Projector Throw Size: Calculate Screen Width and Distance

Projector throw size is the image width produced at a given lens-to-screen distance, or the distance needed for a chosen image width. Enter the projector’s throw ratio and either the distance or screen width to calculate the missing measurement, with results in feet and metres.

Formula

For a fixed throw ratio, image width equals lens-to-screen distance divided by throw ratio. Conversely, the required lens-to-screen distance equals image width multiplied by throw ratio.

• Throw ratio (R): the projector’s stated throw ratio, a unitless number. For example, 1.5 means the lens is 1.5 units from the screen for each unit of image width.
• Distance (D): the distance from the projector lens to the screen, in feet or metres.
• Image width (W): the horizontal width of the projected picture, in the same units as distance.

The formulas are W = D ÷ R and D = W × R. The calculator’s selection combines these into one output: choose “Find width from distance” when you know distance, or “Find distance from width” when you know width. Its second output converts the result from feet to metres.

Worked Example

Using the defaults, the throw ratio is 1.5, the lens-to-screen distance is 10 ft, and the screen image width is 6.667 ft. The selected calculation is to find width from distance.

1. Apply W = D ÷ R: 10 ft ÷ 1.5 = 6.6667 ft.
2. Round the result to two decimal places: the image width is 6.67 ft.
3. Convert feet to metres: 6.6667 × 0.3048 = 2.032 m, or 2.03 m.

The width field is not used in this selected calculation; it is included for the reverse calculation. To calculate the distance for a 6.667 ft-wide image instead, select “Find distance from width”: 6.667 × 1.5 = 10.0005 ft, which rounds to 10.00 ft.

What Throw Ratio Means

Throw ratio describes the relationship between a projector’s lens position and the width of its image. It is usually written as a ratio such as 1.5:1; the “:1” means the distance is compared with one unit of image width. For a 1.5:1 ratio, a 6 ft-wide picture requires a lens-to-screen distance of 9 ft.

Throw ratio is based on image width, not diagonal size or image height. That distinction matters because screens are commonly sold by diagonal measurement, while the basic throw formula uses horizontal width. A screen’s aspect ratio is needed to convert its diagonal into width.

Some projectors have a zoom lens with a range of throw ratios rather than one fixed ratio. A stated range, such as 1.2–1.8, means the lens can be adjusted to produce different image sizes from the same position. Use the ratio corresponding to the lens setting you plan to use; the manufacturer’s specifications or throw chart can identify the available range.

How to Calculate Screen Width or Distance

First decide which measurement is unknown. Measure distance from the projector’s lens to the screen surface—not from the back of the projector, a wall, or the ceiling mount. If the projector is not installed yet, use the planned lens position.

• Known distance, need image width: divide distance by throw ratio. A projector at 12 ft with a 1.5 ratio gives 12 ÷ 1.5 = 8 ft of image width.
• Known image width, need distance: multiply width by throw ratio. A 100-inch-wide image with a 1.5 ratio needs 150 inches, or 12.5 ft, between lens and screen.
• Convert units consistently: calculate in feet throughout or metres throughout. Convert only after the calculation if needed; 1 ft equals 0.3048 m and 1 in equals 2.54 cm.

The calculator reports a mathematical estimate, not a guarantee that every lens can produce that exact image at that position. Zoom limits, focus range, mounting geometry, and the projector’s specifications can constrain placement.

From Screen Diagonal to Width

If you know a screen’s diagonal rather than its width, use its aspect ratio first. For a rectangular screen with horizontal-to-vertical ratio a:b and diagonal S, width is W = S × a ÷ √(a² + b²). For a common 16:9 shape, width is approximately 0.872 times the diagonal. Thus a 100-inch diagonal 16:9 image is about 87.2 inches wide; at a 1.5 throw ratio, the lens distance is about 130.8 inches, or 10.9 ft.

Do not enter a 100-inch diagonal as though it were 100 inches wide. Doing so overestimates the required distance for a 16:9 screen. Also distinguish the visible image from the screen frame or casing: the formula concerns the projected picture area.

Placement Limits and Common Mistakes

A throw calculation is useful for planning, but it does not account for every installation detail. Check the projector’s throw-ratio range, lens shift, focus range, and any manufacturer placement chart before drilling or mounting. Lens shift can move the image without moving the projector, but it does not change the basic width-to-distance relationship. Digital keystone correction adjusts the picture electronically and may reduce usable image resolution; it is not a substitute for checking physical placement.

• Using the wrong distance point: measure from the lens to the screen, not the projector body.
• Mixing units: do not divide a distance in metres by a width in feet or combine inches and feet without conversion.
• Confusing diagonal and width: convert the diagonal using the screen aspect ratio before applying the throw formula.
• Assuming a range is a single value: a zoom range represents a span of possible ratios; actual placement depends on the chosen setting.
• Over-rounding: keep extra decimal places during planning and round the final dimensions to practical installation measurements.

Frequently Asked Questions

How do I calculate projector throw distance?
Multiply the desired image width by the projector’s throw ratio. For example, a 7 ft-wide image at a 1.5 ratio needs 10.5 ft from lens to screen.

How do I calculate projector image size from distance?
Divide lens-to-screen distance by throw ratio. At 12 ft and a 1.5 ratio, the projected image is 8 ft wide.

Is projector throw ratio based on screen diagonal?
No. The standard throw-ratio calculation uses horizontal image width. Convert a diagonal to width using the screen’s aspect ratio first.

What does a 1.5:1 projector throw ratio mean?
It means the lens-to-screen distance is 1.5 times the image width. A 10 ft distance therefore corresponds to a 6.67 ft-wide image, if the lens can produce that size.

Can I use the same throw ratio for any projector?
No. Throw ratio is specific to a projector lens and may be fixed or adjustable within a range. Check the specifications for the exact model and lens setting.

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in ratio | Throw ratio | - | 1.5 | 0
in dist | Lens-to-screen distance | ft | 10 | 0
in width | Screen image width | ft | 6.667 | 0
sel calc | Calculate | Find width from distance=1, Find distance from width=1.5
out size | Image width or required distance | ft | (dist / ratio) * (2 - calc) + (width * ratio) * (calc - 1) | 2
out metric | Matching measurement | m | size * 0.3048 | 2