Grade 3 · Unit 5: Using units of measurement

Area and perimeter problems

Same area, different fence!

Fencing a garden

A garden needs a fence all the way round: that is the perimeter. It needs soil inside: that is the area. Two different measurements!

Same area, different perimeter

All three pens have the same area: 12 square metres. But the perimeters are different. The squarer the pen, the less fence it needs.

Irregular shapes

A pond or a leaf is not a rectangle. Count the whole squares, then join half squares into wholes. This gives a good estimate of the area.

Measure real things

Measure a book: 21 cm by 15 cm (model). Area = 21 × 15 = 315 square centimetres. Perimeter = 21 + 15 + 21 + 15 = 72 cm.

What I learnt

• I know shapes with the same area can have different perimeters.
• I can estimate the area of irregular shapes.
• I can measure area and perimeter of real objects.