Laser Level Accuracy
Is laser level accuracy tolerance rating an important factor when purchasing a laser level? Well yes and no, the important thing is that they are accurate enough for the purpose you want to use it for.
In reality pretty much all lasers on the market today have a tolerance rated between 1mm to 8mm in 20m. Some manufacturers use arc seconds as their rating. This is great if you are an ace at geometry but mm per m or inches per feet is much more meaningful to the average user. For indoor work over short ranges, any rating in this band would be fine. As most of the time, you would not be able to or need to work to any better tolerances.
For long-range outdoor lasers, something within the band of 1mm to 4mm in 20m would be fine as users are likely to generate more error variance by the way receivers and staffs are used than by the tolerance of the laser itself. In the specialised high accuracy engineering environment, the tolerance of a laser becomes far more important and sometimes another device rather than a laser level may be more appropriate.
How accurate are accuracy tolerances themselves
The next points to cover; are the tolerance rating itself and whether the rating quoted has been checked post-manufacture in the country of distribution. Many high-quality brands will calibrate each and every laser level they sell. They will place a calibration certificate and sticker with date calibrated on the unit. Whether the laser maintains this accuracy depends on the quality of manufacture and how it is used and stored by the user. Also, some brands will calibrate their lasers well within their quoted tolerances. Therefore, allowing some margin for change and still, remain within tolerance. Cheaper models may well rely on the factory calibration before shipping. They may also select a rating based more on marketing edge rather than reality. This, I would suggest, is quite prevalent on internet auction sites.
So in summary, when selecting a laser level don’t get hung up on, if one model is half a mm over 20m better than another model stated tolerance. Look at the build quality, the quality of back up service, whether it comes with a local calibration certificate. Also, check over its other features and their relevance to the job you want to use it for.