A standard roll holds around 35 square feet and gives you roughly 27 once pattern matching has taken its cut. Working from the first number instead of the second is why people run out.
Paint covers whatever area you give it. Wallpaper hangs in vertical strips of fixed width, cut to the wall height, and every strip has to line up with the one beside it.
That means you are not buying area. You are buying strips, and the useful question is how many full-height strips you need and how many the roll yields.
A standard roll is about 35 square feet, and by the time pattern matching and trimming have taken their share you are typically getting around 27 usable square feet from it. That is a loss of roughly a quarter, and it is invisible if you simply divide wall area by roll area.
Measure the wall height, add a few inches at top and bottom for trimming, and that is your strip length. Divide the roll length by the strip length to get strips per roll, rounding down. Then divide the total wall width by the paper width to get how many strips you need. This method survives pattern repeats. The area method does not.
A plain paper with no pattern loses very little. Every strip is cut to the same length and there is nothing to align.
A patterned paper has a repeat, the vertical distance before the design starts again. To line the pattern up across a seam, each strip has to start at the right point in the design, so you trim off whatever comes before it. The larger the repeat, the more you waste on every strip.
A large repeat can push waste well beyond the typical figure, and on some papers the offcut from every strip is longer than a foot. The repeat is printed on the roll label and it is worth finding before calculating anything, because it changes the answer materially.
Some patterns are drop-matched rather than straight-matched, meaning alternate strips start at a different point in the design. These waste more again, though offcuts can sometimes be used at the top of the next strip.
Wallpaper is sold in confusingly inconsistent units.
A single roll is commonly around 29 to 31 square feet. A double roll, which is what most retailers actually ship, is around 57 to 61 square feet. Prices are frequently quoted per single roll while the product ships as a double, which means the quantity you order may be half what you expect.
Check what unit you are buying in before ordering. This is the single most common ordering error with wallpaper and it produces a shortfall of exactly half.
Measure the width of each wall separately rather than taking the room perimeter, because strips do not run around corners. Each wall is planned on its own.
Measure the height at several points. Older houses have floors and ceilings that are not parallel, and your strip length has to suit the tallest point on that wall or you will be short at one end.
Add a few inches to the strip length at both top and bottom for trimming. Papering is done with a slight excess that is cut off against the ceiling line and the baseboard once the strip is positioned.
For openings, most decorators do not subtract small windows and doors from the count, because a strip interrupted by a window still has to be cut from a full length and the offcut rarely fits anywhere useful. Large openings, or a wall that is mostly glazing, are worth deducting.
Wallpaper is printed in batches, and batch numbers matter more here than with almost any other material. Two rolls from different batches can differ subtly in colour, and on adjacent strips that difference is obvious.
Order everything in one go, check that all rolls carry the same batch number when they arrive, and buy one more roll than your calculation says. A spare roll from the correct batch is worth having for repairs years later, and it costs far less than rehanging a wall.
Paint works on completely different arithmetic, and the general wall measuring method is here.
If the question is really how to choose a laser measure rather than this specific tool, our front page is the better starting point.