Place a projector for the screen size you want
A 120-inch screen does not fit wherever the projector happens to sit. Work out the lens-to-screen distance before you drill the mount.
Tools used in this guide
The job
Every projector has a throw ratio — distance ÷ screen width. A short-throw unit might be 0.5, a standard one 1.5–2.0. The ratio plus your screen width tells you exactly where the lens must sit.
Size to distance
Enter the diagonal and aspect ratio to get the screen's width and height, then multiply width by the throw ratio — the tool does both steps. A 120-inch 16:9 screen measures 2.66 × 1.49 m; at ratio 1.5 the lens sits 3.98 m from the screen.
Worked example
Living room, 120-inch 16:9 screen, projector rated at throw ratio 1.5: width is 2.657 m, so the lens-to-screen distance is 3.985 m — about 4 m. If only 3 m is available, running the distance-to-diagonal direction shows the same projector tops out near a 90-inch screen at that spot.
Scope
Diagonal 10–500 in, ratio 0.2–10, distance 0.1–50 m. The distance is measured from the lens, and lenses differ — zoom lenses give a ratio range, not one number. Measure to the lens position, not the rear of the cabinet.