The one measurement every quote, listing and material order stands on
Square footage is the quiet unit underneath half of home life: flooring and carpet are priced by it, paint and tile quantities start from it, estate agents list by it, and a landscaping quote is meaningless without it. Yet it is also the measurement people most often take from a single rectangle when the real space is not one — an L-shaped room measured as its bounding box, a round patio guessed as a square, a sloping plot measured along the slant. This calculator does the job properly: pick the shape you actually have — rectangle, circle, triangle or trapezoid — and it returns the area in square feet, square metres, square yards and acres at once, with an optional cost per unit area for turning a measurement straight into a quote.
The formulas, with every term defined
Rectangle = length × width · Circle = π × (diameter ÷ 2)²Triangle = base × height ÷ 2 · Trapezoid = (side A + side B) ÷ 2 × height1 sq yd = 9 sq ft · 1 m² = 10.7639 sq ft · 1 acre = 43,560 sq ft
Two definitions carry all the conversions: a square yard is a yard by a yard, so 9 sq ft, and an acre is 43,560 sq ft — historically one chain by one furlong, kept exact ever since. Height is the trap in the slanted shapes: for both the triangle and the trapezoid it is the perpendicular distance, measured at right angles, never the length of the sloping side. Use the slanted edge of a triangular garden bed (say 9 ft where the true height is 8 ft) and the area is overstated by an eighth before you have ordered anything. Awkward rooms are handled by decomposition, not by a shape button: an L-shaped room is two rectangles (or one big rectangle minus the missing corner), and the count field multiplies any shape when several identical areas — three same-size bedrooms, four office cubicles — need totalling.
Worked example: a 12 × 10 ft room
The defaults above give: 12 × 10 = 120.00 sq ft. The same floor is 120 ÷ 9 = 13.33 sq yd (the unit carpet is sold in), 120 × 0.092903 = 11.15 m², and 120 ÷ 43,560 = 0.002755 acres — a reminder of how small a room is in land units. At a flooring price of $6 per sq ft, the material line is 120 × $6 = $720. Feed that 120 sq ft onward and it becomes 2 gallons of paint's worth of ceiling in the paint calculator, or 128 tiles in the tile calculator. The bar chart above sets the room beside the other shapes' examples below.
The other shapes, worked once each
| Shape | Working | Area |
|---|---|---|
| Rectangle 12 × 10 ft (worked example) | 12 × 10 | 120.00 sq ft |
| Circle, 14 ft across (round patio) | π × 7² | 153.94 sq ft |
| Large rectangle 20 × 30 ft | 20 × 30 | 600.00 sq ft (66.67 sq yd · 55.74 m²) |
| Trapezoid, sides 10 and 14 ft, 6 ft apart (plot) | (10 + 14) ÷ 2 × 6 | 72.00 sq ft |
| Triangle, base 10 ft, height 8 ft (corner bed) | 10 × 8 ÷ 2 | 40.00 sq ft |
| Metric room 5 × 4 m | 5 × 4 | 20.00 m² (215.28 sq ft) |
| Three identical rooms, 10 × 9 ft each | 3 × (10 × 9) | 270.00 sq ft |
The L-shaped case deserves its own line because it is the most common real room: a 12 × 10 ft room with a 4 × 5 ft corner missing is not 120 sq ft — it is 120 − 20 = 100 sq ft, a sixth less flooring than the bounding box suggests. Run the big rectangle, then subtract the missing rectangle, and the two runs of this calculator are the whole method. For the slab that goes under that floor, the concrete calculator starts from the same length × width and adds the thickness.
How to read your result, and the limits — read these
Measure inside the skirting boards for flooring and to the outside walls for a building footprint — the same room honestly gives two areas a few percent apart, and which one is right depends on the job: material orders want the inside figure, listings and plot documents usually state the outside or a surveyed figure. Listed square footage in property documents also follows local conventions (some include, some exclude, garages, lofts and below-grade space), so this calculator's clean geometry will not always match an official figure — that is a difference of definition, not an error on either side. The cost line prices the area only: underlay, adhesive, edging, wastage on patterned flooring and labour all sit on top of a per-square-foot material price, and flooring in particular should be ordered with the same kind of 10% cutting allowance the tile calculator applies. Land areas from the trapezoid and triangle modes assume flat ground; a steep slope's true surface is larger than its map area, which is why surveyors quote the horizontal figure for ownership and the surface figure only for covering it.
Frequently asked questions
How do I calculate the square footage of a room?
Multiply its length by its width, both in feet: a 12 × 10 ft room is 120 sq ft. For a room that is not a clean rectangle, split it into rectangles — an L-shape is a big rectangle minus the missing corner (12 × 10 minus 4 × 5 = 100 sq ft) — and add or subtract the pieces. Measure inside the skirting for material orders, and use the same wall convention throughout.
How many square feet are in an acre, and in a square yard?
One acre is exactly 43,560 sq ft, so the worked example's 120 sq ft room is 0.002755 acres and a 600 sq ft plot is 0.013774 acres — land is normally the only place the acre figure is useful. One square yard is 9 sq ft (a yard is 3 feet, and 3 × 3 = 9), so divide sq ft by 9 for the square yards that carpet and some flooring are sold in: 120 sq ft = 13.33 sq yd.
How do I find the area of a circle or a triangle in square feet?
Circle: halve the diameter to get the radius, square it, and multiply by π — a 14 ft patio is π × 7² = 153.94 sq ft. Triangle: base × height ÷ 2, where the height is the perpendicular distance from the base to the opposite corner, not the slanted side — a bed 10 ft along its base and 8 ft deep at right angles is 40 sq ft. Both shapes are modes in the calculator above.
Does square footage include closets, hallways and stairs?
For a material order, include every patch of floor the material will actually cover — closets yes, the floor under a staircase usually no — and measure each patch as its own rectangle. For a property listing, conventions differ by place and are set by local rules, not by geometry: some totals include hallways and finished below-grade rooms, some exclude them. Use this calculator for the physical area; use the listing rules for the official one.
Why does my measured area differ from the listing or plan?
Usually because of what was measured, not bad arithmetic: plans often use outside wall dimensions while you measured inside (worth roughly 3–6% on a small room), listed figures may follow a convention that includes or excludes whole spaces, and older properties are rarely perfectly square — two opposite walls can differ by several inches. Measure both lengths and both widths; if they disagree, average each pair before multiplying.
