Point up, press the button. Five seconds, and it is a number that quietly scales half the other calculations in the house, which is why guessing it is a bad habit.
Stand the tool flat on the floor, pointing straight up, and take a reading. Set the reference point to the base of the unit so it measures from where it is standing rather than from its top edge, or the reading is short by the length of the tool. The reference point catches people constantly.
Keep it vertical. Tilt it a few degrees and you get the hypotenuse, which reads longer than the true height. Standing it on the floor rather than holding it takes care of this.
Take the reading in more than one place. Floors settle and ceilings are not always level, and in an older house a difference of an inch across a room is unremarkable.
Eight feet is common in newer US construction, so it becomes the default assumption. Nine and ten foot ceilings are not unusual, and older properties vary widely.
The problem is how many calculations this number multiplies. Wall area for paint or wallpaper is perimeter times height. Room volume for heating is floor area times height. Drywall sheet length depends on it. Curtain drops depend on it. Assume eight and find nine, and every one of those is out by more than ten percent.
Worth a moment, because there are several answers in the same room.
Finished floor to finished ceiling is the standard figure and what most jobs need.
Subfloor to ceiling matters if new flooring is going in, because the finished height drops by the thickness of the floor build-up. Measure the intended flooring thickness and subtract, or your door clearances will surprise you.
To the underside of a beam, duct or bulkhead is the constraining dimension if anything is being installed beneath it. This is the one that matters for fitted furniture, tall appliances and anything that has to pass under.
Note that measurement standards may treat these differently. ANSI Z765, used for residential square footage, requires 7 feet of finished ceiling height with an allowance down to 6 feet 4 inches under beams and ducts. Covered in the house square footage guide.
Measure at the lowest point and the highest point, and note where each is.
For volume calculations, use the average of the two. For anything being installed, use the lowest relevant point. For square footage under a standard like ANSI, the rules are specific: at least half the room must have 7 feet of height, and no floor area under 5 feet counts at all.
A loft conversion frequently measures far worse on that test than its floor area suggests, and knowing that before a valuation is better than after.
Where you cannot stand the tool beneath the ceiling, use indirect mode. Shoot the ceiling, shoot the floor, and the tool solves the height between them. This is the situation the mode exists for. Covered here.
A dark ceiling returns less light than a white one and the reading may fail or drift, particularly in a tall room. Hold a piece of white card against the ceiling if you can reach, or measure to a lighter section and account for it.
A textured or coffered ceiling gives a different reading depending on exactly where the dot lands. Aim at a flat area rather than into a recess.
Ceiling fans, light fittings and ductwork all return a reading happily, and none of them is the ceiling. Look where the dot actually is rather than where you pointed.
Wall area for paint uses this number directly, as does room volume and drywall sheet selection.
For the overview, our main laser measure guide covers the picks and the reasoning in one place.