Finding the area of a rectangle
In 3rd grade, children learn to find a rectangle's area by multiplying its length by its width, and to find the area of L-shaped figures by splitting them into rectangles.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 3rd grade practice. Tap the speaker to hear one read aloud, or the light bulb for a hint, and talk about the answer together.
Question 1 of 3
A rectangle is made of square tiles. There are 3 rows with 2 tiles in each row. What is the area of the rectangle?
- 6 square units
- 10 square units
- 5 square units
Why this matters
Area and multiplication meet here, in a picture your child will keep coming back to for years. Splitting a 6 by 7 rectangle into a 6 by 5 and a 6 by 2 becomes a main tool for multiplying bigger numbers like 23 x 14 in 4th grade, and for multiplying fractions in 5th grade. It also answers practical questions, such as how much carpet a room needs.
Where kids can get stuck
- Adding the length and width instead of multiplying, so a 6 by 4 rectangle gets an area of 10 instead of 24.
- Counting an overlapping section twice when splitting an L-shape, or using the wrong side length for one piece.
- Multiplying every side length on an L-shaped figure together, getting a huge number instead of adding two areas.
How you can help
- Measure a rectangular rug, table, or garden bed in feet and ask your child to find its area by multiplying, then check by counting if tiles or squares are visible.
- Look at a chocolate bar or sheet of stickers arranged in rows and ask how many squares there are without counting each one.
- Draw an L-shaped room on paper, label the sides, and ask your child to split it into two rectangles to find the whole area.
If it’s helpful, you can sign up for free, adaptive practice in Bobby's Math Club.
Questions parents ask
Why does length times width give the area of a rectangle?
When a rectangle is covered by squares, they line up in equal rows. A rectangle 5 units long and 3 units wide holds 3 rows of 5 squares, which is 3 x 5 = 15 squares. Multiplying is a fast way to count those equal rows. Your child is expected to understand this, not only use the formula.
How does my child find the area of an L-shaped figure?
Your child draws a line to split the L-shape into two rectangles, finds the area of each, and adds them. For example, a 6 by 4 rectangle and a 3 by 2 rectangle make 24 + 6 = 30 square units. There is often more than one way to split the shape, and every correct split gives the same total.
What is an area model?
An area model is a rectangle used to show multiplication. To find 7 x 8, your child can split the rectangle into 7 by 5 and 7 by 3 pieces, then add 35 + 21 to get 56. In later grades, the same picture helps your child multiply two-digit numbers, such as 14 x 23, and even fractions.
Make learning math fun
Bobby's Math Club turns 3rd grade math practice into something fun to do together.
- Free, unlimited practice for every child in your family
- Questions matched to your child’s level, a few minutes a day
- Hints when they are stuck, and a celebration when it clicks
Written by Robert “Bobby” Barnett
Bobby is an award-winning math teacher, the author of Meet Every Learner’s Needs, and the father of two elementary-school math learners. He uses Bobby’s Math Club with his own kids a few times every week. Learn more or contact Bobby.
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Join the clubNote for teachers: This skill is covered by Common Core math standard 3.MD.C.7: “Relate area to the operations of multiplication and addition. a. Find the area of a rectangle with whole-number side lengths by tiling it, and show that the area is the same as would be found by multiplying the side lengths. b. Multiply side lengths to find areas of rectangles with whole-number side lengths in the context of solving real world and mathematical problems, and represent whole-number products as rectangular areas in mathematical reasoning. c. Use tiling to show in a concrete case that the area of a rectangle with whole-number side lengths a and b + c is the sum of a × b and a × c. Use area models to represent the distributive property in mathematical reasoning. d. Recognize area as additive. Find areas of rectilinear figures by decomposing them into non-overlapping rectangles and adding the areas of the non-overlapping parts, applying this technique to solve real world problems.” Learn how Bobby's Math Club can help your students grow. (Common Core 3.MD.7)