Concept 1 of 4
Perimeter
Today's big idea
Perimeter is the distance around a shape, so P = 2(L + W) for any rectangle.
For an L-shape or any wiggly outline, still just add every straight edge once.
How it works
👀 What's going on?
You already know how to name a rectangle. Measuring the walk right around its edge is a different job, and that walk has a name. Perimeter is the total distance around the outside of a shape.
💡 Here's the trick
Look at the rectangle above. One trip around it covers the long side twice and the short side twice, so P = 2 × (L + W). A square is quicker still, because all four sides match: P = 4 × side.
🪄 How to do it
- 1.Start at one corner of the rectangle above.
- 2.Note the length of each side as you pass it.
- 3.Add those lengths to get the perimeter.
✍️ Try it
What is the perimeter of a rectangle 11 m long and 7 m wide? Then: what is the perimeter of a square of side 13 cm?
The rectangle gives 2 × (11 + 7) = 36 m. The square gives 4 × 13 = 52 cm.
⚠️ Watch out
You might be thinking a shape with a corner cut out must have a shorter perimeter. Cutting a square out of one corner removes two edges but adds two new ones inside. The walk round is exactly as long as it was.
🌟 Why this matters
Fencing a garden, taping the edge of a photo frame and jogging round a park are all perimeter jobs. You are buying or walking a length, so the answer is in metres, never in square metres.
Exam tip: A paper often hides perimeter inside a word like garden or floor, so read what is really being asked.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“A rectangle is 8 m by 5 m. What is the PERIMETER?”
- 1
Add length & width
8 + 5 = 13. 🚶
- 2
Multiply by 2 (rectangle rule)
P = 2(L + W) = 2 × 13
- 3
Compute
2 × 13 = 26.
- 4
Answer ✓
Perimeter = 26 m.
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
Tap every side, one at a time. Each tap adds that side's length to the walk.
RECTANGLE · ROUND 1 OF 3
6 cm × 4 cm
Walked so far: 0 = 0 cm
Quick check
✅ VERIFICATION
A triangle has sides 12 cm, 8 cm and 5 cm. What is its PERIMETER?
⚡ Finish the activity above first to answer.
Keep going!
Up next: Area
L × W · side² · ½ × b × h.