Area and perimeter of rectangles

This resource supports teachers to assess and find appropriate activities for students who need acceleration in their understanding and application of finding the area and perimeter of rectangles.

Two tamariki measuring a box together with measuring tapes.

About this resource

New Zealand Curriculum: Level 3 to early Level 4

Learning Progression Frameworks: Measurement sense, Signpost 5 to Signpost 7

These activities are intended for students who understand how to use metric units of measure to find lengths. When working with units, they should understand the following:

Units relate to the attribute being measured (for example, length is measured with iterations of length).
Identical units need to be used when measuring.
Units should be tiled (or iterated) with no gaps or overlaps to create a measure.
Units can be equally partitioned into smaller units when greater accuracy is needed.

Students should also know how to use a measurement scale, such as a ruler or tape measure. They should be familiar with the most common metric units of length, metres, centimetres, and possibly millimetres, though they may not be able to convert measures (e.g., 45 cm = 450 mm). Students should also have a partial or full grasp of their basic multiplication and division facts.

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Area and perimeter of rectangles

The following diagnostic questions indicate students’ understanding of, and ability to find the areas and perimeters of flat spaces, particularly rectangles. Allow access to pencil and paper and to a calculator if students need it.

The questions should be presented orally and in written form (Area perimeter 1) so that the student can refer to them. The questions have been posed using a money context but can be changed to other contexts that are engaging to your students.

Required materials

See Materials that come with this resource to download:

Area perimeter 1 (.pdf)

Activities

Question 1

Here are three different floors. The numbers show how many tiles are on each side.

Which floor has the biggest number of square tiles?

Show Area perimeter 1, page 1.

Signs of fluency and understanding

Using multiplication to count the number of tiles, i.e., 3 x 6 = 18, 4 x 4 = 16, and 2 x 9 = 18, recognises that the product of numbers of rows and columns gives the number of tiles.

What to notice if they don’t solve the problem fluently

Counting in ones, or by skipping counting in twos, to find the total number of tiles in each floor. This indicates that the student is not yet able to structure the rectangle into rows and columns. It may also indicate a lack of recognition that multiplication can be used to count the number of tiles.

Supporting activity

Finding areas of rectangles

Question 2

Measure the sides of each rectangle in centimetres. Write the measurements down.
Use the measurements to work out the area of each rectangle in square centimetres.
Write the areas using appropriate numbers and units.

Show Area perimeter 1, page 2, and make a calculator available.

Signs of fluency and understanding

Measures the side lengths accurately to the nearest centimetre and correctly writes the measures.
Applies multiplication to find and record the areas, i.e., 2 x 18 = 36 cm², 3 x 12 = 36 cm², 6 x 6 = 36 cm², and 4 x 9 = 3 636 cm².

What to notice if they don’t solve the problem fluently

Draws lines to partition each rectangle into square units of 1 cm2, then finds a way to count all the squares. This indicates an inability to anticipate the structure of the rectangle in rows and columns.
Applies additive strategies to find the number of square units, such as 6 + 6 = 12, 12 = 12 = 24, and 24 + 12 = 36, to find the area of the square. This may indicate that the student has yet to establish multiplication as a binary (two numbers at a time) operation.

Supporting activity

Finding the areas of rectangles using side lengths

Question 3

These rectangles have the same area. The numbers show the length of each side.

Do the rectangles have the same perimeter? Please explain.

You may need to explain that the perimeter is the distance around the outside of each rectangle.

Show Area perimeter 1, page 3, and make a calculator available.

Signs of fluency and understanding

Calculates the perimeter of each rectangle fluently, possibly using a calculator. Recognises that each side measure occurs twice in the perimeter. For example, they may use 2 x 12 + 2 x 4 or 2 x (12 + 4) to work out the perimeter of the 12 x 4 rectangle (example).
Might know that regular rectangles tend to have a smaller perimeter for a fixed area. Regularity means that the sides are equal, so the rectangle closest to a square has the least perimeter.

What to notice if they don’t solve the problem fluently

May use counting or additive strategies to find the perimeters, rather than a combination of addition and multiplication. This suggests the student needs to work on modelling situations that combine operations, like addition and multiplication.

Supporting activity

Finding perimeters

Question 4

Which shape has the greatest area?
Which shape has the greatest perimeter?

Show Area perimeter 1, page 4.

Signs of fluency and understanding

Calculates the area and perimeter of each rectangle and allows for half measures of side lengths. May use mental and paper calculation or a calculator using multiplicative reasoning. For example, the area of the 5½ x 5 rectangle is 5.5 x 5 = 27.5 square units, and the perimeter is 2 x (5.5 + 5) = 2 x 10.5 = 21 units.
Answers are:
- Area: 5.5 x 5 = 27.5 square units for top left, 4 x 6.5 = 26 square units for top right, and 2.5 x 10 = 25 square units for bottom left. So the top-left rectangle has the biggest area.
- Perimeter: 2 x (5.5 + 5) = 2 x 10.5 = 21 units for top left, 2 x (4 + 6.5) = 2 x 10.5 = 21 units for top right, and 2 x (2.5 + 10) = 2 x 12.5 = 25 units for bottom left. So the bottom left rectangle has the biggest perimeter.
Recognises that length (perimeter) and area are different attributes, so the measures are expressed using different units, length units for perimeter and square units for area.

What to notice if they don’t solve the problem fluently

May confuse the two attributes, area and perimeter, when providing answers.
May have difficulty with partial units and either ignore them or be unsure about how to allow for half units in calculation. This might indicate that the student needs experience in finding areas of shapes that include partial units and in calculating areas and perimeters where partial units are involved.

Supporting activity

Working with partial units

Question 5

Find the area and perimeter of this rectangle as accurately as you can.
Use a ruler to measure the side lengths first, then find the answers using a calculator.

Show Area perimeter 1, page 5.

Signs of fluency and understanding

Measures the side lengths accurately and expresses the lengths as 18.5 cm, or 185 mm, and 11.3 cm, or 113 mm.
Correctly applies an appropriate algorithm for both area and perimeter. For the area, they work out 18.5 x 11.3 = 209.05 cm² or 185 x 113 = 20 905 mm². For the perimeter, they work out 2 x (18.5 + 11.3) = 59.6 cm or 2 x (185 + 113) 596 mm. Answers should include the number and unit of measure.

What to notice if they don’t solve the problem fluently

May be unsure about how to accommodate fractional numbers of measurement units. This may show as uncertainty regarding how to express length and width as 18.5 cm and 11.3 cm respectively. This may indicate that the student needs experience with accurately measuring lengths using smaller units in the metric system.
May measure the side lengths accurately, but be unclear about how to find the area and perimeter by calculation and/or express the resulting measure using appropriate units. This indicates the student needs more experience with calculating areas and perimeters with partial units, as provided above in Working with partial units.