How throw distance works
Every projector lens has a throw ratio: the distance from the lens to the screen divided by the width of the image. A projector with a 1.5 throw ratio must be 15 feet away to create a 10-foot-wide picture. Zoom lenses have a range, such as 1.39 – 2.09, which gives you flexibility when mounting. Short-throw models have ratios around 0.4 – 0.8, and ultra-short-throw (UST) projectors under 0.4 sit just inches from the wall.
Throw distance = image width × throw ratio
Screen size from the diagonal
Screens are sold by diagonal, but the throw ratio uses width. For a 16:9 screen the width is about 0.87 × the diagonal; a 100" 16:9 screen is 87" wide and 49" high. ThrowMate converts any diagonal and aspect ratio automatically and also shows the maximum image your room depth allows.
How many lumens do you need?
Brightness requirements rise with screen size and room light. A projector's ANSI lumen rating is spread across the whole screen, so doubling the screen area halves the brightness on it. The calculator multiplies screen area by a target that depends on your lighting and divides by screen gain.
| Room | 100" 16:9 screen | 120" 16:9 screen |
|---|
| Dark home theater | 1,000 – 1,500 lm | 1,500 – 2,000 lm |
| Dim meeting room | 2,000 lm | 2,500 – 3,000 lm |
| Lit office / classroom | 3,000 – 3,500 lm | 4,500 – 5,000 lm |
| Bright room, windows | 5,000 lm + | 6,000 – 7,000 lm + |
Mounting tips
- Center the lens horizontally on the screen. If you can't, choose a projector with lens shift rather than relying on digital keystone, which reduces sharpness.
- Vertical offset: most ceiling-mounted projectors expect the lens slightly above the top of the image. Check the offset value in the manual.
- Screen height: in meeting rooms place the bottom edge 36 – 48 in from the floor so heads don't block it; in theaters, 24 – 36 in.
- Cabling: run HDMI (or HDBaseT for runs over 15 m) and power to the mount before the ceiling is closed.
- Noise and heat: keep the projector away from seated listeners and leave ventilation space.
Projector or flat-panel display?
For meeting rooms up to about 86 – 98 inches, interactive or commercial flat panels are brighter, maintenance-free and readable with lights on. Projectors win above 100 inches, in classrooms and auditoriums, and wherever cost per inch matters. Laser light sources last 20,000+ hours without lamp replacements, making projectors far easier to maintain than older lamp models.
Viewing distance
For presentations, the AVIXA guideline is that the farthest viewer should be no more than about six times the image height away to read normal text; for detailed spreadsheets, four times. For movies, a comfortable seating distance is about 1.5 – 2.5 times the image width for a 4K picture.
Frequently asked questions
How far should a projector be from a 100 inch screen?
With a common 1.5 throw ratio, about 10.9 ft (3.3 m) for a 16:9 100" screen. Multiply the image width (87 in) by your throw ratio.
What is a short-throw projector?
A projector with a throw ratio below about 0.8. It creates a large image from a few feet away, avoiding shadows in classrooms and small rooms.
How many lumens do I need for a conference room?
For a 100 – 120 inch screen with normal office lights, plan on 3,500 – 5,000 ANSI lumens.
Can I use a wall instead of a screen?
Yes, but a smooth white or gray wall usually has a lower gain than a screen, so you need more lumens for the same brightness.
What does lens shift do?
Lens shift moves the image optically up, down or sideways without distortion, making ceiling and off-center mounting much easier than keystone correction.
About ThrowMate
ThrowMate is an independent, free calculator for planning business, education and home projector installations. It uses standard throw-ratio geometry and common brightness guidelines. We are not affiliated with any projector or screen manufacturer.
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The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.
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Last updated: October 28, 2026
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