Copied from Projector Central's web site:
Useful Formulas and Calculations Related to Home Theater Projector Set-Up.
4:3 Aspect Ratio
Width = 1.33 x Height
Height = .75 x Width
Diagonal = 1.25 x Width
16:9 Aspect Ratio
Width = 1.77 x Height
Height = .562 x Width
Diagonal = 1.15 x Width
5:4 Aspect Ratio
Width = 1.25 x Height
Height = .2 x Width
Diagonal = 1.28 x Width
Image Size = Distance / Throw Ratio 20ft/1.5 = 13.3ft Image Size
Note: Because of Variations in Mfg Specifications this will either yield a Width or Diagonal dimension
Throw Ratio (These are approximations when in question use Mfg. Data)
Distance / Image Width
Based on Lens and LCD/DLP Panel Size = (Focal Length/ Panel Diagonal) x 1.15 or 1.25
16:9 screen use 1.15
4:3 screen use 1.25
Area of a Rectangle or Square
Length x Width
Brightness
cd/m2 = Foot-lamberts x 3.43
Screen Brightness
Lumens / Screen Area in ft2
Hope it could help.