Length, height and the openings. Three numbers that feed paint, wallpaper, drywall, panelling and trim, each of which then does something different with them.
Length, corner to corner. Brace the tool flat against one corner and shoot the other. Take it at about waist height rather than along the floor, where the baseboard will shorten your reading by half an inch to an inch at each end.
Height, floor to ceiling. Measure it rather than assuming eight feet. Newer US construction is commonly eight, but nine and ten foot ceilings are not unusual and older properties vary widely. This number multiplies everything else, so an assumption here scales through the whole job.
Openings. The width and height of every door, window, fireplace and built-in on that wall, plus its position along the wall measured from one corner. The position matters for anything that hangs in strips or sheets, since it decides where the cuts fall.
This is the check people skip and it matters more on walls than on floors.
Take the floor to ceiling height at both ends of the wall and in the middle. In an older building these will differ, sometimes by an inch or more, because floors settle and ceilings are not always level.
For paint it makes no difference. For anything cut to length it makes a great deal: wallpaper strips, panelling, drywall sheets and trim all have to suit the tallest point or you get a gap at one end. Knowing the variation before you cut is the difference between a scribe line and a mistake.
Shoot the wall from a fixed point at floor level and again near the ceiling. A meaningful difference tells you the wall leans. For paint, irrelevant. For fitted furniture, panelling or tiling, it is the number that decides how much packing and scribing the job needs.
Paint wants total area, so perimeter times height minus large openings, then divided by a coverage rate that depends heavily on surface texture. Covered in detail here.
Wallpaper wants strip counts rather than area, because paper hangs in fixed-width vertical strips that have to pattern match. Wall width divided by paper width gives strips. Covered here.
Drywall wants sheet counts. Sheets are 4 feet wide in standard lengths of 8, 10, 12 and up to 16 feet, so the wall height determines which length wastes least. Covered here.
Trim and baseboard want linear runs rather than area, plus a count of internal and external corners because each one is a mitre. Covered here.
A laser measure is the obvious answer, and it is genuinely better than a tape for this: braced against one corner, shot at the other, no sag, no second person, no reaching.
If you have neither, there are workable approximations. A standard interior door is close to 80 inches tall and 30 to 36 inches wide, which gives you a reference to step along the wall. A sheet of drywall is exactly 4 feet wide. Your own arm span is roughly your height, which is worth calibrating once and remembering.
These are estimates, useful for deciding whether a sofa will fit or roughly how much paint to price. They are not measurements to cut material from. Covered in more detail here.
Draw the wall as a rectangle, mark the openings in roughly the right positions, and write the numbers onto it as you take them.
A wall elevation sketch takes a minute and makes the difference between a set of numbers and a plan you can order from. It also forces you to notice the things a list would let you forget: the radiator, the light switch, the boxed pipework in the corner.
Everything on LaserMeasure is built the same way, from owner reports rather than spec sheets. The front page is the summary.