Perimeter Calculator (2024)

With this perimeter calculator, you don't need to worry about perimeter calculations anymore. Below you'll find the perimeter formulas for twelve different shapes, as well as a quick reminder about what a perimeter is and a perimeter definition. Read on, give it a try, or check this calculator's twin brother – our comprehensive area calculator.

What is perimeter?

Perimeter is the boundary of a closed geometric figure. It may also be defined as the outer edge of an area, simply the longest continuous line that surrounds a shape. The name itself comes from Greek perimetros: peri meaning "around" + metron, understood as "measure". As it's the length of the shape's outline, it's expressed in distance units – e.g., meters, feet, inches, or miles.

How to find perimeter – perimeter formulas

Perimeter of a square formula

A square has four sides of equal length. To calculate its perimeter, all you need to do is to multiply the side length by $4$4:

$$\begin{split}P_\mathrm{{square}} &= a+a+a+a\\&=4a\end{split}$$Psquare​​=a+a+a+a=4a​

Formula for the perimeter of a rectangle

The formula for the perimeter of a rectangle is almost as easy as the equation for the perimeter of a square. The only difference is that we have two pairs of equal-length sides:

$$\begin{split}P_\mathrm{{rectangle}}& = a+b+a+b\\&=2a+2b=2(a+b)\end{split}$$Prectangle​​=a+b+a+b=2a+2b=2(a+b)​

Perimeter of a triangle formula

$$P_{\mathrm{triangle}} = a+b+c$$Ptriangle​=a+b+c

However, you aren't always given three sides. What can you do then? Well, instead of fretting, you can use the law of cosines calculator to find the missing side:

$$c=\sqrt{a^2+b^2-2ab\times\cos(\gamma)}$$c=a2+b2−2ab×cos(γ)​

This can be incorporated into the perimeter formula:

$$\begin{split}&P_{\mathrm{triangle}} = a+b\\&+\sqrt{a^2+b^2-2ab\times\cos(\gamma)}\end{split}$$​Ptriangle​=a+b+a2+b2−2ab×cos(γ)​​

The other option is to use the law of sines if you have one side and the two angles that are adjacent to that side:

$$b = \sin(\beta)\times\frac{a}{\sin(\beta+\gamma)}$$b=sin(β)×sin(β+γ)a​

And:

$$c = \sin(\gamma)\times\frac{a}{\sin(\beta+\gamma)}$$c=sin(γ)×sin(β+γ)a​

so the triangle perimeter may be expressed as:

$$\begin{split}P_{\mathrm{triangle}} &= a+\frac{a}{\sin(\beta+\gamma)}\\&\times(\sin(\beta)+\sin(\gamma))\end{split}$$Ptriangle​​=a+sin(β+γ)a​×(sin(β)+sin(γ))​

Perimeter of a circle formula (circumference formula)

A perimeter of a circle has a special name – it's also known as the circumference. The most well-known perimeter of a circle formula uses only one variable – circle radius:

$$\mathrm{C} = 2\pi\times r$$C=2π×r

Have you ever wondered how many times your bike wheel will rotate on a ten-mile trip? Well, that's one of the cases where you'll need to use the circumference formula. Input the radius of your wheel (half of the wheel's diameter), and divide 10 miles by the obtained circumference (but don't forget about the conversion of the units of length!). If you want to be even more accurate, you can include the size of the bike tire.

Perimeter of a circle sector formula

Calculating the perimeter of a circle sector may sound tricky – is it only the arc length, or is it the arc length plus two radii? Just keep in mind the perimeter definition! The sector perimeter is the sum of the lengths of all its boundaries, so it's the latter:

$$P_{\mathrm{circle\ sector}} = r\times(\alpha+2)$$Pcirclesector​=r×(α+2)

where $\alpha$α is in radians.

