CalcPro

Area Calculator

Area of common 2-D shapes — rectangle, triangle, circle, trapezoid and more.

Finding the area of any shape

Area measures how much flat space a 2D shape occupies. Whether you're calculating room dimensions for flooring, determining paint coverage, or solving geometry homework, this calculator instantly computes the area of five common shapes using their standard geometric formulas.

Each shape requires different information. A rectangle needs length and width, a triangle needs base and height, a circle needs only its radius, and a trapezoid needs both parallel sides plus the perpendicular height between them. The calculator adapts to whichever shape you select.

The formula

Rectangle: A = length × width | Triangle: A = (base × height) ÷ 2 | Circle: A = π × radius² | Trapezoid: A = ((base₁ + base₂) ÷ 2) × height | Parallelogram: A = base × height

Worked example

Scenario: You're planning to tile a kitchen floor (rectangle), paint a triangular gable wall, and want to know the area of a circular garden bed.

Rectangle (kitchen floor):

  • Length = 12 feet
  • Width = 10 feet
  • Area = 12 × 10 = 120 square feet

Triangle (gable wall):

  • Base = 16 feet
  • Height = 8 feet
  • Area = (16 × 8) ÷ 2 = 128 ÷ 2 = 64 square feet

Circle (garden bed):

  • Radius = 5 meters
  • Area = π × 5² = 3.14159 × 25 = 78.54 square meters

Trapezoid (deck with one sloped side):

  • First parallel side = 20 feet
  • Second parallel side = 14 feet
  • Height = 8 feet
  • Area = ((20 + 14) ÷ 2) × 8 = (34 ÷ 2) × 8 = 17 × 8 = 136 square feet

Notice that the triangle uses half the product of base and height—this is because a triangle is exactly half of a rectangle with the same base and height. The trapezoid averages its two bases before multiplying by height, reflecting that it's narrower at one end.

Common mistakes to avoid

Confusing diameter with radius for circles: The radius is the distance from the center to the edge. Diameter is twice that. If you measure 10 feet across a circle, that's the diameter—divide by 2 to get radius (5 feet) before calculating.

Using slant height instead of perpendicular height for triangles and trapezoids: Height must be measured at a right angle to the base, not along a slanted edge. A tilted triangle's height is the vertical distance from base to opposite point, not the length of the slanted side.

Mixing units: If one measurement is in feet and another in inches, convert everything to the same unit first. Mixing units produces meaningless results.

Forgetting to divide by 2 for triangles: The triangle formula requires dividing by 2. This common slip doubles your answer incorrectly.

Frequently asked questions

What shapes can this calculator handle?

The calculator supports rectangle, triangle, circle, trapezoid, and parallelogram. Each shape requires different inputs—for example, a circle only needs radius, while a trapezoid needs both parallel sides and height.

Do I need to use the same units for all dimensions?

Yes. Whatever unit you use for your inputs (inches, feet, meters, etc.), the result will be in that unit squared. If you enter feet, you'll get square feet; if meters, you'll get square meters.

Why is the area of a circle different from a rectangle?

A circle's area depends on its radius and the constant π (pi ≈ 3.14159). A rectangle's area is simply length × width because its sides form right angles. Different shapes have fundamentally different geometric properties.

How do I find the area of a trapezoid?

A trapezoid has two parallel sides (called bases) of different lengths. The formula averages these two bases and multiplies by the perpendicular height between them. This works because a trapezoid is essentially between a rectangle and a triangle.

What's the difference between a parallelogram and a rectangle?

A rectangle has four right angles (90°), while a parallelogram's angles can be any value as long as opposite angles match. Both use the same area formula: base × height, where height is the perpendicular distance between parallel sides.

Can I use this for real-world projects like flooring or painting?

Yes. Measure your room or wall in feet or meters, enter the dimensions, and you'll get the square footage or square meters needed for materials. Always add 10% extra for waste and cutting.

Related calculators

Surface Area CalculatorSurface area of a box, sphere, cylinder or cone.Volume CalculatorVolume of a box, sphere, cylinder, cone or pyramid.Circle CalculatorArea, circumference and diameter from a circle's radius.Triangle CalculatorArea, perimeter and angles of a triangle from its three sides.