Published 14 July 2026 · Buying Guides · 1 min read
How to Calculate Projector Screen Size
The geometry behind screen sizing, explained — useful even if you just want to understand what the calculator is doing.
The Underlying Formula
A screen's diagonal, width and height form a right triangle, so width and height follow from the diagonal and aspect ratio using the Pythagorean theorem: for an aspect ratio of w:h, width = diagonal × w ÷ √(w² + h²), and height = diagonal × h ÷ √(w² + h²).
Worked Example
For a 120-inch diagonal at 16:9: √(16² + 9²) = √337 ≈ 18.36. Width = 120 × 16 ÷ 18.36 ≈ 104.6 in. Height = 120 × 9 ÷ 18.36 ≈ 58.8 in — which matches the figure on our 120-inch screen size page.
Converting Units
Once you have a dimension in inches, converting to centimetres is a ×2.54 multiplication, and to feet a ÷12 division. Our screen calculator does all of this automatically, including for a custom aspect ratio you enter yourself.
Why Bother Doing It by Hand?
Understanding the formula helps when you're comparing a screen size against a fixed piece of furniture, alcove or wall dimension where you need to reason about the geometry directly, rather than just reading an output number. For everyday sizing, use the calculator — see our calculator walkthrough for how to use every field.
