Unfold a 3D shape flat — then add up all the faces.
Surface area is the total area of all faces (flat surfaces) of a 3D figure. A net is a 2D pattern that folds up to make a 3D shape. Finding the area of each face in the net and summing them gives the total surface area. Surface area is measured in square units.
Rectangular Prism Faces
SA = 2(lw) + 2(lh) + 2(wh)
Box: l = 8 cm, w = 5 cm, h = 3 cm. lw = 40, lh = 24, wh = 15 SA = 2(40 + 24 + 15) = 2(79) = 158 cm² You need at least 158 cm² of wrapping paper.
Remember This!
Drawing the net explicitly before computing helps you visualize all 6 faces and ensures you do not forget any face or count one twice.
A cube has side length 4 cm. What is its surface area?