How far back should the projector sit? Throw ratio, explained

The short version: a projector's spec sheet hides one number that decides where it can live, the throw ratio. Divide the distance by the picture width and you get it.

Most people choose a projector by resolution and brightness, then discover the box has to sit in a place the room does not have. The sofa is 3 metres from the wall, the projector wants 4. This page puts that mistake on a slider first.

The one number: throw ratio

Throw ratio is the distance from the lens to the screen divided by the width of the picture. A ratio of 1.5 means 1.5 metres back for every metre of picture width. Lower numbers sit closer to the wall; higher numbers sit further back.

Zoom lenses give a range rather than a single value, such as 1.3 to 2.0. At the low end the lens is zoomed wide and the projector sits close; at the high end it is zoomed in and it must sit further away. Fixed lenses have one number. The spec sheet may call it throw ratio, throw distance ratio or just "1.2:1", and it is the same thing.

Try it: where does it sit?

What the ratio means in a room

Take a 100-inch, 16:9 picture. That is about 2.2 metres wide. A short-throw lens at 0.7 puts the projector roughly 1.5 metres from the wall. A fixed 1.2 lens puts it near 2.7 metres, and a zoom lens that reaches 1.5 sits around 3.3 metres. Same picture, three different places for the box.

The usual bands are rough, and makers draw the lines in slightly different places. Ultra-short throw means a ratio well under 0.4, with the projector sitting on a low cabinet just under the picture. Short throw is roughly 0.4 to 1.0. Standard throw is roughly 1.0 to 2.0 and beyond, which is where most projectors in the middle of the market live.

The number does not tell you everything

• Lens shift or offset. The ratio sets distance, not height. Some projectors can move the picture up and down, or left and right, without tilting. Without it you are locked to a table height or a ceiling mount that matches the screen.
• Keystone correction. Tilting the box and fixing the trapezoid with software works, but it rescales the picture and can soften it. Placing the projector squarely is better.
• Zoom has a cost. A zoom lens is the cheapest way to gain placement freedom, but the long end of the range is usually a little dimmer than the short end.
• Brightness against size. The same lamp spread over a bigger picture looks dimmer, so a size your room's light can take matters as much as a size your wall can hold.

How to use it

1. Measure from the screen wall to where the projector will sit: a shelf, a table or the ceiling.
2. Pick the picture size from where you sit. Our piece on how big your screen should be does that part.
3. Divide the distance by the picture width. That is the throw ratio you need.
4. Look for a projector whose range covers it, and read the lens shift line while you are there.

If the picture will land on a bare wall rather than a screen, what a projection screen actually changes covers what you give up.