Distance (D), Width (W), Throw Ratio (R) If the distance and width are known, calculate the throw ratio using the formula: R = D / W [1] If the screen width and throw ratio are known, calculate the distance using the equivalent formula: D = W x R Although it is often stated as a single value (or range of values), throw ratio is a comparison of D : W.[2] To reduce this to a single number (as is typically seen in projector/lens specifications), start by dividing both sides by W, leaving us with D / W : 1.
A video projector (lens) with a throw ratio of 2.0 (or "2.0 : 1") would need to be positioned at a distance that is twice the width of the screen.
This provides flexibility in positioning the projector, since the lens can be adjusted to accommodate any throw ratio within that range.
EXAMPLE: A video projector with a throw ratio of 1.2 - 3.0 is to be used on a screen that is 100" wide.
Starting with the smaller value, we use the formula above (D = W x R, or D = 100 x 1.2) to determine that the closest position possible is 120".