To calculate what lens you can use with your projector to fill your screen or surface you need two things:

1. The dimensions of your screen or surface (height and width)
2. The potential location for your projector and the resulting throw distance (D)

Throw distance (D) is shortest distance between the projector and the projection surface (see illustration below)

Once you have these measurements you can enter them into a lens calculator and it will tell you what lens you should use on a specific projector. Note that lenses are specific to every manufacturer or series of projectors by the same manufacturer. It is therefore important not to assume that a specific lens will be availble for projector A just because it exists for projector B.

Consequently every mayor manufacturer has their own lens calculator online. Here are some examples:

Barco
Christie Digital
Panasonic

Please note that these calculators vary in many aspects from one another:
Some may not list discontinued projectors that rental houses still carry or venues have purchased years ago and still use
Some may require you to know the name of lens without telling you what kind of lens it is (like the panasonic). However the manufactueres know how they engineered the optical system that produces the projected images. Thus it is a prudent appoach to use those calculators as opposed to generic lens calculators.

There are many all-around calculators that fill the gap when those by manufacturer fall short. A good example is at projectorcentral.com. For this one to work you need to choose a projector make and model.

The brochure or specification for each projector also susually lists the availble lenses under assecories. However that is only true for professional projectors with interchangeable lenses carried by rental houses. Consumer, business or presentation grade projectors usually have a zoom lens that is not changeble.

rule of thumb calculations: