A 100-inch screen sounds like a single measurement. It is not the measurement that sets projector placement. The 100 inches run diagonally across the picture; the projector's throw ratio uses the picture's width. Before choosing a screen or a shelf, work from the available wall width and the space where the lens can actually sit.
This is a planning guide, with the TOUMEI X5 as a worked example. Its Android 13 manufacturer specification lists a 16:9 picture and a 1.2:1 throw ratio. Those two entries let us estimate the optical distance for several screen sizes. They do not tell us every permissible installation position, image-size limit or result in your particular room.
Manufacturer product render of the X5. It shows the projector's appearance, not a measured throw distance, installed setup or projected-image test.
The calculation you need
Throw ratio = lens-to-screen distance ÷ projected image width.
For a projector with a 1.2:1 ratio, estimate the distance by multiplying the picture width by 1.2. Keep both measurements in the same unit. A 2-metre-wide picture, for example, suggests about 2.4 metres from the lens to the screen plane.
If your screen size is stated as a diagonal, find the width first. For a 16:9 rectangle, width is approximately 0.872 times the diagonal. The numbers below apply that geometry to the manufacturer-listed X5 ratio:
| 16:9 image diagonal | Approximate image width | Estimated lens-to-screen distance |
|---|---|---|
| 80 inches | 69.7 inches / 1.77 m | 83.7 inches / 2.13 m |
| 100 inches | 87.2 inches / 2.21 m | 104.6 inches / 2.66 m |
| 120 inches | 104.6 inches / 2.66 m | 125.5 inches / 3.19 m |
These are geometry estimates, rounded for planning. The X5 sheet does not establish that every size in the table is within an approved image-size range, that the picture will look equally bright at each size, or that a particular shelf position will work. Test the actual unit before fixing a mount or buying a custom screen.
Turn the number into a room plan
1. Mark the picture, then measure its usable width
Mark the intended image rectangle on the wall with low-tack tape, or measure the visible area of a screen you already own. Leave space for the screen frame, furniture, speakers and anything that would block the picture. If you are deciding between sizes, mark both. Sit where you normally watch and decide whether the larger frame is comfortable before designing the rest of the room around it.
2. Measure from the screen plane to a possible lens position
A room may be three metres deep but offer less usable throw distance. A sofa, doorway or passage may prevent the projector from sitting where the calculation suggests. Measure from the projection surface to the proposed lens, rather than assuming a measurement to the back of a shelf equals the optical throw. Also reserve room for the projector body, its power adapter, cable bends and ventilation. The X5 sheet lists a 19V power-adapter input; it does not document an internal battery.
For a 100-inch 16:9 image, the estimated X5 lens position is roughly 2.66 metres from the screen. If the only safe position is closer, try a smaller picture on paper before relying on digital resizing to solve the mismatch.
3. Check both picture size and placement
A calculated throw distance answers one question: approximately how wide the picture will be. It does not determine the height of the picture on the wall or whether furniture blocks the optical path. Set the projector on a stable temporary surface, display your normal source and check where all four corners land. Move the projector physically toward the screen to reduce the picture or away from it to enlarge it, provided the position remains safe and practical.
The X5 sheet lists ToF laser focus and automatic keystone correction. Those are alignment aids, not substitutes for sufficient room depth. Heavy correction or digital resizing can change the usable picture area; start with a sensible physical position, then make small adjustments.
4. Check the whole experience from your seat
Read subtitles and fine text, play both a bright and a dark scene, and listen from your usual seating position. A larger image spreads the projector's light over a larger area, while ambient light and screen material also affect what you see. The distance formula alone cannot promise image quality. If the picture looks too dim or too large for your viewing distance, test a smaller rectangle instead of assuming a specification number will compensate.
What if the room is too shallow?
First, calculate the largest width that the available lens-to-screen distance suggests: picture width ≈ distance ÷ throw ratio. With 2.0 metres of usable throw and a 1.2:1 ratio, the geometric width is about 1.67 metres, or a roughly 75-inch 16:9 diagonal. That is a planning estimate, not an X5 certified image-size claim.
If the desired screen is much wider, compare projectors with a different documented throw ratio. Ultra-short-throw placement is a separate setup problem: the TOUMEI S2 placement guide explains its close-to-screen measurements. Do not reuse the S2 ratio or furniture distances for the X5.
A pre-purchase measurement checklist
- Record the screen's usable width, aspect ratio and diagonal.
- Record the clear distance from the screen plane to each realistic lens position.
- Multiply width by the exact model's stated throw ratio; compare the answer with those positions.
- Allow room for the body, adapter, cables, airflow and people moving through the room.
- Check where the image lands vertically with the real projector before committing to furniture or a fixed screen.
- Try the intended content at the intended size and ambient light level.
- Confirm the configuration in the sales listing matches the specification used for your calculation.
When this guide fits, and when it does not
This method is useful when you have a flat screen or wall, a documented throw ratio, a 16:9 picture and enough freedom to move the projector. It is particularly useful for deciding whether an 80-, 100- or 120-inch plan is physically plausible before spending money.
It is not a complete installation drawing. An angled projection, optical-zoom range, unusual screen shape, ceiling mount, obstruction or ultra-short-throw cabinet needs model-specific installation information. A ratio alone does not establish exact vertical offset, focus range, screen limits or screen brightness. If any of those decide your purchase, request the matching manual or test the unit in the actual space.
The TOUMEI X5 product page is a starting point for checking the current retail configuration. Compare its model and version with the manufacturer's sheet and your measured room before ordering; you can also compare the projector range if your space calls for a different throw style. The right screen size is the one your room, projector and viewing position can support together.
