Area is the amount of surface a flat shape covers, always measured in square units. Each common shape has its own formula: a rectangle is length × width, a square is side², a circle is π × radius², a triangle is ½ × base × height, and a parallelogram is base × height. The companion measure is the perimeter (the distance around the edge — the circumference for a circle). Keep every dimension in the same unit and the answer falls out cleanly.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: standard plane-geometry area formulas.
The area formulas
Rectangles and squares tile space directly, so their area is just the product of the sides. A circle's area uses π, the ratio of circumference to diameter (≈ 3.14159), giving π r². A triangle is exactly half the rectangle that boxes it, hence ½ × base × height — and the height must be the perpendicular distance to the base, not a slanted side. A parallelogram shears that rectangle without changing its area, so it stays base × height.
Worked example — a rectangle
Scenario: a rectangle 8 units long and 5 wide.
The area is 40 square units, the perimeter 26, and the corner-to-corner diagonal about 9.434. Switch shapes and the same inputs feed different formulas: a circle of radius 5 has an area of about 78.54 and a circumference of 31.42; a triangle of base 6 and height 4 has an area of 12; a square of side 5 has an area of 25. Whatever the shape, the area is reported in the square of your input unit — feed metres and you get square metres.
Frequently Asked Questions
length × width. 8 × 5 = 40 sq units; perimeter 26; diagonal ≈ 9.434.
π r². r = 5 → ≈ 78.54; circumference 2π r ≈ 31.42.
½ × base × height. b 6, h 4 → 12. Use Heron's formula for three sides.
Square units matching your inputs (m → m²). Keep all dimensions in one unit.
Area = surface (square units); perimeter = distance around the edge (length units).