Most people get a usable number out of one within thirty seconds. The gap between that and a number you can actually build from comes down to about four things, and the first one is not in any manual I have read.
Switch it on, hold the back of the tool flat against your starting surface, point the dot at the far surface, press the measure button once. The number is the distance. Everything below is about making that number reliable rather than approximate.
This is the single most common source of wrong readings, and almost nobody checks it. Your laser measure does not measure from the dot. It measures from a fixed point on its own body, and manufacturers do not agree on where that point should be.
Some measure from the front edge. Some from the back. Some let you switch between the two, and a few add a third option for measuring from a tripod thread. If the tool is set to reference from the back and you are holding the front against the wall, every reading you take is wrong by the length of the tool. That is usually somewhere between two and five inches, and it does not average out. It is the same error every single time.
Bosch ships the GLM400CL referencing from the back by default, and enough owners consider that the wrong choice that it comes up repeatedly in reviews. The LEXIVON laser and tape hybrid measures from behind its tape housing, which quietly adds roughly three inches to everything unless you know to account for it.
Look for a small icon on the display showing a rectangle with a mark at one end. That mark is your reference point. Change it with the reference button, which is usually the one nobody presses. Do this once when the tool is new and check it again after any battery change, because some units forget the setting.
The tool will give you a number almost regardless of what you do. Getting a number worth using is about removing three variables.
Brace it, do not hold it. Freehand, your hand introduces a small angle you cannot feel. Over a short span that is irrelevant. Across a forty foot room a few degrees of tilt returns the hypotenuse rather than the distance you wanted, and the number comes back longer than the truth. Press the back of the tool flat against the wall, the floor, or a door frame. If there is nothing to brace against, hold it with both hands against your chest rather than out at arm's length.
Give it something to bounce off. The tool needs light to come back. A matte white wall returns plenty. Black asphalt, polished tile, glass and anything wet return much less, and at that point the reading either fails outright or drifts. If you are measuring to a dark or shiny surface, hold a piece of white card at the far end and measure to that instead. A target plate costs a few dollars and does the same job with a magnetic back.
Take the reading more than once. Press the button five times from slightly different hand positions. If the number is identical every time, trust it. If it moves by a quarter of an inch, the tool is telling you that something in your setup is marginal, usually the angle or the target surface.
Most laser measures ship with more modes than anyone uses. Four are genuinely worth learning.
Area. Take two readings, length and width, and the tool multiplies them. This is the mode that pays for the tool if you are pricing flooring, paint or carpet, because it removes the arithmetic step where mistakes creep in.
Volume. Three readings, and you get cubic feet. Useful for concrete, mulch, gravel and anything sold by the yard.
Continuous or tracking. The tool measures repeatedly while you move, showing the number live. This is how you find a specific distance rather than measure a known one, which is what you want when positioning something or finding the centre of a wall. Most units also show minimum and maximum while tracking, which is the trick for measuring a room diagonal accurately: sweep the corner and take the minimum.
Indirect or Pythagoras. This calculates a distance you cannot shoot directly. Point at the top of a wall, then the bottom, and the tool solves the height from the two readings and the angle between them. It is how you get ceiling height over a stairwell, or a roof edge from ground level, without a ladder. It needs a steady hand and a tool with an angle sensor, and it is worth ten minutes of practice before you rely on it. Indirect measurement is covered in more detail here.
Reading through trade discussions, there is a split that never appears in manufacturer marketing. Tradespeople divide fairly cleanly into people who trust a laser enough to cut from it and people who use it only for estimating.
One electrician described their Klein as a tool they do not trust at all, but which is excellent for sizing up a job, pricing it, or working out whether a ladder will reach before hauling it off the truck. A carpenter in the same discussion said they had cut crown and base off a Bosch with no trouble at all. Both are right, and the difference is not really the tool.
It comes down to what a wrong number costs. If a bad reading means a slightly wrong quote, an occasional error is survivable and the speed is worth it. If a bad reading means a mitre cut in expensive trim, the tape comes back out. A sensible default is to use the laser for anything over about twenty five feet, where a tape sags and needs a second person, and to use a tape for anything under a few feet, where the laser has no hook to catch an edge and positioning it precisely is fiddlier than just pulling a tape.
The other thing a handheld laser measure is not is a precision instrument. Someone on a tools forum asked for one accurate to under a millimetre for machining work and got the correct answer, which is that this is not what these tools are. For that you want calipers, or a survey grade instrument at ten times the price. The accuracy question deserves its own answer, because the number on the box is not the number you get.
Nearly every handheld laser measure is a Class 2 laser. That means the beam is visible and low powered enough that your blink reflex protects you under normal circumstances. It does not mean it is harmless. Do not look into the aperture, do not point it at anyone's face, and be careful on reflective surfaces where the beam can come back at you from an angle you did not expect. It is a small risk, but it is a real one and it costs nothing to respect.
Most bad readings are technique rather than fault. In rough order of how often they turn out to be the cause: you are not square to the surface, the target is too dark or too shiny, the batteries are low enough to reduce pulse power without triggering a warning, or the reference point is set to the end you are not using.
Genuine calibration problems are rarer than people assume, and they look different. A calibration fault gives you the same wrong number repeatedly against a known distance. Technique problems give you numbers that scatter. If your readings are consistent but wrong, that is a calibration question. If they are inconsistent, work through the four causes above first. The full troubleshooting sequence is here, and calibration has its own guide.
If you would rather just be told what to buy, our best laser measures are on the front page.