How do I set up and use a laser level properly?

Getting a laser genuinely level and genuinely where you want it: the self-levelling range, why the tripod matters more than the laser, checking the unit against itself, and what to do when the line is invisible outdoors.

Difficulty: beginner
Time: 25 min
Read: 5 min
Safety: caution

Short answer

Set it on a firm tripod or a bracket within its self-levelling range, wait for the line to stop moving and the warning to clear, then check it by rotating the unit 180 degrees and confirming the line lands in the same place. If the line is faint, the fix is a green laser, a detector, or shade — not squinting.

A laser level is a very accurate instrument mounted on whatever you happened to stand it on, and almost every problem with one comes from the mounting rather than the laser. The other half comes from not knowing what the self-levelling mechanism is doing, which is why a unit that beeps or flashes is telling you something important rather than being annoying.

Safety

Never look into the beam or point it towards anyone's face, and do not view it through binoculars, a camera or a telescope — magnifying optics defeat the assumption that the blink reflex protects you. Domestic units are usually Class 2; anything marked Class 3R needs more care and eye protection is sensible. Switch it off when you leave the room and keep it away from children.

What you'll need

Cross-line or rotary laser level
Tripod or a wall bracket
Pencil, for marking the line at intervals
Laser detector and target card, for outdoor or long-range work (optional)
Safety glasses matched to the laser colour, for Class 3R units (optional)

Step by step

Unlock the pendulum before you start.Self-levelling units have a transport lock that clamps the pendulum so it cannot swing and damage itself. If the line appears but does not self-correct when the unit is tilted, the lock is still on.
Stand it on something that will not move.A tripod on a solid floor, or a proper wall bracket. A laser on a stepladder, a stack of boxes or a plank across two trestles will move when someone walks past, and it will not tell you that it moved.
Get it roughly level first.Self-levelling only works within a limited range, typically three or four degrees. Outside it the unit flashes, beeps or shuts the beam off. Level the tripod head by eye or with its own bubble before switching on.
Wait for it to settle.The pendulum takes a few seconds to stop swinging. A line marked while it is still settling is not level, and this is a common error when working quickly.
Check it against itself before you trust it.Mark the line on a wall, rotate the unit through 180 degrees on the tripod without moving the tripod, and compare where the line now falls. Any difference is the unit's error and it is worth knowing before you tile a bathroom from it.
Set the height to suit the job, not the tripod.The line is most useful at the height you are actually working — the top of a tile course, the underside of a shelf, the datum for kitchen units. Set the height once, mark it at several points along the wall in pencil, and you can re-establish it later even if the laser is moved.
Mark the line, do not work to the light.Put pencil crosses on the line at intervals and work to those. It frees the laser to be moved, it survives someone knocking the tripod, and it gives you a record if the job runs over more than one day.
Use the plumb and cross functions properly.A cross-line unit gives a horizontal and a vertical simultaneously, which is what makes it worth having for tiling and units. Many also project a plumb dot up and down, which transfers a point from floor to ceiling directly — useful for stud walls and light fittings.
Deal with a faint line rather than guessing.Outdoors and in bright rooms the line becomes hard or impossible to see. A green beam is several times more visible than red, laser glasses increase contrast without making the line more accurate, and a laser detector clamped to a staff picks the line up well beyond the range of the eye. On a long outdoor run the detector is the real answer.

Tips

An elevator tripod, or a pole clamped between floor and ceiling, is the accessory that makes a laser genuinely quick to use — most of the setup time is height adjustment.
Check the batteries before a long job. A dying laser dims gradually rather than switching off, so the line just becomes harder to see and you assume the room got brighter.
For a level line right round a room, the laser is the tool that does not accumulate error, because every wall is marked from the same instrument setting rather than from the previous wall.
Laser glasses do not protect your eyes. They are a contrast filter. Eye protection for a Class 3R laser is a different, specified product.

Common mistakes

Ignoring the out-of-level warning — It means the pendulum has hit the end of its travel and the line is no longer level — it is just a line. Some units keep projecting while flashing, which makes it easy to work from a sloping beam.
Standing the tripod on a floor that flexes — A chipboard floor deflects when you walk across it, moving the beam by several millimetres at the far wall. Set up in a corner, or on a solid floor, and stop walking about while marking.
Trusting a laser out of the box without checking it — Lasers survive being dropped less well than they look. The self-check by rotation takes a minute and catches a unit that has been knocked in transit.
Leaving it running and pointed at head height across a room — People walk into it. It is a low-power beam and a momentary flash is not the issue — repeated exposure and someone stopping to look at it is.

Questions people ask

Why can I not see my laser line outside?

Daylight overwhelms it. Nothing about the unit is wrong. The solutions in order of effectiveness are a laser detector on a staff, which reads the beam electronically well beyond visual range; a green beam instead of red; working in shade or at dusk; and enhancement glasses, which help a little at short range.

Is a self-levelling laser more accurate than a manual one?

In practice yes, because the levelling is done by gravity on a pendulum rather than by you and three bubbles. The specification of both is similar; the difference is that a self-levelling unit cannot be set up slightly wrong.

Can I use a laser level to set a fall?

A standard self-levelling laser gives you level, not a slope. Some units allow the pendulum to be locked so the unit can be deliberately tilted, which is how a fall is set with a laser — but you then have no automatic reference, so the slope has to be set and checked another way.