Throw distance is the throw ratio multiplied by the image width. A 100-inch 16:10 image is 2.15 m wide, so a 1.5:1 lens must sit about 3.2 m from the screen and a 0.25:1 ultra-short-throw lens about 0.54 m. This guide shows the conversions, ready-made tables, and how zoom, lens shift and mounting height affect where a projector can go.
Key takeaways
- Throw distance = throw ratio × image width. Everything else is converting your screen size into width.
- Image width = diagonal × 0.848 for 16:10, × 0.872 for 16:9 and × 0.8 for 4:3.
- A zoom lens gives a throw-distance range; plan the mount in the middle of it, not at the limit.
- Use optical lens shift for height and side offsets — digital keystone softens text and wastes pixels.
Throw ratio in one sentence
Throw ratio is the distance from the projector lens to the screen divided by the width of the image it produces. A 1.5:1 lens needs 1.5 m of distance for every metre of image width; a 0.25:1 ultra-short-throw lens needs only 0.25 m. Manufacturers state it as a single number for fixed lenses or a range (for example 1.1–1.8:1) for zoom lenses.
- Ultra-short throw (UST): below about 0.4:1 — sits above or below the screen. See UST vs standard throw.
- Short throw: about 0.4–1.0:1 — small rooms and rear projection.
- Standard throw: about 1.0–2.0:1 — most ceiling-mounted meeting rooms and classrooms.
- Long throw: above 2.0:1 — auditoriums, houses of worship and projection booths, usually with interchangeable lenses.
The formula, step by step
- Decide the screen diagonal and aspect ratio (match your main source — most business laptops are 16:10 or 16:9).
- Convert diagonal to width: 16:10 → × 0.848, 16:9 → × 0.872, 4:3 → × 0.8. Multiply inches by 0.0254 for metres.
- Multiply the width by the throw ratio: distance = ratio × width.
- For a zoom lens, repeat with the minimum and maximum ratio to get the allowed range.
Example: a 120-inch 16:10 screen is 120 × 0.848 = 101.8 inches wide, or 2.58 m. With a 1.2–1.5:1 lens the lens must sit between 3.10 m and 3.88 m from the screen.
Throw distance table for 16:10 screens
| Diagonal | Image width × height | 0.25:1 (UST) | 0.8:1 | 1.2:1 | 1.5:1 | 2.0:1 |
|---|---|---|---|---|---|---|
| 80" | 1.72 × 1.08 m | 0.43 m | 1.38 m | 2.07 m | 2.58 m | 3.45 m |
| 100" | 2.15 × 1.35 m | 0.54 m | 1.72 m | 2.58 m | 3.23 m | 4.31 m |
| 120" | 2.58 × 1.62 m | 0.65 m | 2.07 m | 3.10 m | 3.88 m | 5.17 m |
| 150" | 3.23 × 2.02 m | 0.81 m | 2.58 m | 3.88 m | 4.85 m | 6.46 m |
| 200" | 4.31 × 2.69 m | 1.08 m | 3.45 m | 5.17 m | 6.46 m | 8.62 m |
| 300" | 6.46 × 4.04 m | 1.62 m | 5.17 m | 7.75 m | 9.69 m | 12.92 m |
Lens-to-screen distance. 16:9 images of the same diagonal are about 3% wider — add 3% to the distance.
Zoom, lens shift and keystone — what they really do
Zoom range
A 1.6× zoom lets you place the projector anywhere within a range of distances. Planning the mount in the middle of the range leaves room for a later screen change. At the long end of the zoom, many lenses lose some brightness.
Lens shift
Lens shift moves the image optically — up, down or sideways — without distorting it. It is how a ceiling-mounted projector fills a screen whose centre is lower than the lens. Each model specifies its shift as a percentage of image height or width; check it against your mounting height. The PSL-5000U and the engineering range offer horizontal and vertical shift.
Digital keystone and geometry correction
Keystone reshapes the image digitally when the projector is tilted. It is fine for small corrections, but it uses fewer pixels for the image and softens small text. Treat it as fine-tuning, not as a substitute for correct placement.
Mounting height and offset
Most projectors are designed with an offset: the image starts above (or, when ceiling-mounted upside down, below) the lens centre. A typical fixed-offset design places the bottom of the image near the lens height. To check a design:
- Find the image height (table above) and the offset percentage from the datasheet.
- Calculate where the image will sit relative to the lens at your mounting height.
- If it does not land on the screen, use lens shift, a drop pole of a different length, or a different mount point.
- Keep the bottom of the image at least 1.0–1.2 m above the floor in rooms where people sit, so heads do not block it.
Worked example: a boardroom with a ceiling beam
A boardroom needs a 120-inch 16:10 image (2.58 m wide, 1.62 m high). A beam limits the ceiling mount to 3.5 m from the screen. With a 1.1–1.8:1 zoom lens the allowed distance is 2.84–4.64 m, so 3.5 m sits comfortably inside the range. With a fixed 1.5:1 lens the projector would need to be 3.88 m away — behind the beam.
Next, height: the ceiling is 2.9 m and the screen bottom should be at 0.9 m, so the image runs from 0.9 to 2.52 m. If the lens is at 2.6 m after the mount drop, the image must sit almost entirely below the lens — check that the model's offset and lens-shift range can do this; otherwise choose a model with more vertical shift or a different drop pole.
Ultra-short-throw and rear projection
UST projectors are specified differently: the datasheet usually gives the distance from the wall or screen to the back or front of the unit for each image size, because the mirror geometry is built in. Use the manufacturer's table for the exact model, and allow for the wall arm's adjustment range.
Rear projection places the projector behind a translucent screen. It hides the projector and removes shadows but needs space behind the screen; short-throw lenses or a mirror reduce that depth. The same formula applies to calculate the throw distance; the mirror path counts as part of the distance.
Common placement mistakes
- Calculating with the diagonal instead of the width.
- Using a 16:9 width for a 16:10 screen (or the other way round) and ending up with black bars or overshoot.
- Ignoring the projector's own depth and the mount's adjustment range.
- Relying on keystone to fix a projector placed off-centre.
- Placing a zoom projector at the extreme end of its range, leaving no margin.
- Forgetting the brightness side of the equation — see the lumens guide.
Choosing between models by throw
| Room situation | Recommended throw type | Example models |
|---|---|---|
| Classroom, teacher at the board | UST 0.22–0.25:1 | PSE-3800UST, PSE-4200UST |
| Meeting room, ceiling mount mid-room | Standard zoom 1.1–1.8:1 | PSL-4500U |
| Boardroom with off-centre mount | Standard zoom + H/V lens shift | PSL-5000U |
| Auditorium / rear booth | Long-throw interchangeable lens | PSP-10000 |
Recommended models
Frequently asked questions
Questions on this topic
How far should a projector be from a 100-inch screen?
Is throw ratio measured from the lens or the back of the projector?
Can I use keystone instead of lens shift?
Does throw ratio change brightness?
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