How to Calculate the Surface Area of Irregular Shapes

Written by charlotte johnson
How to Calculate the Surface Area of Irregular Shapes
Use graph paper to help you keep your sketch fairly accurate. (Brand X Pictures/Brand X Pictures/Getty Images)

When you think of regular shapes -- squares, rectangles, circles and triangles probably come to mind. However, in the real world, many objects possess shapes that are not regular. The surface area of a shape is the two-dimensional space contained within that shape. You might have to measure surface area as an assignment in a math class. You also might have to find surface area in real life if you are measuring a home or piece of land. In this case, surface area is often referred to as "square feet" or "square footage." You can determine the surface area of irregular shapes through a series of basic calculations.

Sketch the irregular shape onto a piece of graph paper if you haven't already been provided with a sketch. If necessary, measure the dimensions of the shape with a ruler or tape measure, depending on the size of the shape.

Divide your sketch into regular shapes. Use rectangles and squares first. If necessary, use triangles and circles as well.

Calculate the area of each shape division that you created on the sketch. If you have rectangles or squares, multiply the width times the length. In the case of triangles, multiply the base times the height and divide your product by the number 2. If you have a circle, multiply the radius squared times pi, which equals 3.14. The radius is half of the diameter. The diameter is the width of the circle. When you square the radius, you multiply it times itself.

Add the surface areas of the individual shapes that you calculate in Step 3 in order to find the surface area of the entire irregular shape.

Things you need

  • Graph paper
  • Pencil
  • Ruler or tape measure
  • Calculator

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