How to measure square footage

1. Decide what you are measuring for. Flooring, paint and sod all want different numbers from the same room, and the finished-area figure a realtor uses is different again. Settle this before you pick up the tape or you will measure twice.

2. Measure at the floor, wall to wall. Hook the tape on the baseboard face at one wall and pull to the opposite baseboard. Read to the nearest inch. Do not measure at chair height across a room with a bay or a chimney breast — you will miss the bump.

3. Convert inches to decimal feet, not fractions. 14 ft 7 in is 14.583 ft, because 7 divided by 12 is 0.583. Do the division once, write it down, and stop trying to multiply mixed fractions in your head.

4. Multiply length by width per rectangle. 14.583 x 11.25 = 164.06 sq ft. Round at the end, never partway through.

5. Split anything irregular at the inside corner. An L-shaped room is two rectangles. Draw the split line, measure both, add. The trap: after splitting, one rectangle's dimension is the *remaining* length, not the full outside wall. In the drawing below, B runs the full 24 ft, so A is only 18 ft, not 24.

6. Add the rooms, not the whole footprint. Interior walls, stud cavities and chases eat 5 to 8 percent of a floor plate. Measuring outside dimensions and calling it living area overstates by roughly that much.

7. Add waste last, and only for materials. Flooring gets 10 percent for straight lay, 15 for diagonal or herringbone, and more on a cut-up plan. That number belongs on the order, not in the square footage you report to anyone.

Triangle: half of base times height, where height is the perpendicular distance to the point, not the length of a sloping side. A triangular dormer alcove 8 ft across the base and 5 ft deep is 20 sq ft.

Circle: 3.1416 times the radius squared. Radius, not diameter — the single most common error in this whole subject. A 16 ft round patio is 8 x 8 x 3.1416 = 201 sq ft, not 804.

Half-round bay: treat it as half a circle and add it to the rectangle behind it.

Trapezoid: average the two parallel sides, multiply by the distance between them.

Curves you cannot name: grid it. Chalk or tape a 1 ft grid on the floor, count whole squares, count the partials as a half each. It is crude and it lands within two percent, which is closer than most tape work.

ANSI Z765 is the standard most appraisers work to for detached houses. Under it, finished area is space that is heated or cooled by a permanent system, finished with walls, floor and ceiling in materials matching the rest of the house, and reachable from other finished area without going outside or through an unfinished space.

Counted: - Finished rooms with permanent heat, including a finished attic with a stair - Stair openings, counted on the floor the stair descends from - Interior wall thickness within the measured envelope

Not counted: - Garages, always. Convert one and it still does not count until it is heated, finished, and usually permitted. - Unfinished basements. Finished below-grade space is real, but ANSI reports it separately as below-grade finished area — never folded into the above-grade number. - Open two-storey spaces. A vaulted great room's volume is counted once, at the floor. - Covered porches, screen rooms and unheated Florida rooms. - Any area with a ceiling under 5 ft. Under a sloped ceiling, you count only floor where the ceiling is 7 ft or higher, and at least half the finished area must have that height for the room to count at all.

Detached space with its own roof and a walk across the yard is not part of the house. It gets its own line, whatever the listing says.

A homeowner measures 2,180. The tax card says 2,046. The old listing said 2,340. Nobody is lying.

The assessor measures the outside of the house from the sidewalk or from an aerial, on a cycle that might be seven years old, and the county's own rules on what counts vary by state. That number exists to allocate tax, not to describe your floors.

The appraiser measures outside dimensions per storey, ANSI method, excludes below-grade, and their number is the one that moves a mortgage.

The agent may have taken the builder's plan figure — which is measured to the outside face of sheathing and includes the wall thickness the appraiser will hold to a different rule — or added a finished basement straight into the headline.

If you are pricing a house, use the appraiser's method and say which method you used. If you are buying flooring, none of this matters and you want the wall-to-wall interior figure plus waste.

Living room 18 ft 4 in x 14 ft 0 in. 18.333 x 14 = 256.7. Kitchen and dining, L-shaped: A is 12 x 10 = 120, B is 16 x 11 = 176. 296. Hall 4 ft x 19 ft = 76. Powder room 5 x 7 = 35. Study 11 ft 6 in x 10 ft 9 in. 11.5 x 10.75 = 123.6.

Total 787 sq ft of measured room area. Add roughly 6 percent for interior partitions if you are reconciling against an exterior-measured figure and you land near 834 — which is why the tax card and your tape never agree even when both are careful.

For flooring in that same space, ignore the partition allowance and order to the 787 plus 10 percent: 866 sq ft, or 39 boxes at 22.4 sq ft.

• 1 square yard = 9 square feet. Carpet is still quoted per yard in places, and dividing by 3 instead of 9 is a mistake I have watched a supplier make.
• 1 square (roofing and siding) = 100 square feet.
• 1 square metre = 10.764 square feet.
• Square inches to square feet: divide by 144, not 12. Tile schedules live in square inches and this catches people constantly.

And the one that is not a conversion at all: linear feet are not square feet. Trim, gutter and fence are sold by the running foot. If a supplier asks for linear feet, walk the perimeter, don't multiply anything.