How to Calculate Square Footage: Rooms, Houses, Roofs & Triangles

The formula for calculating square footage is straightforward:

Area (sq ft) = Length (ft) × Width (ft)

Both measurements must be in feet. If you measured in inches, convert to feet first by dividing by 12. For example, a wall that is 10 feet 6 inches long is 10.5 feet (6 ÷ 12 = 0.5). Working in decimal feet keeps the math simple and prevents unit-mixing errors.

For projects with multiple areas — like a home with several rooms or a floor with an L-shaped layout — calculate each section separately and add the results. If you are estimating material (flooring, paint, tile), add a waste allowance of 5–10% on top of the total for cuts and mistakes.

Measuring a room is the most common square footage calculation. Here is the step-by-step process:

1. Measure the longest wall (length) in feet and inches.
2. Measure the perpendicular wall (width).
3. Convert any inches to decimal feet (divide by 12).
4. Multiply length × width to get square feet.
5. For multiple rooms, repeat and add the totals.

Example: A bedroom that is 12 ft × 10 ft = 120 sq ft. A living room that is 15 ft × 12 ft = 180 sq ft. Combined total = 300 sq ft. With 10% waste for flooring, order material for 330 sq ft.

For L-shaped rooms, divide the space into two rectangles. Measure each rectangle separately, calculate both areas, and add them. An L-shaped room with a 10 × 8 main section and a 6 × 4 alcove is 80 + 24 = 104 square feet.

To calculate a home's total square footage, measure each finished room individually and add the results. The ANSI Z765 standard — used by US real estate professionals — defines what counts:

• Include: Finished, heated, enclosed living space.
• Exclude: Garages, unfinished basements, open porches, and attics.
• Single-family homes: Measure from exterior walls (wall thickness included).
• Attached homes and condos: Measure from interior unit boundaries.

For a two-story house, measure each floor separately and add the totals. Stair openings are counted on only one floor (typically the lower level). The total is the ground-floor area plus the second-floor area — not the ground floor doubled. A 1,000 sq ft ground floor with an 800 sq ft second floor is a 1,800 sq ft home.

Triangular areas appear in gable ends, corner gardens, and bay windows. The formula is:

Area = ½ × base × height

The base and height must be perpendicular to each other. A triangle with a 10-foot base and 8-foot height has 40 square feet (½ × 10 × 8 = 40). For a right triangle, the two legs forming the right angle are the base and height.

Example: A triangular garden bed with a 12-foot base and 9-foot perpendicular height: ½ × 12 × 9 = 54 sq ft.

Roof square footage is larger than the building footprint because pitched roofs slope. To calculate:

1. Measure the length and width of each roof plane.
2. Multiply to get the footprint area.
3. Multiply by the pitch multiplier (see table below).
4. Divide the total by 100 to get roofing squares.

Roofing materials are sold in squares — one square covers 100 square feet of roof surface. After calculating total roof square footage, divide by 100, then add 10–15% for waste and overlaps.

Example: A 40 × 30 foot house (1,200 sq ft footprint) with a 6:12 pitch roof: 1,200 × 1.118 = 1,342 sq ft of roof surface, or about 13.4 squares (round up to 14, then add waste).

For painting, wallpapering, or siding, calculate wall area and subtract openings:

Wall area = Width × Height − (doors + windows)

A standard interior door is about 21 sq ft (3 × 7 ft). A standard window is about 12 sq ft. For a 12 × 8 foot wall with one door: 96 − 21 = 75 sq ft of paintable surface.

One acre equals 43,560 square feet. This conversion is essential for lot sizing, lawn care, and land projects.

For circular areas — pools, round patios, and circular garden beds — use the formula Area = π × radius², where the radius is half the diameter. A 10-foot-diameter circle (5 ft radius) has π × 5² = 78.54 square feet. A 20-foot-diameter pool (10 ft radius) has π × 10² = 314.16 square feet.

For irregular curved shapes, break the area into simpler parts — rectangles and circles — calculate each, and add the results.

If your measurements are in inches, convert to feet first by dividing by 12. A wall that is 144 inches × 120 inches: 144 ÷ 12 = 12 ft, 120 ÷ 12 = 10 ft, so 12 × 10 = 120 sq ft.

Alternatively, multiply inches × inches to get square inches, then divide by 144 (the number of square inches in a square foot). This shortcut is useful for countertops and small surfaces measured in inches.

To find the cost of a project, multiply the total square footage by the price per square foot. For 300 sq ft of flooring at $5/sq ft = $1,500. To find the price per square foot from a total quote, divide the total cost by the square footage: a $2,400 quote for 400 sq ft = $6/sq ft.

Common US price ranges per square foot (installed): flooring $3–$15, paint $2–$6 per coat, countertops $40–$200, siding $3–$15, roofing $3–$8, concrete $4–$15, tile $10–$30.

Linear feet measure length (1D); square feet measure area (2D). To convert, divide the square footage by the material’s width in feet: Linear feet = Square feet ÷ Width (feet). For 100 sq ft of 8-inch-wide siding (0.667 ft): 100 ÷ 0.667 = 150 linear feet.

Countertops are priced per square foot, so accurate measurement is essential. Measure each section in inches (length × width), divide by 144 for square feet, and add all sections. A standard L-shaped kitchen counter runs about 40–60 square feet. Don’t forget the backsplash — typically 4–18 inches tall — measured separately.

Convert inches to decimal feet by dividing by 12, then multiply. For a room that is 12 ft 6 in × 10 ft 3 in: 12.5 ft × 10.25 ft = 128.13 sq ft. Common decimal conversions: 3 in = 0.25 ft, 6 in = 0.5 ft, 9 in = 0.75 ft, 1 in = 0.083 ft.

The square footage calculator supports inches natively — select “in” from the unit dropdown and the calculator handles the conversion automatically.

For volume calculations (concrete, mulch, gravel), use the cubic yard calculator. For driveway material, try the gravel calculator. For instant results, use our free square footage calculator.