If you know the perimeter and some side lengths, subtract to find the missing side.
Missing side = Perimeter - sum of known sides
For rectangles: Perimeter = 2 x (Length + Width), so Length + Width = Perimeter / 2.
Perimeter • Topic 2 of 2
Find the Missing Side Given Perimeter
Solved Examples
1
Worked Example
Square perimeter is 36 cm. What is each side?
SolutionSquare side = Perimeter / 4 = 36 / 4 = 9 cm.
2
Worked Example
Rectangle perimeter 30 m, length 10 m. Find the width.
SolutionP = 2(L+W). 30 = 2(10+W). 15 = 10+W. W = 5 m.
3
Worked Example
Triangle perimeter 21 cm. Two sides are 7 cm and 8 cm. Find the third side.
SolutionThird side = 21 - 7 - 8 = 6 cm.
Key Points
- Missing side = Perimeter - all known sides.
- For rectangles: L+W = P/2, then find the unknown.
- Always check your answer by recalculating the full perimeter.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.To find a missing side, ___ the known sides from the perimeter.
Explanation: Subtract.
Q2.A square's side equals perimeter ___ $4$.
Explanation: $\div4$.
Q3.A square has perimeter $20$ cm. One side is:
Explanation: $20\div4=5$.
Q4.The perimeter is the sum of all:
Explanation: Sides.