How to Calculate Projector Throw
Most manufacturers specify a maximum throw distance that is the furthest a projector can be from the screen and still achieve a clear picture; beyond this distance, the image blurs or fades. Because projected images spread out from the lens at an angle, the picture becomes larger the further the projector is from the screen. The direct correlation between the throw distance and the projected size is called the throw ratio, which you can calculate with a few measurements. If the projector also contains a zoom feature for widening the image, it has a range of throw ratios. Armed with these ratios, you can calculate the throw distance required to achieve a specific screen size.
Steps

1Calculating the Throw Ratio: Position the projector within the maximum throw distance from the screen and turn it on.

2Set the zoom to the maximum value to calculate the lowest throw ratio. The zoom settings should be accessible through the projector's menu, but the projector could also have dedicated buttons on the case or remote control.

3Measure the distance between the projector's lens and the screen.

4Measure the width of the projected image. Using the width measurement instead of the diagonal measurement commonly used for TVs and monitors circumvents variation due to the aspect ratio, which is the ratio of the picture's width to its height. As an example, you might project a widescreen or fullscreen image; the diagonal measurements between the two aspect ratios differ, but the width remains the same.

5Divide the distance by the width to calculate the lower throw ratio. As an example, if the projector is 84 inches from the screen and creates a 60inch image, divide 84 by 60 to calculate the lower throw ratio of 1.4, which is displayed as 1.4:1. This ratio means that for every 1.4 feet of throw distance, you gain 1 foot of screen width.

6Set the zoom to the lowest setting and repeat the measurement. Continuing with the example, if the lowzoom width is 44 inches, divide 84 by 44 to calculate a upper throw ratio of 1.9:1. Therefore, the throw ratio ranges from 1.4:1 to 1.9:1, which means you can have a range of screen widths from the same throw distance.

7Calculating Throw Distance: Determine the screen size you wish to achieve. As an example, if you wish to fill a 64inch wide projector screen, your target width is 64 inches.

8Multiply the target width by the lower throw ratio to calculate the minimum throw distance. Continuing with the previous examples, multiply 64 inches by 1.4 to calculate the minimum throw distance of 90 inches, or 7.5 feet.

9Repeat the calculation with the upper throw ratio to calculate the maximum distance required to achieve the desired screen size. In the example, multiply 64 inches by 1.9 to calculate a maximum distance of 122 inches, or 10.1 feet. These results mean you can position the project anywhere from 7.5 to 10.1 feet from the screen and still achieve your target width.

10Calculating Screen Width: Measure the prospective distance between the projector and the screen. As an example, if you're considering a projector for an 8foot long room, the longest throw distance you could use would be 8 feet.

11Divide the distance by the lower throw ratio to calculate the largest screen size possible. Continuing with the previous examples, divide 8 feet by the lower throw ratio of 1.4 to calculate the maximum screen width of 5.7 feet, or 68 inches.

12Divide the distance by the upper throw ratio to calculate the smallest screen size at that distance. In the example, divide 8 feet by the upper throw ratio of 1.9 to calculate the minimum screen width of 4.2 feet, or 51 inches.
Tips
 If you're setting up a projector in a small room, consider getting a shortthrow projector with a low throw ratio. These projectors spread the image in a wide angle to achieve a large picture with a relatively short throw distance.