Length times width. That is genuinely the whole formula, and you already knew it. The reason people search for this is that the formula keeps giving them a number that turns out to be wrong.
Square feet equals length in feet multiplied by width in feet. A room 12 ft by 15 ft is 180 square feet.
The trap is mixed units, and it is responsible for more wrong answers than anything else here. A room measuring 12 ft 6 in by 15 ft 3 in is not 12.6 by 15.3. Inches are twelfths, not tenths.
Convert properly. Six inches is 6 divided by 12, which is 0.5, so 12 ft 6 in is 12.5 ft. Three inches is 0.25, so 15 ft 3 in is 15.25 ft. Multiply those and you get 190.6 square feet. Using 12.6 by 15.3 gives 192.8, which is over two square feet out on a small room and gets worse as rooms get bigger.
The clean way to avoid this entirely is to set your laser measure to decimal feet rather than feet and fractional inches before you start. The tool then hands you 12.52 and you multiply directly with nothing to convert. Changing units takes one long press.
1 in = 0.083 ft. 3 in = 0.25 ft. 6 in = 0.5 ft. 9 in = 0.75 ft. To convert square inches to square feet, divide by 144, not 12. To convert square metres to square feet, multiply by 10.764.
The room is not actually rectangular. Length times width assumes four right angles. Older houses frequently do not have them. Measure both diagonals: if they differ, the room is skewed, and your area figure is an approximation rather than a fact. Small skews are fine for material ordering and not fine for cutting a rug to size.
You measured along the floor. A reading taken at floor level often hits the baseboard rather than the wall, losing half an inch to an inch at each end. Measure at waist height instead.
You forgot the intrusions. Chimney breasts, boxed pipework, kitchen islands and built-in wardrobes all remove floor area. For material ordering people often deliberately ignore them to build in slack, which is defensible, but do it knowingly rather than by accident.
You are measuring the wrong surface. Square feet of floor, of wall, and of ceiling are three different numbers. Paint coverage needs wall area, which means perimeter times height minus openings, not floor area.
For painting or wallpapering you want wall area rather than floor area, and the method is different enough to catch people out.
Add up the length of every wall to get the perimeter, multiply by the ceiling height, then subtract the doors and windows. A standard interior door is roughly 20 square feet and a typical window 12 to 15, so on an average room the openings account for a meaningful share.
Whether to subtract them at all is a judgement call. Professional decorators often do not, because the saving is smaller than the waste factor and the extra paint is useful for touch ups. If you are pricing a large job the openings are worth subtracting. For one room they usually are not.
Less than most people assume, and knowing that saves a lot of time.
Anything you order by square footage comes with a waste factor. Straight-laid carpet, laminate or vinyl in a simple room is typically 5 to 10 percent over. Hardwood and large format tile around 10 percent. Diagonal layouts 15 percent, herringbone or chevron 15 to 17, natural stone 15 to 20 because the fitter discards pieces.
On a 200 square foot room, a 10 percent factor adds 20 square feet. Getting your measurement right to the nearest inch instead of the nearest sixteenth changes your answer by well under a square foot. The overage is the dominant term by a very wide margin.
So measure carefully enough to avoid inch-scale mistakes, which means the baseboard, the skewed walls and the alcove you nearly forgot. Then stop. Applying this to a specific room, including which intrusions to count, is covered here.
For what we recommend overall across every use case, the front page is the place to look.