# How to choose the right screen size

## Is your telly too small - or too big - for HD?

### The fear of all sums

There are 1080 pixels vertically on a full HD set, so we already know that p = H/1080. Calculating the value for the tangent allows us to put in a real number, and we come up with the following formula for the distance:

A quick calculation suggests that for a 37in display, you need to be 57.74 inches away, or a bit short of five foot. Any further and most people won’t notice the difference.

For comparison, using the ITU recommendation of 3H for viewing distance would come up with a distance of 54.41 inches for the same size screen. And if you want to know the viewing distance in metres, starting with your screen size in inches, then use:

If that seems a bit close for comfort, consider 3D viewers, where the recommendation is to sit only 2H away, according to Panasonic. That, I suspect, is unlikely to happen in many living rooms.

Finally, if you have the reverse question, ‘what size TV do I need to buy for my living room’, it’s easy enough to turn around our last formula. Start with the viewing distance, d in metres, and you can find out the diagonal screen size D you need, in inches, using this formula, again only for a 16:9 set. If you want to buy one of Philips’ super-wide models, do your own sums!

Alarmingly, that suggests that if we really are all watching at the 2.7m distance suggested in the BBC internal survey, we need 68in sets to be able to make the most of full HD.

So, how big is yours, and can you really tell the difference? Tell us in the comments. ®