screen size calculation
(Last Updated On: June 3, 2019)

It’s common knowledge that a projector will give you a larger screen for your home theater than your average television. But how much bigger—and how can you tell? The measurements listed on the product description are only as helpful as your ability to interpret them, and they don’t always make it obvious how to compare them.

The good news is, it’s pretty easy to figure out a projector’s screen size, both in terms of the maximum potential dimensions and the change in size you’ll get when you move the projector further from or closer to the screen. Read on below for all the answers!

How are projector screens measured?

For most products, it’s typical to see the size listed in terms of its dimension: one number for the length, and another for the width. With projectors, though, you’ll see only a single measurement listed under the screen size. Figuring out which dimension this represents—and how to extrapolate the rest of the image size from this information—is your first step.

Similar to televisions, projector images are measured on the diagonal. This is also the measurement that will be used for the screens themselves. If you want to figure out the size so you can match your projector to the right screen, or for the sake of comparing one screen to another, you can generally do so with just this information.

Extrapolating length and width

For some situations, just knowing the diagonal won’t be enough. You might be trying to figure out where in your home will be the best fit for the screen, for example. If you want to get the screen’s dimensions from the screen size, you’ll need to think back to your high school geometry classes.

Drawing a diagonal across a rectangular screen breaks it into two right triangles, with the diagonal serving as the hypotenuse of each. This diagonal can be the hypotenuse of an infinite number of right triangles, though. Just like in geometry class, you’ll need both the length and the slope of the hypotenuse to figure out the screen’s dimensions.

This is where the projector’s aspect ratio comes into play. This spec tells you the relationship of the image width and height, expressed as a mathematical ratio. If the aspect ratio is 1:1, the width and height are equal. If the aspect ratio is 2:1, the width is twice the height.

Different forms of content have different aspect ratios. HD movies and TV shows have an aspect ratio of 16:9. If you want to replicate a true theater experience, you’ll want a cinema aspect ratio of 2.35:1. If you remember your Pythagorean theorem, you can use that and the aspect ratio together to determine the exact dimensions of the image or screen. You can also find an aspect ratio calculator online if you’re not mathematically inclined.

What about throw ratio?

project throw ratio

Another factor complicating the question of screen size is the fact that, unlike a television, a projector’s screen size is not static. The size of the image produced by a projector lamp will change depending on the distance between the lamp and the screen.

The good news is that the aspect ratio will not change, only the scale—in other words, both the length and width will increase or decrease in the same proportion. You can tell how much the screen size will change when it’s moved using the throw ratio.

The throw ratio of a projector is expressed as a decimal, which you find by dividing the width of the image by the distance. As you can see looking at a diagram, bringing a zoom into the equation can complicate the matter, but generally speaking the more distance between the lens and the screen, the larger the screen you’ll need.

Remember that alignment can affect the picture as well as distance. Moving the projector up and down or side to side in relation to the screen—or changing its angle of projection—can change the size and quality of the image as well.

So how do you calculate a projector screen’s size?

As you can see, there are quite a few factors involved, and you may need to do some math to get the answer. There are three equations you’ll need to know:

  • The throw ratio: You can express this as T=W/D, where the throw ratio (T) is equal to the width (W) divided by the distance (D).
  • The aspect ratio: For HD content, this is 16:9; for cinema screens, this is 2.35:1.
  • The Pythagorean Theorem: Typically written as A2+B2=C2, it might be more helpful to think of it as W2+H2=S2, or width2+height2=screen size2.

If you know where you’ll be putting your projector, measure the distance between the lens and the wall, then multiply this by the throw ratio. This will tell you the width of the projected image. Use the aspect ratio to get the height, then use the Pythagorean Theorem to find the screen size.

You can also go the other way. If you have a space on the wall in mind, measure out the area and decide how large you want your image. You can use painter’s tape to mark out where you envision the image (using the aspect ratio to get the dimensions). Get the screen size using the Pythagorean Theorem. You can also divide the width by the throw ratio to figure out how far away your projector needs to be.

It might seem complicated at first, but in practice, it’s pretty easy to figure out your projector’s screen size. Taking the time to make the calculations for your space before you buy and install your equipment can save you a lot of hassle and guesswork in the long run. Hopefully, the info in this article is helpful as you’re setting up your home theater!