Projection Screen Size and Viewing Distance Calculator:

The calculator needs three pieces of information.  The first is how far you sit from the screen, measured in feet.  The second is what shape your screen is, and the third is screen size.  You may enter either the diagonal screen size or the screen width. See the sketch below for details.  (If you enter screen width, don't forget to check the appropriate radio button) If you do not specify a viewing distance, the calculator will tell you what the optimal seating distances are for the selected screen size. 

Current viewing angle: Based on the inputs, this is the viewing angle or arc for your set up.

Maximum recommended viewing distance: Electrohome suggests a viewing distance of  three to six screen widths for video. Probably too far away to be effective for home theater, OK for everyday TV viewing. 

Maximum SMPTE recommended viewing distance: A de facto standard for front projection home theaters. 

Maximum and Recommended THX viewing distances:  THX requires that the back row of seats in a theater have at least a 26 degree viewing angle and recommends a 36 degree viewing angle.

SMPTE and THX screen widths:  The minimum screen widths required to meet the SMPTE and THX recommendations.

Viewing Distances based on Visual Acuity:  Calculated based on the resolving power of the human eye, or visual acuity. These distances represent the point beyond which some of the detail in the picture is no longer able to be resolved and "blends" with adjacent detail.