Square footage calculator for rooms and shapes

Square footage is the flat area of a surface, in squares one foot on each side. For a rectangle it is the length times the width: a room 14 ft long and 12 ft wide is 14 × 12 = 168 sq ft.

Most real spaces are not one rectangle. The way through is always the same: break the space into simple shapes, work out each one, and add them together. Take out any part you will not cover, such as a fireplace hearth or a kitchen island, by working out its area and subtracting it.

1. Sketch the space and mark where it changes direction.
2. Split it into rectangles first, then triangles or curves for whatever is left.
3. Measure each piece in feet. Convert inches to feet by dividing by 12: 6 in is 0.5 ft, 9 in is 0.75 ft.
4. Work out each area with the formulas below and add them up.

Measure wall to wall at floor level for flooring, and round each measurement up to the next inch. It is easier to trim a little than to come up short.

Write each measurement on the sketch as you take it, and measure each wall in two places if the room looks out of square. Old houses are rarely true, and a room that is 12 ft at one end and 12 ft 4 in at the other is better worked out with the larger figure.

The height of a triangle or a trapezoid is measured square to the base, not along a sloped side. For a bay window or an angled wall, measure straight out from the main wall to the farthest point.

For a circle, the diameter is the distance across through the center. If you only have the radius, the distance from the center to the edge, double it. A round patio 12 ft across is π × 144 ÷ 4 = 113.1 sq ft.

Carpet, some sod and some fill are sold by the square yard, and many product sheets and plans abroad use square meters. The conversions are exact:

• Square yards = square feet ÷ 9, because a yard is 3 ft and 3 × 3 = 9.
• Square meters = square feet × 0.09290304, because 1 ft is defined as exactly 0.3048 m and 0.3048² = 0.09290304.
• Acres = square feet ÷ 43,560.

An L-shaped living and dining room. The living room is 20 × 12 ft and the dining area off one end is 10 × 8 ft. Split at the corner: 20 × 12 = 240 sq ft, 10 × 8 = 80 sq ft. Total 320 sq ft, 35.56 sq yd, 29.73 sq m. For laminate with 10% waste, order 320 × 1.10 = 352 sq ft.

A kitchen with a bay. The main kitchen is 12 × 15 ft = 180 sq ft. The bay is a trapezoid 8 ft wide at the wall, 4 ft wide at the glass and 2 ft deep: (8 + 4) ÷ 2 × 2 = 12 sq ft. Total 192 sq ft.

A patio with a round end. A 16 × 12 ft slab with a half circle 12 ft across on one side: 192 + π × 144 ÷ 8 = 192 + 56.55 = 248.55 sq ft.

A gable end to side or paint. A wall 24 ft wide and 8 ft to the eaves, with a gable rising 6 ft to the peak: rectangle 24 × 8 = 192 sq ft, triangle 24 × 6 ÷ 2 = 72 sq ft. Total 264 sq ft before windows and doors.

The measured area is what you price labor by. The material you order is the measured area plus an allowance for cuts, breakage and pattern matching. The allowance depends on the material and the layout, so use the figure from the product's instructions or from your own jobs:

• Straight-lay flooring in a simple rectangle wastes little. Diagonal and herringbone layouts, and rooms with many corners and doorways, waste more.
• Tile and stone add breakage and the cuts at every edge.
• Carpet comes off a roll of fixed width, so seams and the roll width decide the order more than the area does.
• Sod, mulch and gravel are ordered by the pallet or the cubic yard; see the mulch, soil and gravel calculator for depth.

Enter your allowance in the calculator and it shows the area to order, rounded up to the next whole square foot.

Every trade measures a slightly different surface, so the same room can give different numbers. Be clear which one you mean when you quote:

• Flooring measures the floor wall to wall, including closets and under the toe kicks of cabinets if the floor runs there, and leaves out islands and built-ins that sit on the subfloor. Doorway thresholds are usually included up to the middle of the door.
• Painting and drywall measure the walls, not the floor: the distance around the room times the wall height, less doors and windows. A 12 × 14 room has 168 sq ft of floor but 416 sq ft of 8 ft wall.
• Roofing measures the sloped surface of the roof, which is larger than the footprint the house sits on. Multiply the footprint by the pitch factor, or use the roofing materials calculator.
• Concrete and pavers measure the flat area, then multiply by the thickness to get volume. A 4 in slab is a third of a foot thick, so 300 sq ft is 100 cubic feet, about 3.7 cubic yards.
• Lawn care, sealing and washing measure the treated surface outside, where satellite measuring saves a site visit.
• Real estate listings use the finished, heated living area measured from the outside of the walls, which is not the same as the floor you lay inside them.

When you write the estimate, name the surface and the method: "168 sq ft of floor, wall to wall" leaves no room for a dispute at the end of the job.

Many trades price by the square foot: flooring, painting, drywall, decks, concrete, pressure washing, roofing by the square of 100 sq ft. Enter your price per square foot and the calculator multiplies it out on the order quantity, then "Save as an estimate in Koira" carries it into an estimate with each area listed, ready to send.

For the materials on a specific job, the trade tools go further than area: the drywall calculator counts sheets, compound and tape, the paint calculator works out gallons for walls and ceilings, and the roofing materials calculator adds pitch to a roof's footprint. To measure a lawn or a roof from above without a tape, use the satellite measuring tool.