Laser level or spirit level: what accuracy really changes
What does a spirit level actually measure?
A slope, over the length of its body. A 60 cm level tells you whether those 60 cm are horizontal; it says nothing about what happens three metres further on. Its reach is the reach of its own body, not a metre more.
To cover a longer distance you “walk” the level: move it one body length and start again. Every transfer adds its own error, and those errors add up instead of cancelling out.
Why does the error pile up?
Because a transfer is an operation: you set the level on the previous mark, with a small imprecision in placing it and a small one in reading it. Over one transfer that is negligible. Over ten it becomes visible. Ten transfers are ten times two imprecisions.
| Method | Over 1 m | Over 4 m | Over 10 m |
|---|---|---|---|
| 60 cm spirit level, successive transfers | Excellent | Depends on the number of transfers | Not reliable |
| 2 m straightedge | Excellent | Good | Transfers still needed |
| Laser level | Excellent | Excellent | Stated error, with no transfer |
The laser has no transfer: the reference is thrown in one piece. Its error is the instrument's, proportional to distance, and known in advance — on our 12-line as on the 16-line, it reads in millimetres for a given distance.
From what point does the laser become necessary?
Four thresholds: under a metre the spirit level is quicker and enough; between one and three metres a straight, checked straightedge does the job; beyond three metres the laser becomes clearly safer; and as soon as the same reference is needed in several places, it has no rival left.
- Under a metre: the spirit level is quicker, and enough. Getting a laser out for a shelf is a waste of time.
- Between one and three metres: a straightedge does the job, provided it is straight and has been checked.
- Beyond three metres: the laser becomes markedly safer, and above all quicker.
- As soon as you need the same reference in several places: the laser has no rival, whatever the distance.
Is the spirit level finished, then?
No, and it stays in the box for three reasons. Its battery never runs out. It sits on the object itself, which a laser does not do: you check a washing machine, a worktop, a stud, by laying it on them. And it serves to check the laser, when you have doubts.
The two tools do not replace each other: one measures an object, the other throws a reference.
How do you tell whether a spirit level is true?
Set it on a roughly horizontal surface, note where the bubble sits, then turn it end for end in the same place. The bubble should sit in the same place. If it moves, the real error is half the movement you see.
That is exactly the reasoning of the reversal test applied to a laser level: the error reverses when you turn the instrument round, and comparing the two reveals it.
And a laser's stated accuracy?
It is given in millimetres at ten metres and scales in proportion: an instrument stated at 3 mm at 10 m drifts in theory by 1.2 mm at four metres. In a room at home that is under a millimetre and a half — beyond what laying tiles or a stud wall demands.
The criteria in detail are in how to choose a line laser, and the full section in laser levels.
- The spirit level measures the length of its body, no further.
- Every transfer adds its error; the laser has no transfer.
- Beyond three metres, the laser becomes the sensible choice.
- Both are checked with the same reversal test.