Area, Perimeter and Telling Time: Grade 3 Study Guide
Perimeter is the distance around a shape. Area is the space inside it. Time is how long something takes.
Two measurements are easy to confuse. Perimeter is the length of the boundary, like a fence around a garden. Area is how much flat surface is inside, like the grass in the garden. Perimeter is measured in units of length (feet, centimeters); area is measured in square units (square feet, square centimeters).
Grade 3 also introduces telling time to the nearest minute and solving problems about elapsed time, meaning how long it takes from a start time to an end time.
Area by tiling
Cover a rectangle with unit squares and count them. A rectangle that is 4 units long and 3 units wide holds 4 rows of 3 squares, so its area is 4 × 3 = 12 square units. This is why multiplication and area are linked.
Perimeter by adding
Add all the side lengths. A rectangle that is 7 cm by 4 cm has sides 7 + 4 + 7 + 4 = 22 cm. Two shapes can have the same perimeter but different areas, or the same area but different perimeters.
| Rectangle | Perimeter | Area |
|---|---|---|
| 2 by 6 | 16 | 12 |
| 3 by 4 | 14 | 12 |
| 1 by 12 | 26 | 12 |
Telling time and elapsed time
On an analog clock the short hand shows hours and the long hand shows minutes. Count by fives around the clock face to read minutes: 1 is 5 minutes, 2 is 10, and so on.
A class starts at 9:15.
Add 30 minutes to reach 9:45, then 20 more to reach 10:05.
9:15 to 10:05 is 50 minutes.
Worked examples
Example 1. A rug is 6 feet long and 4 feet wide. Area = 6 × 4 = 24 square feet. Perimeter = 6 + 4 + 6 + 4 = 20 feet.
Example 2. A rectangle has an area of 18 square centimeters and a width of 3 centimeters. Its length is 18 ÷ 3 = 6 centimeters.
Example 3. Practice starts at 3:45 and ends at 4:30. From 3:45 to 4:00 is 15 minutes. From 4:00 to 4:30 is 30 minutes. In total, 45 minutes.
Common mistakes
The biggest mix-up is using perimeter units for area. Always write square units for area. Also, do not add only two sides when finding the perimeter of a rectangle; there are four. On a clock, remember that the hour hand sits between numbers, so 4:50 has the hour hand close to 5 but it is still 4 o'clock.
Check your understanding: model answers
1. Perimeter = 5 + 8 + 5 + 8 = 26 inches. Area = 5 × 8 = 40 square inches. 2. 24 ÷ 6 = 4 feet. 3. From 2:40 to 3:00 is 20 minutes, then to 4:00 is 60 minutes, then 15 minutes more. The total is 20 + 60 + 15 = 95 minutes, or 1 hour 35 minutes.
✨ Key Takeaways
- Perimeter is the distance around; area is the space inside.
- Area of a rectangle = length × width, in square units.
- Different shapes can share an area but have different perimeters.
- Elapsed time can be found by jumping from the start time to the end time.
💬 Questions for Discussion & Reflection
- A rectangle is 5 inches by 8 inches. Find its perimeter and area.
- A garden has area 24 square feet and one side of 6 feet. What is the other side?
- A movie begins at 2:40 and ends at 4:15. How long is it?
Print graph paper for area drawings →