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Grades 3–5 · Mathematics

Can equal areas have different perimeters?

Two gardens can have exactly the same space for plants but need different amounts of fencing. Nothing has gone missing. The ground inside a garden and the distance around its edge are different things to measure.

Imagine twelve square paving tiles, each one unit long on every side. Arranged in three rows of four, they cover twelve square units of ground. This is the area. A fence following the outside would travel four units across, three down, four back, and three up. Its total length, the perimeter, would be fourteen units.

The same twelve tiles can form two rows of six. This rectangle still covers twelve square units: no tiles have been added or removed. But its outside edges measure six, two, six, and two units. The fence now needs sixteen units of length. The longer, narrower arrangement has two extra units of boundary around the same amount of ground.

The tile edges inside the rectangle explain the change. When two tiles sit beside one another, their touching edges become an inside seam. That seam helps divide the tiled surface but does not need an outside fence. The three-by-four arrangement has more shared seams than the two-by-six arrangement, so fewer tile edges remain exposed around its boundary.

The pale grid lines are shared inside seams. Only the orange outside border counts toward the fence length.

Equal area and different perimetersTwo rectangles use the same twelve unit squares at the same scale. The four-by-three rectangle has perimeter fourteen; the six-by-two rectangle has perimeter sixteen. Only the orange outside boundaries count.Same tiles, different fenceEach tile: 1 square unit3 rows of 44 units3Area: 12 square unitsPerimeter: 14 units2 rows of 66 units2Area: 12 square unitsPerimeter: 16 unitsOutside fenceInside seam
Both arrangements use twelve identical tiles at the same drawing scale. Orange outer edges total fourteen units for the three-by-four rectangle and sixteen for the two-by-six rectangle.

Area uses square units because it counts a surface. Perimeter uses length units because it measures a line. A carpet covers an area; trim follows an edge. The distinction matters whenever someone plans a floor, frames a picture, or lays out a garden. Knowing how much space something covers does not, by itself, tell us how long its border will be.

About this explanation

The rectangles use exact square tiles with one-unit sides. Equal areas can have different perimeters, but they do not always have to.

Starting knowledge: Multiply small whole numbers and distinguish a square unit from a length unit.

Sources

  • Common Core State Standards Initiative — Grade 3: Area and perimeter comparisons (3.MD.D.8)
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