How to Calculate Projector Throw Distance: How Far for 100 Inches, Reading the Throw Ratio, Screen Size for Your Room and Mounting Height

The short answer: throw distance = throw ratio × image width, so 100 inches needs about 2.66 m at a 1.2 throw ratio

How big the image gets at a given distance depends on the projector's throw ratio, which is fixed for each model (or for each end of its zoom). The throw ratio is throw distance ÷ image width. With a throw ratio of 1.2, every 1.2 m (or 1.2 ft) between lens and screen gives you 1 m (or 1 ft) of image width. So: throw distance = throw ratio × image width.

A 16:9 100-inch image is about 221 cm (87.2 in) wide, so a projector with a 1.2 throw ratio needs 221 × 1.2 ≈ 266 cm, and one with a 1.5 throw ratio needs about 332 cm. Here are the distances for popular sizes and common throw ratios.

Getting image width and height from the diagonal (16:9, 16:10, 4:3, 21:9)

A projector's "100 inches" is the diagonal of the image, just like a TV size. 100 inches is 254 cm diagonally, but the throw calculation needs the width and the mounting calculation needs the height. For an aspect ratio a:b, width = diagonal × a ÷ √(a² + b²) and height = diagonal × b ÷ √(a² + b²). For 16:9 the width is about 0.872 × the diagonal and the height about 0.490 ×.

At the same "100 inches", 4:3 is about 18 cm narrower and 27 cm taller than 16:9. If you put a 16:9 projector on a 4:3 meeting room screen you get black bars at the top and bottom, and the usable image is smaller, so match the screen's aspect ratio to the projector's picture. Inch-to-centimeter conversion itself is covered in how to convert inches to cm.

What is throw ratio? Standard, short throw and ultra short throw compared

Throw ratio describes the lens's angle of view as a ratio of distance to width. The smaller the number, the wider the lens and the closer you can be for a large image; the larger the number, the farther back the projector has to go. Typical values by type are below, but models vary, so use the figure from your spec sheet in the end.

With optical zoom you can hit the same image size from anywhere between the wide end (smaller throw ratio) and the tele end (larger throw ratio). A 1.4–2.2 projector can fill 100 inches from anywhere between 3.10 m and 4.87 m. Without zoom, size and distance are locked together: once you decide on the shelf, the size is decided too. Digital zoom can shrink the image, but it throws away pixels and softens the picture.

Throw distance chart from 40 to 150 inches

Distances for a 16:9 image at common throw ratios, measured from lens to screen. A zoom projector can produce that size anywhere between its wide-end and tele-end columns.

The chart shows that even a 1.2 throw ratio projector needs almost 3.2 m (10 ft 6 in) for 120 inches, and a 1.5 model needs about 4 m (13 ft). In a room about 3.5 m (11 ft 6 in) long, once you allow for the depth of the projector and its cables, 120 inches is out of reach for most home projectors. If you want a big picture in a small room, a short throw model at around 0.5 puts the same 120 inches on the wall from 1.33 m (4 ft 4 in).

Working backwards: what size do you get from the distance you have?

When the projector's spot is already fixed, flip the formula: image width = throw distance ÷ throw ratio, then, for 16:9, inches = width in cm ÷ 0.872 ÷ 2.54 (or width in inches ÷ 0.872). For example, a 1.2 throw ratio projector on a shelf 2.5 m (8 ft 2 in) from the wall gives a width of 250 ÷ 1.2 ≈ 208 cm, which is about 94 inches.

On a zoom projector, the wide-end throw ratio gives the largest size at that distance and the tele-end ratio gives the smallest. A 1.2–1.5 projector placed 3 m back can be adjusted anywhere from about 90 to 113 inches. Even when the math says 200+ inches, as in the short throw column, wall width, ceiling height and brightness cap the size in practice, so check mounting height and room size next.

