Area Calculator
Calculate the area of a rectangle, circle, or triangle.
About the Area Calculator
Different shapes require different area formulas — this calculator covers the three most common shapes in home and construction projects: rectangles, circles, and triangles, switching formulas automatically based on your selection.
How It Works
For a rectangle, the calculator multiplies length by width. For a circle, it uses the radius squared multiplied by pi. For a triangle, it multiplies base by height and divides by two, matching the standard geometric formula for each shape.
Rectangle: L × W. Circle: π × r². Triangle: 0.5 × base × height.
Worked example
Scenario: A 10 × 6 rectangle, and a circle with a radius of 5.
- Rectangle: 10 × 6 = 60 square units; perimeter 2 × (10 + 6) = 32.
- Circle: π × 5² = 78.54 square units; circumference 2 × π × 5 = 31.42.
- Triangle (base 10, height 6): ½ × 10 × 6 = 30 square units.
Result: 60 for the rectangle, 78.54 for the circle and 30 for the triangle.
Tips and common mistakes
- Use the same unit for every dimension; the result is in that unit squared.
- For a circle, enter the radius — half the diameter.
- A triangle's height is measured at a right angle to its base, not along a side.
Assumptions & Limitations
- For circles, dim1 is treated as the radius, not the diameter — divide a known diameter by 2 before entering it.
- For triangles, dim1 is the base and dim2 is the height measured perpendicular to that base.
- Does not cover irregular polygons or compound shapes — break those into simpler rectangle, circle, and triangle sections and sum the results.
Choosing the right area measurement
Area is always a two-dimensional measurement. Keep every dimension in the same unit before multiplying, and split irregular spaces into simple rectangles, triangles or circles when needed. For material purchasing, calculate the usable area first and then apply the waste allowance recommended for that material rather than inflating the room dimensions.