Area calculations: square footage and acreage

Measure the surface, not the boundary

Area is the surface inside a boundary, expressed in square feet or acres. Perimeter is the distance around it, expressed in linear units such as feet.

Choose the formula

• Rectangle: length × width. A 120-foot by 200-foot lot contains 120 × 200 = 24,000 square feet.
• Square: side × side.
• Triangle: base × perpendicular height ÷ 2. A 100-foot base and 80-foot height give 100 × 80 ÷ 2 = 4,000 square feet. A sloping side is not necessarily the perpendicular height.
• Trapezoid: (parallel side one + parallel side two) ÷ 2 × perpendicular height.

Divide an irregular outline into nonoverlapping familiar shapes and add their areas, or subtract a missing section from a surrounding rectangle.

The rectangle is 220 feet by 198 feet: 220 × 198 = 43,560 square feet, or one acre. An acre measures area; it does not prescribe a shape.

Convert consistently

Memorize 43,560 square feet per acre and 5,280 feet per mile.

• Square feet to acres: divide by 43,560. Thus 87,120 ÷ 43,560 = 2 acres.
• Acres to square feet: multiply by 43,560. Thus 0.5 × 43,560 = 21,780 square feet.
• One square yard: 3 feet × 3 feet = 9 square feet.
• One square mile: 5,280 × 5,280 = 27,878,400 square feet; 27,878,400 ÷ 43,560 = 640 acres.

Convert dimensions to matching units before multiplying. Keep intermediate precision and round the final result as requested. Geometric area alone does not establish surveyed boundaries or determine which building spaces qualify as reported living area; use the dimensions and inclusion instructions supplied in the question.