A room does not have one square footage. It has several, and which one is correct depends entirely on why you are asking. Nearly every guide skips this, which is why people end up ordering the wrong amount.
Take a 12 by 15 bedroom with a chimney breast projecting 12 inches across a 4 foot width, and a 2 by 5 foot closet off one side.
Same room, same tool, three legitimate numbers. The mistake is not measuring badly. It is measuring accurately for the wrong purpose.
Whatever the purpose, start the same way. Measure length at both ends of the room and width at both sides, at about waist height rather than along the floor. Take both diagonals to confirm the room is actually square. If the two lengths differ, use the larger for material and the average for records.
Multiply length by width for the base rectangle. Then adjust for what is actually in the room, which is where the purpose comes in.
Chimney breasts and boxed pipework. Subtract for hard flooring, where you genuinely cut around them. Do not subtract for carpet, where the roll width governs. Do not bother subtracting for paint, where the wall area is what matters.
Closets and wardrobes. Add if they are being floored and are open to the room. A built-in wardrobe with a solid base is not being floored, so it comes off. Walk-in closets are usually measured and quoted separately.
Bay windows and alcoves. Add them, measured as their own rectangle or trapezoid. They are frequently forgotten because you measure the main room wall to wall and the bay sits outside that line.
Kitchen islands and fixed units. Subtract for flooring if the units sit on the subfloor, which is common in fitted kitchens. Do not subtract if flooring runs underneath, which is also common. This one genuinely depends on the install and is worth asking about rather than assuming.
Doorway thresholds. A small thing that adds up across a house. Flooring usually runs into the doorway to the centre of the door. On one room it is negligible. On a whole floor it is several square feet.
For material ordering, an over-measure costs you a little money and an under-measure costs you a second delivery, a dye lot mismatch and a delay. The asymmetry is severe, which is why the industry builds in a waste factor rather than measuring more precisely.
Loft conversions and attic rooms need a decision about where the room stops.
For flooring, the floor is the floor and the slope is irrelevant. Measure the full footprint.
For anything describing the room as living space, the convention in most standards is to count only floor area under a ceiling height above a stated minimum, commonly around seven feet, with a reduced allowance for the area beneath. Standards differ by country and by purpose, so if the number is going into a listing or a valuation, use whichever standard your local market expects rather than a generic one. Whole-house measurement is a separate discipline with its own conventions.
Once you have the area, add the overage before ordering. Typical industry guidance runs 5 to 10 percent for straight-laid carpet, laminate or vinyl in a simple rectangular room, around 10 percent for hardwood and large format tile, 15 percent for diagonal layouts and rooms with multiple angles, 15 to 17 percent for herringbone or chevron, and 15 to 20 percent for natural stone because the fitter culls pieces.
Wide plank hardwood, meaning 7 inches or wider, commonly adds another 3 percent on top, because the offcuts are too large to reuse elsewhere.
Worth noticing what this means for your measuring effort. On a 180 square foot room, a 10 percent factor is 18 square feet. Measuring to the nearest sixteenth rather than the nearest inch changes your figure by well under one square foot. Spend the effort on remembering the bay window, not on the fraction. The underlying square footage method is here, and awkward shapes have their own guide.
If you have not settled on a unit yet, the laser measures we recommend are gathered on our front page, eight of them, each with the reason it is there.