Projector Throw Distance Calculator (screen size, throw ratio, zoom)

Understanding the projector throw distance calculator — concept and principle

This calculator works out how far a projector must be from the screen to fill a given screen size, from the projector's throw ratio, and the picture width and height. It also shows the range of screen sizes possible from the distance you have. Home cinema builders use it before buying a projector or drilling a ceiling mount.

Throw ratio × screen width: with the default inputs the result is 3.01–3.63 m (projector distance).

How it is calculated

Image width = diagonal × a ÷ √(a² + b²) for aspect ratio a:b (16:9 gives 0.872 × diagonal).
Throw distance = throw ratio × image width.
With a zoom lens, the farthest distance = tele throw ratio × width.
From a fixed distance: largest screen = distance ÷ wide throw ratio ÷ width factor; smallest = distance ÷ tele ratio ÷ width factor.
1 in = 0.0254 m; 1 ft = 0.3048 m.

The principle

The throw ratio is a property of the projector's lens: the distance to the screen divided by the width of the picture it makes. Light spreads out in a cone, so doubling the distance doubles the picture width. A zoom lens changes the cone's angle, so it has two throw ratios and can fill the same screen from a range of distances. The width follows from the diagonal by Pythagoras, because the diagonal, width and height form a right triangle in the ratio of the aspect ratio.

Worked example

Default inputs: 100 in diagonal, 16:9, throw ratio 1.36–1.64, available distance 3.5 m.

Width factor = 16 ÷ √(16² + 9²) = 0.8716; width = 100 × 0.0254 × 0.8716 = 221 cm; height = 125 cm.
Throw distance = 1.36 × 2.214 = 3.01 m (9.9 ft).
Farthest with zoom = 1.64 × 2.214 = 3.63 m.
From 3.5 m: largest screen = 3.5 ÷ 1.36 ÷ 0.8716 ÷ 0.0254 = 116 in; smallest = 3.5 ÷ 1.64 ÷ … = 96 in.

Each point is the calculator re-run with only “Screen diagonal (in)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Screen diagonal (in)	Throw distance, lens to screen (m)
25	0.75
50	1.51
75	2.26
100	3.01
125	3.76
150	4.52
200	6.02

How to read the result

Place the projector between the shortest and longest distances for your screen size. Distances are measured from the lens, not the back of the projector, so allow for its depth and any cables. Lens shift and keystone settings change the picture position and quality; check the manufacturer's distance chart before fixing a mount.

Common mistakes

Using the diagonal instead of the width with the throw ratio.
Measuring from the wall behind the projector instead of the lens.
Ignoring the aspect ratio of the screen.
Relying on keystone correction, which can soften the picture.

Keep in mind: Mount projectors securely to suitable structures, and follow the maker's instructions for ventilation and fixing.

Frequently asked questions

How far should a projector be from a 100 in screen?

About 3.0 m with a 1.36 throw ratio, up to 3.6 m at the tele end of a 1.36–1.64 zoom.

What is throw ratio?

Throw distance divided by picture width.

How wide is a 100 in 16:9 screen?

About 221 cm (87 in).

Sources: BenQ Knowledge Center, "What is throw ratio for projectors" · NIST Special Publication 811 (2008), Appendix B (1 in = 0.0254 m)

Checked 9 Nov 2026. Throw ratio = throw distance ÷ projected image width, as used in projector specifications (e.g. BenQ Knowledge Center, “What is throw ratio for projectors”); screen geometry from the diagonal and aspect ratio; 1 in = 0.0254 m and 1 ft = 0.3048 m exactly (NIST SP 811, 2008).

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