Throw ratio is a dimensionless relationship between the lens-to-image distance and the projected image width. It describes ideal horizontal geometry; it does not by itself prove that a particular projector can focus or produce a suitable picture there.
Start with D ÷ W
Write D for the manufacturer-defined lens-to-screen distance and W for image width. The throw ratio is r = D ÷ W, so distance is D = W × r and width is W = D ÷ r. Units cancel only when both lengths describe the same physical unit.
A zoom lens usually supplies a range
A specification such as 1.2–1.5 means the same image width may be produced from a span of distances. For a known width, multiply by 1.2 for the near end and by 1.5 for the far end. In the inverse calculation, divide distance by 1.5 for minimum width and by 1.2 for maximum width.
Throw ratio uses image width, not diagonal
A 100-inch 16:9 screen is not 100 inches wide: its width is about 87.16 inches. Using the diagonal directly would overstate placement distance by roughly fifteen percent. Convert diagonal through the aspect ratio first, then apply the projector’s range to the resulting width.
Treat the calculation as a geometry check
ThrowFit does not know zoom detents, focus limits, optical offset, lens shift, projector body depth, cable clearance, brightness, or screen gain. A number inside the geometric range is a reason to continue checking the exact manual and room, not permission to drill or buy a mount.