A clear answer before the details
Area measures the amount of surface inside a boundary. Choose a shape, enter its dimensions, and this calculator applies the correct formula while preserving the units as square units.
Choose the right area calculator for your shape
Area is the amount of two-dimensional space inside a boundary, measured in square units. This area calculator brings several common tools together: use the rectangle area calculator for length × width, the circle area calculator for πr², the triangle area calculator for ½ × base × height, and the trapezoid area calculator for the average of two parallel bases multiplied by height. Keeping the shape selector beside the inputs makes it easier to choose the right formula before you start.
Area, surface area, and square units
A surface area calculator measures the total outside area of a three-dimensional object, not the space inside it. Use the surface-area mode for cubes, rectangular prisms, cylinders, spheres, and cones. A body surface area calculator is a specialised health and science application that may use height and weight; this page focuses on geometric surfaces. Convert every dimension to the same unit first, because mixing centimetres and metres changes the result by a scale factor.
Formula guide
The key equations are separated below. Start with the calculator mode, then choose the shape or model that matches your problem.
2D area
Rectangle
Multiply length l by width w.
Square
Multiply the side s by itself.
Triangle
Multiply base b by height h, then take one-half.
Circle
Use the radius r, not the diameter.
Trapezoid
Average the parallel bases b₁ and b₂, then multiply by height h.
Parallelogram
Base b times its perpendicular height h.
Ellipse
a and b are the major and minor radii.
Sector
Take the fraction θ/360 of the circle area.
Rhombus
Multiply the diagonals d₁ and d₂, then take one-half.
Surface area
Cube
Six congruent square faces with side s.
Rectangular prism
Add the three face-pair areas and double them.
Cylinder
Two circular bases plus the curved side.
Sphere
Use radius r for the entire outside surface.
Cone
r is radius and ℓ is slant height.
Worked example
For a circle with radius 3, A = π × 3² = 9π ≈ 28.2743 square units. For a 10 by 5 rectangle, A = 50 square units.
Formulas for common use cases
Use the matching equation for the problem you are solving, then enter its variables in the calculator above.
Flooring, paint, and fabric
Use matching linear units; the answer is in square units.
Circular surfaces
Use half the diameter as the radius.
Curved solids
Separate the two circular ends from the curved side.
Outside of a box
Useful for wrapping, painting, or covering a rectangular prism.
3D surface versus volume
Surface area uses square units; volume uses cubic units.
How to use this calculator
- Choose a shape from the geometry selector.
- Enter positive dimensions in one consistent unit.
- Review the formula, substituted values, and result.
- Use the related perimeter calculator when you need the distance around the shape.
Where it helps
Area is useful for flooring, paint, fabric, land measurement, classroom geometry, and estimating the surface of a solid. The calculator keeps the interface compact so you can switch from a rectangle to a circle or triangle without opening another page. For 3D shapes, use the surface-area option when you need the total outside area rather than the volume.
Common mistakes
The most common mistakes are mixing units, using diameter as a circle radius, or forgetting the one-half factor in triangle and trapezoid formulas. For a triangle with three sides, check the triangle inequality before using Heron’s formula. Every dimension must be positive, and a circle’s diameter must be divided by two before applying πr².