How it's calculated
Square footage is the flat area of a floor, wall, lawn or roof plane. The calculator finds the area of up to three sections, each a rectangle, triangle, circle or L-shape, and adds them together. That covers most rooms and yards: break an odd-shaped space into simple pieces, measure each one and let the calculator sum them.
Enter every length in the same unit: feet for US units, meters for metric. The total comes out in square feet or square meters, with the other unit and square yards alongside. Add an optional percentage for waste if you are buying material, and a price per square foot (or per m²) to get a cost.
The formula
- Rectangle: area = length × width.
- Triangle: area = base × height ÷ 2, where the height is measured at a right angle to the base.
- Circle: area = π × (diameter ÷ 2)².
- L-shape: split it into two rectangles and add them: (length₁ × width₁) + (length₂ × width₂).
- Total = the sum of every section. With waste = total × (1 + waste %). Cost = area with waste × price per unit of area.
- Conversions: 1 square yard = 9 square feet; 1 square foot = 0.09290304 m² exactly, because NIST defines the foot as exactly 0.3048 m NIST.
Worked example
A 12 × 14 ft bedroom is 168 sq ft, which is 15.61 m² or 18.67 square yards. At $5 per square foot that is $840.
Total area: 168 square feet
An L-shaped living area made of a 12 × 14 ft section and a 6 × 8 ft nook is 216 sq ft. Add a round 10 ft patio as a second section and a triangular planting bed with a 10 ft base and 8 ft height as a third, and the total becomes 334.54 sq ft.
Assumptions and limitations
- Flat, horizontal area. Sloped surfaces such as roofs have more surface area than their footprint; use the roofing calculator, which applies the pitch.
- Measure to the finished edges. For flooring, measure wall to wall; for paint, use wall area instead (the paint calculator does this).
- Triangles need a true height. Measure the height square to the base, not along a sloping side.
- Same unit everywhere. Mixing feet and inches is the most common error: convert inches to feet (divide by 12) before entering them.
- Prices are examples. Material and labor are often quoted per square foot; enter your own figure.
For tips on measuring awkward rooms and adding a waste allowance, see our guide to estimating materials for home projects.