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Perimeter formulas
Need a perimeter formulas within reach? Print it for the wall, then hand off to the practice sheets below.
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Reference chart
Perimeter formulas
Rectangle
2 × l + 2 × w
16 + 6 = 22 cm
Square
4 × s
4 × 7 = 28 cm
Triangle
a + b + c
3 + 4 + 5 = 12 cm
Any shape
Add every side
6 + 4 + 5 + 5 + 4 = 24
Circle
π × d
(the circumference)
3.14 × 10 = 31.4
Missing side
Halve P, take l
11 − 8 = 3 cm
Perimeter is a length, so the answer is in cm or in — never square units. If you are filling the inside rather than walking around the edge, you want area.
What's on this chart
A formula sheet without figures asks a child to do three things: picture the shape, decide which of its numbers is which letter, and then apply the rule. Two of those are the work the chart was supposed to remove. So every shape here is drawn, its sides are labelled on the drawing, and the edge being measured is traced in colour — because a perimeter figure and an area figure are otherwise the same picture with different words underneath.
Why this chart helps
That tracing is the whole distinction this page exists to protect. Perimeter and area get confused constantly, and not because the arithmetic is hard: a child who writes 2(8) + 2(3) for the area of a rectangle knows both formulas perfectly well and reached for the wrong one. Seeing the line run round the outside, with the middle left empty, is a better reminder than any sentence.
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