Image height for table and ceiling mounting, and what offset means

Even with the right distance, an image that sits too high or too low is uncomfortable to watch. The vertical position is set by the offset. Most projectors are designed to throw the image above the center of the lens, so a projector on a low table doesn't aim at the floor, and when mounted upside down on the ceiling the image lands below the lens.

Offset is the distance from the center of the lens to the far edge of the image, as a percentage of the image height. At 100%, the bottom of the image sits exactly at lens height when the projector is on a table. At 110% the bottom edge is 10% of the image height above the lens, and at 50% the lens is level with the middle of the image. Manufacturers express this differently, so the diagram in the spec sheet showing the lens center relative to the image is the most reliable reference.

Seated eye height is around 100 cm (3 ft 3 in). With the lens on a table at 60 cm, the middle of a 100-inch image is about 122 cm high, so you look up only slightly. Ceiling mounting puts the top of the image near the ceiling, so at 120 inches and above the bottom edge drops below 70 cm and is easily hidden by the back of the sofa or the heads of people in front.

How big can you go in a small or medium room?

The largest image that fits a room is the smallest of three limits: (1) throw distance, the size you can reach from the room's depth minus about 30 cm for the projector body and cables; (2) wall width, the screen wall's width minus side margins; and (3) ceiling height, the ceiling minus the height of the image's bottom edge and a margin at the top.

The calculator also accepts room sizes in Japanese tatami mats, but any room works the same way. The table below uses a 3.52 × 2.64 m room (11 ft 7 in × 8 ft 8 in, about 9.3 m², a Japanese 6-tatami room) and a 3.52 × 3.52 m room (11 ft 7 in square, about 12.4 m², 8 tatami), with a 240 cm (7 ft 10 in) ceiling, the image bottom 50 cm above the floor, 30 cm behind the lens, 10 cm on each side and 10 cm at the top (16:9).

Projecting along the long side of the 3.52 × 2.64 m room, a 1.2 throw ratio reaches about 121 inches by distance, but the 2.64 m end wall only fits about 110 inches, so the real limit is around 110. A 100-inch screen in a room this size is entirely realistic with a 1.2-ish projector at the back of the room. Projecting across the short side, or using a 1.5 model, drops the limit into the 70s–80s, so decide where the projector goes before choosing a model.

Viewing distance: what size suits how far away you sit

Bigger is more exciting, but sitting too close means turning your head to see the edges and missing subtitles or game HUDs. Viewing distance guidelines are based either on the horizontal angle the screen fills in your field of view or on a multiple of the image height.

Working from your seat instead: from a sofa 2.5 m (8 ft 2 in) away, SMPTE suggests about 61 inches, THX about 73, the THX maximum about 82 and the Full HD guideline about 67. From 3 m (9 ft 10 in), it is about 73 inches for SMPTE, 88 for THX and 99 for the THX maximum. Choose SMPTE for relaxed film watching and lean toward the THX maximum for games and impact. The 4K guideline is the distance at which pixels stay invisible even up close, so with a 4K projector the THX distances are fine.

How many lumens do you need? Why bigger images get darker

A projector's light output (lumens) is fixed, so a bigger image spreads the same light over a larger area and looks dimmer. A 100-inch image has about 1.56 times the area of an 80-inch one, so it is only about 64% as bright. You can estimate image brightness in cd/m² (nits) as lumens × screen gain ÷ (π × image area in m²). Projectors rarely deliver their full rated output, so the figures below assume 80% reaches the screen.

Rough targets by room: about 50 cd/m² in a fully dark room at night with blackout curtains, 100 cd/m² with only indirect lighting, 200 cd/m² with the lights on, and 300 cd/m² in a bright living room or meeting room by day. For 100 inches in a dim room (100 cd/m²) you need about 1,100 ANSI lumens; with the lights on, a bit over 2,000. If you want a big picture from a small mobile projector, plan on darkening the room.

How to choose screen size and projector placement without regrets

1. 1

   Decide where you sit and which wall gets the image

   Fix the sofa or chair position and the screen wall first. From the viewing distance, work out the comfortable size range using the SMPTE and THX guidelines.

