Cross line, 3D, 4D or rotary: which laser to choose
What does each family actually throw?
The difference is not a question of price bracket but of geometry. What the instrument draws in the room decides what it lets you do. A cross draws two lines, a 3D three 360° planes, a 4D four, and a rotary one plane that turns. Everything else follows from that.
| Family | What is thrown | Coverage | Detector |
|---|---|---|---|
| Cross | 1 horizontal + 1 vertical line | One section of the wall | Rarely |
| 3D | 3 planes at 360°: 1 horizontal, 2 vertical | The whole room | Optional |
| 4D | 4 planes at 360°: 2 horizontal, 2 vertical | Room and ceiling | Optional |
| Rotary | 1 spinning dot, drawing a plane | The whole site | Essential |
What does a 360° plane change in practice?
An ordinary line stops where the beam runs out of its fan angle: you have to move and reset the instrument to deal with the next wall, and every reset is a chance to introduce an error. In a four-walled room that means four set-ups instead of one.
A 360° plane goes round the room in one sweep. All four walls get the same reference, at the same moment, from one setting up. On repeated setting out — plasterboard track, tiling, framing — that is where the time is won.
Is the fourth plane worth anything?
It adds a second horizontal plane, usually higher up. Its point is to set out a ceiling without moving the instrument or changing its height: suspended ceiling, track, downlights. Without it you have to raise the instrument and build a new reference at every change of level.
It changes nothing about accuracy. On our 16-line, the manufacturer states the same accuracy of ±1.5 mm at 5 m as on the 12-line. You pay for comfort in use, not for correctness.
Why does a rotary laser reach so far?
Because it does not spread its energy out. A line laser stretches its power along the whole length of the line; a rotary concentrates it into a single dot, which it spins very fast. The eye sees a plane, but at any instant there is only one dot — and that dot is far more intense.
The direct consequence: at those distances the line cannot be seen by eye at all. The detector is not a comfort accessory, it is part of the setup. Without it, a rotary laser is of no use.
Which instrument for which job?
Four cases, four answers: a shelf, a splashback or picture frames call for a cross, which fits in a pocket; a room to tile or stud walls call for a 3D; floor and ceiling in the same visit justify a 4D; a slab, a terrace or earthworks demand a rotary with a detector.
- A shelf, a splashback, some pictures: a cross will do, and it fits in a pocket.
- A room to tile, stud walls: 3D. Three planes cover the floor, the walls and the corners.
- Floor and ceiling in the same visit: 4D. The second horizontal saves setting up again.
- A screed, a terrace, groundworks: rotary with a detector, and nothing else.
Can you buy bigger “just in case”?
A 4D does everything a cross does, so technically yes. But an instrument with four planes lit draws more, and the many lines clutter the wall when you only use one. Every serious model lets you switch off the planes you are not using: check that before you buy.
The real over-buying is elsewhere: taking a rotary for indoor work. It throws no vertical line, it wants a tripod and a detector, and it costs as much as three line lasers.
And the colour of the beam?
It is independent of the family: there are red crosses and green 4Ds. The subject has its own guide: green laser or red laser. For the rest of the criteria, see how to choose a line laser and our laser levels section.
- You choose the family on the geometry thrown, not on the price bracket.
- A 360° plane covers the room from one setting up.
- The fourth plane buys time, never accuracy.
- A rotary laser without a detector is of no use.