(A) 50 INCHES DIAGONAL 

VIEWING DISTANCE = DIAGONAL x 1.63 = 50 x 1.63 = 81.5 INCHES (6 FEET-10 INCHES)

(B) 100 INCHES DIAGONAL = 87 INCHES WIDE

MINIMUM VIEWING DISTANCE = WIDTH x 1.54 MIN (36° ANGLE) = 154 INCHES 

MAXIMUM VIEWING DISTANCE = WIDTH x 2.2 MAX (26° ANGLE) = 220 INCHES

WHERE TO SIT?

Groups like SMPTE (Society of Motion Picture and Television Engineers) and THX have developed guidelines for seating distance based on subtended viewing angles, or how much your field of view is occupied. SMPTE recommends a 30º viewing angle, while THX recommends a maximum 36º viewing angle for a more involving image, and a minimum of 26º. But even with the formula and a scientific calculator, deciphering subtended viewing angles is, frankly, a real pain.

Instead of hateful trigonometry, use these simple formulas. For 16 x 9 screens, multiply the screen diagonal by 1.63 or the width by 1.9 to determine your SMPTE-recommended seating distance. With a 50-inch-diagonal screen, multiplying by 1.63 and then dividing that result by 12 (converting inches to feet) suggests sitting 6.79 feet from the screen.

Know how far away you’re sitting and want help choosing the ideal screen size? Divide the seating distance in inches by 1.63 for the screen diagonal or by 1.9 for the width. So, 12 feet away equals 144 inches divided by 1.63 equals a recommended screen size of 88 inches.

As a general guideline, technical editor Al Griffin suggests multiplying the screen width by 1.54 for 36º viewing, and the width by 2.2 for maximum distance. So, an acceptable seating range for a 100-inch-wide screen would be 154 (12.83 feet) to 220 inches (18.33). THX’s Master Jedi, John Dahl, suggests multiplying the viewing distance by 0.84 for max screen size and dividing the screen diagonal by 0.84 for optimal distance.

Armed with these simple formulas, you can use the calculator in your cellphone for some quick figuring in a store and determine the set size that’s perfect for you.