Projector screens are commonly sold by diagonal, while throw ratio uses horizontal image width. The conversion is geometry, not a model lookup, and the chosen aspect ratio is part of the input rather than an assumption hidden by the calculator.
Resolve the aspect-ratio triangle
For aspect values a:b and diagonal d, compute n = √(a² + b²). Width is d × a ÷ n and height is d × b ÷ n. The same formula works for 16:9, 4:3, 2.35:1, or another positive ratio entered consistently.
A 100-inch 16:9 example
For 16:9, √337 links the diagonal to both sides. A 100-inch diagonal produces about 87.16 inches of width and 49.03 inches of height, or roughly 221.38 by 124.53 centimetres. Those rounded display values should not become the input to later chained calculations.
Aspect ratio changes width at the same diagonal
A 100-inch 4:3 image is 80 inches wide, narrower than the 16:9 example. Because throw distance is proportional to width at a fixed ratio, copying a placement number between aspect ratios is unsafe even when both products advertise the same diagonal.
Separate ideal geometry from physical measurement
A screen’s advertised diagonal may refer to the viewable image, while a frame adds border and mounting depth. Measure the intended image rectangle and the manufacturer’s distance reference point. Leave installation tolerance instead of designing to a rounded endpoint with no room for adjustment.