EstimatorKitSquare footage of a room, shape by shape
Measure a building in the pieces you can reach with a tape — rectangles, L-shapes, triangles, circles and walls that run out of parallel — and this adds them into one running total in square feet, square yards and square meters, with doors, windows and stair openings taken back off. It holds up to 20 named areas, each entered the way a tape reads: 12' 6 3/8", 150 in or 4.1 m. The measured figure and the figure with a cutting allowance sit side by side, because only the first one can be checked against a quote.
- 5 shapes
- 20 areas per total
- Openings deducted
Each line is one shape. Name it the way you would say it out loud — the names come out in the copied list, and a list that says “back bedroom” is worth more at a counter than one that says Area 3.
Four square corners: a bedroom, a wall, a landing, a run of decking.
+234 sq ft
Measure the full outline as if the room were whole, then the rectangle a chimney breast, pantry or stair bulkhead takes back out of the corner.
+168 sq ft
Four square corners: a bedroom, a wall, a landing, a run of decking.
−35 sq ft
A 3 by 7 opening is 21 sq ft — a standard 3 × 7 door opening. The window preset uses 15 sq ft, which is a stand-in only: actual window size — deduct real ones where known.
Sides are read off a tape, so write them as one: 13, 12' 6 3/8", 10'4 — an unmarked number after the foot mark is taken as inches — 150 in or 4.1 m. A line that will not read says so instead of guessing. Up to 20 lines.
Net floor or wall area from 3 lines
367 sq ft
- Measured
- 402 sq ft
- Taken off
- 35 sq ft
- Square yards
- 40.8
- Square meters
- 34.1
Cutting waste
- Measured, no allowance
- 367 sq ft
- Order for, at 0%
- 367 sq ft
Both figures stay on screen so the one you check a quote against and the one you hand over at the counter are never the same number by accident.
What this area turns into
Cartons are not a standard size, so this one has to come off the box in front of you rather than from us.
If these lines are walls rather than floors, 1 gal covers them once and 2 gal covers them twice, on the net area after the openings came off.
Paint at 375 sq ft per gallon — the coverage stated on a can of interior latex, mid-range of the 350–400 usually printed. It moves with surface porosity, color change and how it is applied, so treat the gallon count as a starting point.
What counts as finished square footage
For flooring and paint you measure what you are covering and nothing else. For a valuation there is a published rule, ANSI Z765, which Fannie Mae has required on most single-family appraisals since April 2022 — and it disagrees with common sense in seven specific places.
| Space | In the figure? | The condition |
|---|---|---|
| A room with a sloped ceiling | Partly | Floor under a ceiling of 5 ft or more counts, but only if at least half the room's finished area has 7 ft of headroom. |
| Stairs | Yes, once | The tread area counts on the floor the stair descends from, not on both floors. |
| A two-story foyer or vaulted opening | No | There is no floor at that level to measure, so the opening is subtracted from the upper story. |
| A finished basement | Not in the same number | Anything below grade is reported as below-grade finished area on its own line, however good the finish is. |
| Interior partitions and closets | Yes | The standard measures to the outside face of the exterior walls, so everything inside the shell is already in the figure. |
| An attached garage | No | Excluded whatever it has been converted into, unless the conversion was permitted and finished to the same standard as the house. |
| A screened or covered porch | No | Not enclosed and conditioned, so it is described separately rather than added in. |
From the tape to a net square footage
Shapes in, openings off, allowance last.
Break the floor into shapes a tape can actually reach
Add one line per piece and name it the way you say it out loud. A plain room is a rectangle; a kitchen with the pantry cut out of the corner is one L-shape rather than two rectangles; a gable wall above the plate is a triangle; a room with one wall out of square is the angled-wall option, which wants both parallel walls and the distance across rather than the sloping side. The count field handles repeats, so eight identical stud bays are one line, not eight.
Take the openings back off
Mark anything you are not covering as a deduction and it is subtracted from the running total. The door preset loads a 3 by 7 opening at 21 square feet and the window preset a 3 by 5 at 15, both editable, and a stair well or a chimney breast goes in as a plain rectangle set to deduct. The starting example shows the mechanism: a 234 sq ft living room plus a 168 sq ft L-shaped kitchen less a 35 sq ft stair opening is 367 square feet net.
Read the net figure, then add the cutting allowance beside it
The total appears in square feet with square yards and square meters under it, and the waste allowance sits in its own panel with the bare number still visible. Tap the allowance that matches the layout — 8 percent for plank, 10 for straight-laid tile, 15 for diagonal, 20 for herringbone — or type your own. Copy gives a plain-text list with every named area and both totals on it.
Inside the walls, and outside them
Length times width answers this question for maybe a third of the rooms it is asked about. The rest have a chimney breast, a bay, a stair well, a bulkhead over the boiler or one wall that the original builder never brought square, and the standard advice — “divide it into rectangles and add them up” — quietly moves the hard part onto a person holding a tape in one hand and a phone in the other. That is why this calculator is a list rather than two boxes: the pieces stay on screen with their own names, so a mistyped 8 shows up as a room that is obviously the wrong size instead of disappearing into a total.
Before multiplying anything, check the corners. Two tapes run diagonally across a room should read the same; when they do not, the room is a parallelogram and every length-times-width figure taken from it is high. The other habit worth having is measuring at floor level rather than at waist height, because a wall that bows in the middle costs you the difference across the whole run. Neither check takes a minute, and between them they catch nearly every square-footage argument that ends with a short flooring delivery.
The number this page produces is a measured area, and what happens to it next depends on what you are buying. Anything sold by the yard needs a depth added first, which is what the cubic yard calculator asks for; anything sold by the stick — baseboard, trim, decking, fence rail — is a perimeter rather than an area and belongs on the linear feet calculator. Flooring cartons and paint gallons come straight off the total here, with the flooring calculator carrying the layout waste in more detail. And if the surface you are measuring is land rather than a building, stop here and use the acre calculator — a lot is measured to the boundary, quoted in acres, and follows none of the rules above.
When two square footages disagree
How do I calculate the square footage of a room that is not a rectangle?
Split it into shapes with square corners, work each one out and add them. Almost every awkward room is a rectangle with a rectangle missing from a corner, which is one L-shape line here: measure the outline as though the room were whole, then measure the bite the chimney breast or pantry takes out of it. Curved bays are close enough to a half-circle to use the circle option on the bay alone, and a wall that runs out of parallel is a trapezoid, not two triangles glued together.
Why is my measured square footage smaller than the listing says?
Because a listing is measured to the outside of the exterior walls and you are measuring inside them. On a 30 by 40 ft house with a 7 in wall assembly the interior comes out at 28.83 by 38.83 ft, which is 1,120 sq ft against the 1,200 the deed or the appraisal quotes — a gap of nearly 7 percent that has nothing to do with either measurement being wrong. Flooring and paint want the inside number; a valuation wants the outside one.
What ceiling height does a space need before it counts as finished area?
Seven feet, dropping to 6 ft 4 in under beams and ducts, under the ANSI Z765 standard most US appraisals now use. Sloped ceilings get their own rule: the floor under any part of the ceiling at least 5 ft high counts, but only where at least half the room already has the full 7 ft. That is why a converted attic can lose a third of its floor from the official figure while still being a room you can stand up in.
Can I add floors together, and does the stair count twice?
Add them, and count the stair once. Finished area on each level is measured separately and summed, and the stair opening is subtracted from the upper floor while the treads themselves count on the floor they descend from — otherwise the same few square feet is sold twice. The same logic applies to a two-story entrance hall: the upper level has a hole in it, so the hole comes off.