Is projector throw distance measured from the wall or lens?

It is measured from the projector's manufacturer-defined lens reference point to the image plane, not automatically from the wall, mount, or rear of the cabinet. Read the installation diagram for the exact model. Lens position inside the chassis varies, so using cabinet depth can move the planned image size or mount location enough to matter.

Why is screen width used instead of diagonal?

Throw ratio describes the relationship between projection distance and horizontal image width. A diagonal does not uniquely specify width unless the aspect ratio is also known. Convert the chosen diagonal and aspect ratio to active width first, excluding borders, then apply the ratio. Changing aspect ratio can change width even when the marketed diagonal stays the same.

What does a projector throw-ratio range mean?

It usually indicates the image-size range available through the optical zoom at a given distance, or the distance range for a chosen width. The lower ratio creates a wider image at the same distance. Verify the exact model because zoom position may also affect light output, lens-shift availability, sharpness, and installation margin.

Projector Planning

How to Calculate Projector Throw Ratio

A Beam & Backwall guide · Updated 2026-07-19 · 4 min read

Quick answerCalculate projector throw ratio as lens-to-screen distance divided by active image width. To find width, divide throw distance by throw ratio. To find distance, multiply image width by throw ratio. Keep all lengths in the same unit, use image width rather than diagonal, and follow the manufacturer's lens reference point. For a zoom lens, calculate both ends of the official ratio range, then verify placement, tolerances, offset, lens shift, and geometry with the exact manual or installation calculator before mounting.

Contents

Use image width, not diagonal
Define all three variables
Calculate from a known room distance
Calculate placement from a chosen image width
Keep aspect ratio in the conversion
Account for zoom and special lenses
Record the real image plane
Verify vertical geometry separately

Use image width, not diagonal

Projector throw ratio equals lens-to-screen throw distance divided by image width. Rearranged, image width equals throw distance divided by throw ratio, and throw distance equals image width multiplied by throw ratio. Use one unit throughout and the active image width—not the diagonal. Epson's EB-PU2220-series specifications label throw ratio as projection distance divided by screen width and show that lens-specific ratios and distance formulas differ.

Use active image width in the calculation. A diagonal measurement cannot be substituted without converting it to width for the intended aspect ratio.

Define all three variables

Throw distance: the distance from the projector's specified lens reference point to the image plane.
Image width: the horizontal width of the projected picture, excluding a screen border.
Throw ratio: a dimensionless relationship, often given as one value for fixed lenses or a range for optical zoom lenses.

Check the manual because the manufacturer may define the measurement reference and tolerances. Cabinet depth and mount location do not substitute for lens position.

Calculate from a known room distance

When placement is fixed, divide the measured throw distance by each end of the official throw-ratio range. Because width is distance divided by ratio, the lower ratio produces a wider image at the same distance and the higher ratio produces a narrower one.

Write units beside the distance and width before division, then confirm they match. A unit mismatch produces an invalid placement result and must be corrected before any installation decision.

Calculate placement from a chosen image width

When screen size is established, multiply the active image width by the minimum and maximum throw ratios. The results form the permitted lens-distance range for that image size, subject to the official installation calculator and other lens restrictions.

Do not place the mount at the theoretical edge without margin. Construction tolerances, lens reference, screen frame depth, mount adjustment, and desired zoom position all need room.

Keep aspect ratio in the conversion

Screen size is often marketed by diagonal, so convert the intended aspect ratio to active width before applying throw ratio. Use a trusted calculator or the Pythagorean relationship for the chosen width-to-height proportion, then verify the result against manufacturer tools.

A change from one aspect ratio to another can preserve diagonal while changing width. Throw ratio follows the width.

Account for zoom and special lenses

An optical zoom range permits several image widths at one distance. It does not guarantee equal brightness, sharpness, lens shift, or installation freedom at every point. Short-throw and ultra-short-throw products can be especially sensitive to small placement or surface changes; use their exact templates.

Compare short-throw and standard designs without transferring one model's ratio to another.

Record the real image plane

Record whether the screen surface sits forward of the wall. A framed screen can move the image plane enough that measuring only to drywall introduces a placement error.

Verify vertical geometry separately

Throw ratio answers horizontal size and distance, not image height, lens offset, vertical placement, or lens shift. Use the manual diagrams for those. A mathematically correct width can still put the image partly on the ceiling.

Before any drilling, use a temporary image or physical mock-up to check geometry, ventilation, and cable access at the planned ceiling or shelf location. Rerun every number independently. Do not drill for or adjust an overhead mount yourself. The Epson safety instructions state that ceiling or wall mounting should be performed by qualified technicians using hardware designed for the projector. The exact projector and mount manuals govern hardware, clearances, safety fittings, and installation method.

Projectors, mounts, screens, ladders, mains power, cables, lasers, lamps, heat, glass, batteries, and overhead installations can create shock, fire, fall, eye-injury, burn, or falling-object hazards. Follow the exact product manuals. Route overhead mounting, structural fixing, concealed mains wiring, and other non-user-service work to qualified local professionals. Never look into an operating projection lens or open equipment beyond user-service instructions. An independent publication. Not affiliated with any prior owner of this domain.