GRADE 3
http://www.math3.nelson.com/studentcentre/studtryout.html#ch8- Grade 3 area and grid activities.
http://ca.ixl.com/math/grade-3/area - Determine area of shapes.
https://ca.ixl.com/math/grade-3 - Grade 3: Go to the section called "Geometric measurement" and do any of the activities.
GRADE 4
http://www.math4.nelson.com/studentcentre/studtryout.html#ch8 - Grade 4 area and grid activities.
https://ca.ixl.com/math/grade-4 - Grade 4: Go to the section called "Geometric measurement" and do any of the activities.
http://www.adaptedmind.com/p.php?tagId=1158# - Determine area of shapes.