Square Footage Calculator

Length times width, and where the 144 comes from

Area is two lengths multiplied, so both must be in the same unit. The sheet opens on 12 ft 6 in by 14 ft 3 in. Six inches is half a foot and three inches a quarter, so the sum is 12.5 × 14.25 = 178.125 square feet, 19.79 square yards or 16.55 m². Type the dimensions the way you read the tape, 12' 6″ or 12 ft 6 in, and the sheet converts before it multiplies.

Measured in inches, divide by 144, not 12: dividing by 12 converts only one of the two dimensions and prints an area twelve times too big. Square yards divide square feet by 9, three by three.

In metric nothing needs converting: 3.8 by 4.35 m is 16.53 m², printed beside as 177.93 square feet. Switch units and every length you typed follows.

Six lines, four shapes, and the ones you deduct

Each line of the sheet takes its own shape. A circle uses the diameter: π × D² ÷ 4, so a 10 ft round is 78.54 sq ft. A triangle is base × height ÷ 2 with the height square to the base: 10 by 6 ft is 30. A trapezoid, the shape of a bay or a tapered lot corner, averages its two parallel sides and multiplies by the distance between them: (10 + 6) ÷ 2 × 8 = 64.

Any line from the second on can deduct instead of add. A 20 by 15 ft area with a 10 ft round bay, less a 3 by 4 ft hearth, is 300 + 78.54 − 12 = 366.54 sq ft. The plan redraws every line to scale, deducted ones dashed, and the takeoff lists each line.

Outside dimensions to the inside floor

A plan or a tape run along the siding gives the building’s outside size, and the floor you cover inside is smaller by the walls. Pick outside to inside floor and each rectangle loses the wall thickness twice in each direction: (L − 2t) × (W − 2t). The framing depth opens at 3.5 in, the minimum dressed dry width of a nominal 4 in NIST’s softwood lumber standard PS 20-25: the wide face of a 2x4. Your wall may be a 2x6 or a block wall: change it. The finish layers, drywall, sheathing and siding together, have no default, because they vary with the wall: type yours.

A 30 by 40 ft outline with framing alone is 1,159.51 sq ft inside instead of 1,200; with one more inch of finishes, 1,148.06. That is 52 sq ft the walls take. Each rectangle or circle is read as one room walled on every side; a triangle or a trapezoid counts as typed, with a caution, because offsetting its corners would be a guess.

A house, measured the way Fannie Mae applies ANSI Z765-2021

Two square footages for one house usually include different spaces. The method appraisers use is the American National Standard ANSI Z765-2021, and Fannie Mae publishes a summary of how it applies it to appraisals. This mode applies that summary and nothing else. The standard itself is a paid document: we did not open it, we link its page, and every rule below is cited from Fannie Mae’s summary.

Ceilings first. Fannie Mae writes: All finished areas must have a ceiling height of at least 7’. In a room with a sloping ceiling, at least 50% of the finished square footage of the room must have a ceiling height of at least 7’ and no portion of the finished area can have a ceiling height of less than 5’. A room that fails is not dropped: the appraiser must report this area separately as nonstandard finished area. The sheet asks each finished line for its lowest ceiling and the share of its floor at 7 ft or more. Beside a 1,200 sq ft floor, a 12 by 20 ft room under a slope that is 60 % at 7 ft with its lowest point at 5 ft 6 in counts, so the floor reports 1,440. The same room with a low side at 4 ft 6 in reports 1,200 plus 240 nonstandard; at 40 % over 7 ft, 240 nonstandard again. A flat ceiling at 6 ft 6 in has none of its floor at 7 ft, so it lands there too: 240.

Grade next: Any space that is partially or completely below-grade is required to be reported as below-grade area. A walk-out basement is below grade. Garages and unfinished space are listed on their own lines of the result. Openings are deducted lines: The standard does not allow openings to the floor below, e.g., two-story foyers, to be included in the reported square footage. Stairs go where Fannie Mae puts them: Staircases are included in the square footage of the floor from which they descend. The rounding comes last: the final square footage calculations are reported to the nearest whole square foot. The opening room as a house is 178 sq ft, not 178.125.