2. 2

   Check the largest size the room allows

   The smallest of throw distance, wall width and ceiling height sets the limit. Also check that curtain rails, the air conditioner and lights won't be in the picture.

3. 3

   Work out the throw ratio you need from where the projector can go

   Once you know the shelf, stand or ceiling position, throw ratio = throw distance ÷ image width tells you what to look for. For 100 inches (221 cm wide) from 2.3 m, you need a throw ratio of 1.04 or less.

4. 4

   Check the top and bottom of the image with height and offset

   Use lens height and offset to find the bottom and top edges, and make sure the image stays clear of the ceiling, floor and furniture. If it doesn't, change the stand height or consider a model with lens shift.

5. 5

   Make sure the lumens suit your room's lighting

   Check that the ANSI lumens are enough for your chosen size and room lighting. If not, go a little smaller or plan to darken the room.

The Projector Throw Distance Calculator lets you check all five at once just by picking a throw ratio and aspect ratio. You can switch between size from distance and distance from size, a side view for table and ceiling mounting, the largest size from your room dimensions, viewing distance, brightness from lumens and a 40–150 inch chart on the same page.

FAQ

How far does a projector need to be for a 100-inch screen?

It depends on the throw ratio. A 16:9 100-inch image is about 221 cm (87.2 in) wide, so you need about 2.66 m (8 ft 9 in) at a 1.2 throw ratio, 3.32 m (10 ft 11 in) at 1.5, 1.11 m (3 ft 8 in) for a 0.5 short throw and about 55 cm (1 ft 10 in) for a 0.25 ultra short throw. Distance is measured from the lens to the screen.

What is throw ratio?

Throw distance divided by image width. At 1.2, each 1.2 m of distance gives 1 m of image width. Smaller numbers mean you can project a larger image from closer. Spec sheets show it as "1.2:1" or as a range like "1.36–2.18" for zoom lenses.

Can I get a 100-inch image in a small room?

In a 3.52 × 2.64 m room (about 9.3 m²) projecting along the long side, yes, with a projector of about 1.2 throw ratio at the back of the room: distance allows about 121 inches and the end wall about 110. Projecting across the short side or using a 1.5 model limits you to the 70s–80s, so consider a short throw model.

How do I work out screen size from the distance I have?

Image width = throw distance ÷ throw ratio; for 16:9, inches = width in cm ÷ 0.872 ÷ 2.54. A 1.2 throw ratio projector 2.5 m away gives about 208 cm of width, or about 94 inches.

What does projector offset mean?

It describes where the image sits vertically relative to the lens, as a percentage of image height. At 100%, the bottom of the image is at lens height on a table, and the top of the image is at lens height when ceiling mounted. Manufacturers express it differently, so check the diagram on the spec sheet.

How high will the image be with a ceiling mount?

With 100% offset, a 240 cm ceiling and the lens at 215 cm, a 100-inch image (about 125 cm high) spans about 90–215 cm from the floor, and a 120-inch image about 66–215 cm. The bigger the image, the lower its bottom edge, and the easier it is for heads in front to block it.

What's the difference between short throw and ultra short throw?

The throw ratio. Short throw is roughly 0.4–0.8 and projects 100 inches from around 1 m. Ultra short throw is below 0.4 (usually 0.2–0.3); it sits a few tens of centimeters from the wall and projects steeply upward. Ultra short throw is very sensitive to uneven walls, so it needs a flat wall or a screen.

How many lumens do I need for a projector?

For 100 inches, about 550 ANSI lumens in a fully dark room, 1,100 in a dim room and 2,200 with the lights on (with a gain 1.0 screen and 80% of the rated output reaching it). "LED lumens" are roughly 2.4 times the ANSI figure, so convert before comparing.

Can keystone correction fix any placement?

Keystone correction fixes the shape digitally by dropping pixels, which blurs text and fine detail. Keep corrections small, place the projector level, and use a stand or lens shift to position the image.