Perimeter of Rectangle Calculator
Enter the length and width of a rectangle to find its perimeter.
Updated
Perimeter of Rectangle Calculator
Our Perimeter of Rectangle Calculator finds the perimeter of a rectangle from its length and width, in any unit. It also shows the working, the area, the diagonal and the perimeter in other units, with a drawing to scale. Use it for homework, fencing, trim, frames and room layouts.
Quick answer
The perimeter of a rectangle is 2 times the length plus the width: P = 2 × (8 + 5) = 26 cm.
- Formula: P = 2 × (l + w) = 2l + 2w
- Missing side: w = P ÷ 2 − l
- Square: P = 4 × side
- Units: the perimeter is a length, so cm stays cm (not cm²)
What Is the Perimeter of a Rectangle?
A rectangle is a four-sided shape with four right angles. Its opposite sides are equal and parallel: two long sides (the length) and two short sides (the width, or breadth). The perimeter is the total distance around the outside, so it is the four sides added together.
Perimeter of a Rectangle Formula
P = 2l + 2w
Both forms give the same answer. Adding the length and width first gives half the perimeter (one long side and one short side), and doubling it covers the other two.
- Add the length and the width. 8 + 5 = 13.
- Multiply by 2. 2 × 13 = 26.
- Write the answer in the same unit. 26 cm.
Rectangle Perimeter Examples
| Length × width | Perimeter |
|---|---|
| 8 cm × 5 cm | 2 × 13 = 26 cm |
| 12 m × 9 m | 2 × 21 = 42 m |
| 15 in × 10 in | 2 × 25 = 50 in |
| 7.5 ft × 3.5 ft | 2 × 11 = 22 ft |
| 6 m × 6 m | 2 × 12 = 24 m |
The last row is a square, so the shortcut P = 4 × 6 = 24 m gives the same answer.
Sides in Different Units
Convert both sides to the same unit before adding. A shelf 2 m long and 50 cm deep: 50 cm = 0.5 m, so P = 2 × (2 + 0.5) = 2 × 2.5 = 5 m, which is 500 cm. Pick a unit for each side in the calculator and it converts the width to the length's unit for you.
Finding a Missing Side
30 ÷ 2 − 9 = 6
Halve the perimeter to get one length plus one width, then take away the side you know. A rectangle with a perimeter of 30 m and a length of 9 m has a width of 15 − 9 = 6 m. The same works the other way: l = P ÷ 2 − w.
Perimeter vs Area
The perimeter is the distance around a shape (a length, such as m). The area is the space inside it (square units, such as m²). Rectangles with the same perimeter can have very different areas. For a fixed perimeter, a square encloses the most area.
Common Mistakes
- Forgetting to double: l + w is only half the perimeter. 8 + 5 = 13, but P = 26.
- Multiplying the sides: l × w is the area, not the perimeter.
- Square units: a perimeter is a length (cm, m, ft), not an area (cm²).
- Mixing units: convert both sides to the same unit before adding.
- Counting a shared wall: for fencing along a house, leave out the side that is not fenced.
Key Terms
- Length
- The longer pair of equal sides, l.
- Width
- The shorter pair of equal sides, w (also called breadth).
- Perimeter
- The distance all the way around, P = 2(l + w).
- Area
- The space inside, A = l × w, in square units.
How to Use the Perimeter of Rectangle Calculator
- Enter the length and width. Type the two sides, such as 8 and 5. Decimals and fractions such as 3/4 work.
- Pick the units. Choose a unit for each side if you want one (mm, cm, m, km, in, ft, yd or mi). Mixed units are converted for you.
- Read and copy the result. The perimeter appears as you type, with a drawing to scale, the working, the area, the diagonal and the perimeter in other units. Press Copy to copy it, or Clear or Esc to start again.
Example: a garden is 12 m long and 7.5 m wide. Enter 12 and 7.5 and pick m. The perimeter is 2 × 19.5 = 39 m, so you need 39 m of fencing to go all the way around.
Who Can Use This Calculator?
Frequently Asked Questions (FAQs)
How do you find the perimeter of a rectangle?
Add the length and the width, then multiply by 2. A rectangle 8 cm long and 5 cm wide has a perimeter of 2 × (8 + 5) = 26 cm.
What is the formula for the perimeter of a rectangle?
P = 2 × (l + w), which is the same as P = 2l + 2w, where l is the length and w is the width.
How do you find the width from the perimeter and length?
Halve the perimeter and subtract the length: w = P ÷ 2 − l. A perimeter of 30 m and a length of 9 m give a width of 6 m.
What is the difference between perimeter and area?
The perimeter is the distance around the rectangle, 2 × (l + w), in units such as m. The area is the space inside, l × w, in square units such as m². An 8 m by 5 m rectangle has P = 26 m and A = 40 m².
Can two rectangles have the same perimeter but different areas?
Yes. A 10 by 2, an 8 by 4 and a 6 by 6 rectangle all have a perimeter of 24, but their areas are 20, 32 and 36. For a fixed perimeter, the square has the largest area.
Is the perimeter of a square found the same way?
Yes, a square is a rectangle with equal sides, so 2 × (l + w) works. With side s it simplifies to P = 4s: a 6 m square has a perimeter of 24 m.
Is the Perimeter of Rectangle Calculator free?
Yes. It is completely free, needs no sign-up, and can be used as many times as you like.
Try our Perimeter of Rectangle Calculator to find the perimeter of any rectangle from its length and width, in any unit.