Put together: a 30 by 40 ft main floor, the same 30 by 40 ft upstairs with a 10 by 12 ft foyer open to the floor below, a 30 by 20 ft finished basement and a 20 by 22 ft garage report 2,280 sq ft finished above grade, 600 below grade and 440 of garage. Added together they make 3,320, a total this method never reports.

Waste, whole boxes and price

Sheathing, underlayment and pavers sell by the coverage printed on the box, bundle or pallet: the sheet asks for it and never assumes one. Your waste percentage goes first, then the count rounds up to whole boxes. The opening room with 10 % waste is 178.13 × 1.10 = 195.94 sq ft; at 20 sq ft a box that is 9.8, so 10 boxes. Type a price per square foot and the cost is on what you buy, the 200 sq ft in those boxes: at $3.49 that is $698.00. Without a box size the order line is the area with waste rounded up to the next whole square foot.

What the five pages ranking for this search do

We read the five results Google showed in the US for square footage calculator on 13 October 2026. Calculatorsoup offers thirteen shape choices, one at a time, including a wall less one window. Calculator.net has nine shapes on separate forms and no formula in its text. Ls-usa takes a width, a length and a quantity. Mcmillanfloors is a widget that adds rooms. Squarefootagecalculator.cc covers ten shapes, several rooms and a scale preview.

Counted across the five: none applies a house-measuring method or a ceiling-height rule, none takes a wall thickness off outside dimensions, and none turns a box coverage into whole boxes. This sheet does all three, with deductions on any line and a CSV export.

Rectangle  A = L x W
Circle     A = pi x D^2 / 4
Triangle   A = b x h / 2
Trapezoid  A = (a + b) / 2 x h
Net        N = sum(added) - sum(deducted)
Inches     sq ft = sq in / 144
Inside     (L - 2t) x (W - 2t)     t = framing + finishes
Order      ceil( N x (1 + w / 100) / box )
House      each finished line counts at >= 7 ft, else apart

Frequently asked

How do I calculate sq ft?

Multiply the length by the width, both in feet. A room 12 ft 6 in by 14 ft 3 in is 12.5 × 14.25 = 178.125 square feet. Split any other outline into rectangles, circles, triangles and trapezoids, add them, and deduct what is not floor.

How many sq ft is in a 12x12 room?

144 square feet: 12 × 12. That is 16 square yards, or 13.38 m².

How many sq ft is 30x40?

1,200 square feet. The neighbours of the same question: 30 × 50 is 1,500 and 30 × 60 is 1,800.

What is 12 ft by 8 ft in square feet?

96 square feet, 10.67 square yards.

Why divide by 144 for square footage?

A square foot is 12 in by 12 in, so it holds 12 × 12 = 144 square inches. Measure in inches, multiply, and divide by 144: 150 in × 171 in = 25,650 sq in, ÷ 144 = 178.125 sq ft.

How do I calculate the square footage of my house?

Pick the house mode, type each level and space as a line, and the sheet reports finished area above grade, below grade, nonstandard finished, garage and unfinished apart, each to the whole square foot. A 30 × 40 ft main floor, a 30 × 20 ft finished basement, a 20 × 22 ft garage, plus the same floor upstairs with a 10 × 12 ft foyer open to the floor below, report 2,280 sq ft above grade and 600 below. The stairs stay in: they count on the floor they descend from; only the open foyer comes out of the upper floor.

Limits of this sheet

G-102 knows four flat shapes and six lines: split anything else into them. It measures area, not volume; carry the square feet to the cubic yard calculator for a depth. The house mode is a reading of a summary, not a substitute for an appraisal.

Sheet G-102 prepared by TakeoffMetric Editorial; no author, credential or field measurement is claimed.