Projector Throw Distance

A projector position is fixed by two numbers you can measure and one you cannot. The depth of the room and the image width are yours. The throw ratio belongs to the lens, and it has to be read from the maker.

Deciding factors

Depth of the room from the screen wall to the back wall
The image width you want, which is not the diagonal
The throw ratio of the projector, which is a property of its lens
Whether the projector sits on a table, a shelf, or the ceiling
How much light is in the room at the hour you actually watch

How to measure

Measure the depth along the center lineRun the tape from the screen wall straight back, at roughly the height the lens will sit. Doors that swing into that line matter as much as the wall behind it.
Tape the image width on the wallMark the width you actually want and leave the tape up for an evening. Wanting a wall-sized picture and living with one are different experiences.
Find where the projector can physically goA shelf has a depth, a table has a position, a ceiling has joists. Write down the lens distance each option would give you before doing any arithmetic.
Work out the ratio the room needsFeed the depth and the width into the calculator below. The output is a range of ratios, because the projector can sit at more than one distance.
Take that range to the manufacturerLook up the throw ratio for the exact model in its own documentation and see whether it falls inside your range. This is the step that cannot be done on this site.

Throw ratio your room needs

Inputs

Room depth from the screen wall to the back wall (ft)

Range 6 to 30

Image width you want on the wall (in)

Range 60 to 150

The result shown is calculated for the values above. Changing the fields needs JavaScript; the printed formula lets you run the same calculation by hand.

Result

Required throw ratio: 1.09 to 1.60
Usable lens distance: 9 ft 1 in to 13 ft 4 in

None of the projectors listed on this site states a throw ratio, so this number cannot be checked against them here. Check it against the manufacturer documentation for the exact model.

Formula

Throw ratio = lens distance ÷ image width, both in the same units
Room depth is converted to inches first: depth in inches = depth in feet × 12
Usable lens distance = depth in inches × 0.65 to depth in inches × 0.95
Required throw ratio = usable lens distance ÷ image width

Does not account for

Lens shift
Keystone correction
Ceiling mounting and the offset it introduces
Ambient light in the room at the hour you watch
Where the seats go, which is measured separately

Without buying anything

A projector is mostly moved rather than replaced. These are the adjustments to exhaust before you decide the room is wrong.

Move the projector before you change the image size. A foot of lens distance does more than any setting in the menu.
Cover the window opposite the screen wall with what you already own and watch the same scene again.
Put the projector on a stack of books at the height you are considering and check the picture lands where you want it.
Turn off the light behind the seats rather than the one beside the screen. The one behind you is usually the one in the picture.

Common errors

Common errors when measuring Projector Throw Distance
Error	Consequence	What to do
Using the diagonal where the formula wants image width	The required ratio comes out low and the projector lands too close to the wall.	Convert to image width first, or measure the width you taped on the wall.
Assuming a projector will fit because it is described as portable	The lens needs a distance the room cannot give, and keystone correction is used to hide it.	Check the throw ratio in the manufacturer documentation before buying.
Planning the lens position without the seats	The projector sits exactly where somebody has to sit, or shines past their head.	Mark the seats on the floor before deciding where the projector goes.
Filling the ratio gap with a figure from a similar model	A number nobody published is used to make a decision about a room.	Treat an unstated ratio as unknown and ask the maker for the model you own.

Before drilling

Stop

Do not look into the lens of a running projector, and keep children away from the front of it. That holds at every distance, in every room, and for every model.

Stop

A ceiling-mounted projector is held by the ceiling, not by the bracket. The rule for a wall-mounted panel applies here too: if you cannot confirm what is above the surface, stop and bring in an installer.

Note

No projector listed on this site states a throw ratio, a brightness figure, or a contrast ratio. Any comparison between them on those grounds would be invented, so none is made.