Where should the projector go?
Before you buy a projector or drill holes for a ceiling mount, you need to know how far from the screen it must sit to fill it. That depends on one number from the specifications, the throw ratio, and on the size and shape of your screen.
The formulas
Throw distance = throw ratio × image width
The image width comes from the diagonal and the aspect ratio:
- Width = diagonal × r ÷ √(r² + 1)
- Height = diagonal ÷ √(r² + 1)
where r is the aspect ratio, 16 ÷ 9 = 1.778 for a standard home theater screen. The distance is measured from the front of the lens to the screen.
Projectors with a zoom lens have a range of throw ratios, such as 1.5 to 1.8, so they can sit anywhere in a range of distances.
Worked example
A 100-inch 16:9 screen and a projector with a 1.5 to 1.8 zoom:
- Image width: 87.2 inches; height: 49.0 inches.
- Closest position: 1.5 × 87.2 = 130.7 in, about 10.9 ft.
- Farthest position: 1.8 × 87.2 = 156.9 in, about 13.1 ft.
- Brightness for a dark room: about 475 lumens on a gain 1.0 screen; for a room with some light, about 1,400 lumens.
Throw ratio categories
- Long throw: above about 2, for large rooms and auditoriums.
- Standard: about 1.2 to 2, most home theater projectors.
- Short throw: about 0.4 to 1, a 100-inch image from 3 to 7 feet away.
- Ultra-short throw: below about 0.4, placed just in front of the wall.
How much brightness?
Cinemas aim for about 16 foot-lamberts (55 cd/m²) on screen. The lumens needed equal the target in foot-lamberts times the screen area in square feet, divided by the screen gain. Real projectors often deliver less than their rated lumens in their most accurate color mode, so a margin of 20 to 50% is wise. In a living room with lights on, plan on about three times more, or choose an ambient-light-rejecting screen.
Mounting tips
- Height: check the projector’s vertical offset. Most project the image above the lens axis when on a table, or below it when ceiling-mounted upside down.
- Lens shift moves the image without distortion; keystone correction fixes a tilted image digitally but costs sharpness.
- Seating: for 16:9 screens, viewers usually sit about 1.2 to 1.6 times the diagonal away.
- Cables and ventilation: plan power and HDMI runs, and leave space around the vents.
Manufacturers’ online calculators account for their exact lens offsets, so confirm the final position with the model’s manual.