# How to calculate projector throw distance

Date Posted:25 July 2017  Calculating throw distance and image size.

The throw distance of a projector is a very important consideration, particularly if you are replacing an existing projector. Not all projectors can project from the same distance, just like with cameras the projectors can have different lenses and therefore must be positioned at a different distance even though you are projecting the same size image.
Finding the correct installation point is important to make sure you do not wire up a projector in the wrong position. Every projector comes with a number called the throw ratio which can be used to calculate the installation point of a projector. If the projector has a zoom (as most do) there will be 2 throw ratios’, the ratio when zoomed in and the ratio when zoomed out.

So how do you interpret the throw ratio?
The first number in the throw ratio represents the distance and the second represents the screen width. For example, if the advertised ratio is 1.15:1, this means for a 1metre wide picture, the distance the projector must be is 1.15 metres back. If your image is 2metres wide then the projector must be 2.3m back.

If it is a throw ratio is advertised as 1.15:1 - 1.5:1, and you want to fill a 100inch 16:9 screen (which is approx. 2.13m wide) then the calculations would look like this:

 1.15 X 2.13m = 2.45 metres - Closest installation point 1.50 X 2.13m = 3.20 metres - Furthest installation point

Most manufacturers will then advise to round these numbers up and down by 5% in case there is a slight variance among models. After rounding it would be 2.6metre to 3.1metres.

You can also use these figures to work out image width if your projector is positioned at a certain distance. Just divide your distance by the throw ratio. For example, if your throw ratio is 1:15:1 - 1.50:1 and you want the projector placed 4 metres away then the calculation would look like the below.

 4 / 1.15 = 3.48 metres - Largest image width possible. 4 / 1.50 = 2.67 metres - Smallest image width possible

Keep in mind Throw Ratio is related to image width, not diagonal.