get 30.6". So you need a 31" widescreen TV.

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**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\
**Post date:** [December 22, 2006, 1:34pm UTC](https://boards.straightdope.com/t/how-do-i-do-this-conversion-sdtv-to-hdtv/385213/3 "2006-12-22T13:34:52Z")

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A standard TV is 4:3. A right triangle with legs of 4" and 3" will have a diagonal of 5". Your 25" diagonal should be 20" wide and 15" high.

Assuming you want the same height, a 15" high screen would be 26.6" wide in 16:9 which gives me a diagonal of 30.5", so I’d go with a 32" screen, that’s pretty popular and will give you a lot of options.