Pick a perimeter to work on
Drag on the grid to build a rectangle — or use the buttons below.
Perimeter — around the outside
0units
Area — inside the rectangle
0square units
Width ↕ down the side
1
Length ↔ across the top
1
My chart
Rectangles I made
0
Stuck? Try a strategy
For teachers — how to run Concept Development with this tool
Part 1 · perimeter of 10, then 14
- Share your screen with 10 selected. Ask what the perimeter is on the first chart, then build one rectangle with a perimeter of 10 and press Count around so students see the 10 unit edges get counted.
- Ask: "Previously we added the length and width, then multiplied by 2." Turn on the equation line under Perimeter to show P = 2 × (L + W).
- Ask what the opposite of doubling is, then use Halve it: 10 ÷ 2 = 5, so find pairs of numbers with a sum of 5.
- Students record on their own chart. Then send them to 14 to repeat the method with a partner.
Part 2 · perimeter of 12, then 16, 18, 20
- Students work in pairs on 12, then compare charts. If their widths and lengths differ, they figure out why and come to an agreement.
- "How many different rectangles did you find?" — the chart header shows the count.
- "Are any of your rectangles squares? How do you know?" — a square row is flagged green in the chart.
- Perimeters of 16, 18, and 20 finish the data sheet.
Completing a chart opens the comparison strip: same perimeter, different areas, with the smallest and greatest area marked. That is the Part 2 discussion, made visible.
The margin-box strategy
The Teacher Edition offers a second strategy in the margin box, and says to read both and decide which fits your class — or teach both. It lives in the Doubles tab: list doubles up to the perimeter, find pairs of doubles that add to it, then halve each one to get the width and length.
The TE demonstrates it with a perimeter of 22. Choose Any perimeter and set it to 22 to model that exact example.
Teaching this virtually
- Model first, then release. Share your screen for one rectangle, then paste the link in chat so every student has their own grid. No login, nothing to install.
- This replaces the physical materials in the lesson: square unit tiles, grid paper, and the personal white board all live on one screen.
- Breakout rooms: pairs build on the same perimeter, then one student shares their screen to compare charts.
- Checking work: ask students to press Save a copy and turn in the file, or Print my charts for a paper copy. The TE asks you to keep the data sheet for Lesson 17.
- Ask for the count, not the answer. "Type in chat how many rectangles you found for 18" surfaces who is still hunting.
Supports built in
- Double means two times as much. The Doubles tab shows 1 tile becoming 2, and 4 becoming 8, for students who need that clarified.
- Every rectangle can be turned sideways — the tool names that as the same rectangle instead of just rejecting it.
- Counting is one edge and one square at a time, so students who lose track can watch it again.
Sentence stems for pair talk
The length/width is ___ because…
I disagree because…
Can you explain…?
What can we do first?
Debrief prompts
- Why are the areas of the rectangles different even though the perimeters are the same?
- What are the widths and lengths you found? Explain to a partner how you found them.
- Which perimeter let you draw a square? How do you know?
Squares showed up for 12, 16, and 20 — the perimeters you can split into four equal sides. Let students notice the pattern before you name it.