Perimeter of an ellipse formula (ellipse circumference formula)

Although the formula for the area of an ellipsis really simple and easy to remember, the perimeter of an ellipse formula is the most troublesome of all the equations listed here. We've chosen to implement one of the Ramanujan approximations in this perimeter calculator:

$$\begin{split}&P_\mathrm{ellipse} = \pi\times\bigl(3(a+b)\\&-\sqrt{(3a+b)\times(a+3b)}\bigr)\end{split}$$​Pellipse​=π×(3(a+b)−(3a+b)×(a+3b)​)​

Where $a$a is the shortest possible radius and $b$b in the longest possible radius of an ellipse. The other, more accurate Ramanujan approximation is:

$$\begin{split}P_\mathrm{ellipse} =\ &\pi\times(a+b)\\&\times\frac{1+3\frac{(a-b)^2}{(a+b)^2}}{10+\sqrt{4-3\frac{(a-b)^2}{(a+b)^2}}}\end{split}$$Pellipse​=​π×(a+b)×10+4−3(a+b)2(a−b)2​​1+3(a+b)2(a−b)2​​​

There is also a simpler form, using an additional variable $h$h:

$$h = \frac{(a-b)^2}{(a+b)^2}$$h=(a+b)2(a−b)2​

That is:

$$\begin{split}P_\mathrm{ellipse} =\ &\pi\times(a+b)\\&\times\frac{1+3h}{10+\sqrt{4-3h}}\end{split}$$Pellipse​=​π×(a+b)×10+4−3h​1+3h​​

Or you could just use our calculator!

Perimeter of a trapezoid formula

If you want to calculate the perimeter of an irregular trapezoid, there's no special formula – just add all four sides:

$$P_{\mathrm{trapezoid}} = a+b+c+d$$Ptrapezoid​=a+b+c+d

Perimeter of a parallelogram formula

$$\quad\ \ \begin{split}P_{\mathrm{parallelogram}} &= a+b+a+b\\&=2(a+b)\end{split}$$Pparallelogram​​=a+b+a+b=2(a+b)​

$$\quad\ \ \begin{split}&P_{\mathrm{parallelogram}} = 2a\\&\qquad \ \ \ +\sqrt{2e^2+2f^2-4a^2}\end{split}$$​Pparallelogram​=2a+2e2+2f2−4a2​​

$$\ \ \ P_{\mathrm{parallelogram}} = 2\left(b+\frac{h}{\sin(\alpha)}\right)$$Pparallelogram​=2(b+sin(α)h​)

Perimeter of a rhombus formula

The perimeter of a rhombus formula is not rocket science, so let's make it concise – it's the same as the perimeter of a square formula!

$$P_{\mathrm{rhombus}} = 4a$$Prhombus​=4a

Another solution to finding the rhombus perimeter requires the diagonal lengths:

$$P_{\mathrm{rhombus}} = 2\times\sqrt{e^2+f^2}$$Prhombus​=2×e2+f2​

Try deriving the formula yourself. You know that the two diagonals of a rhombus are perpendicular to and bisect each other so that you can divide the shape into four congruent right triangles. Each triangle has legs that are e/2 and f/2 long – all you need to do is find the triangle's hypotenuse, which is, at the same time, the rhombus side. Then multiply the result by four to find the final perimeter of a rhombus formula.

Perimeter of a kite formula

The formula for the perimeter of a kite is pretty straightforward – just sum up all of the sides:

$$\begin{split}P_{\mathrm{kite}} &= a+a+b+b\\&=2(a+b)\end{split}$$Pkite​​=a+a+b+b=2(a+b)​

Perimeter of an annulus formula

As the perimeter is defined as the boundary, an annulus requires us to add the circumference of both concentric circles:

$$\begin{split}P_{\mathrm{annulus}} &= 2\pi \times R+2\pi\times r\\&=2\pi\times (R+r)\end{split}$$Pannulus​​=2π×R+2π×r=2π×(R+r)​

Perimeter of a polygon formula (regular pentagon, hexagon, octagon, etc.)

In our perimeter calculator, we've also implemented a simple formula for a regular polygon perimeter:

$$P_{\mathrm{polygon}}=n\times a$$Ppolygon​=n×a

$$P_{\mathrm{polygon}} = \sum a_i$$Ppolygon​=∑ai​

$$\begin{split}P_{\mathrm{polygon}} = &\sum_{i=1}^n\Bigl((x_{i-1}-x_i)^2\\&\qquad +(y_{i-1}-y_i)^2\Bigr)\end{split}$$Ppolygon​=​i=1∑n​((xi−1​−xi​)2+(yi−1​−yi​)2)​

With $x_{n+1}=x_1$xn+1​=x1​ and $y_{n+1}=y_1$yn+1​=y1​.

FAQ

How do I calculate the perimeter of irregular shapes?

Can I determine area given perimeter?

In general, no, it's not possible to calculate area from the perimeter. This is particularly true for rectangles, parallelograms, kites, and trapezoids. However, for some specific shapes, like squares, hexagons, regular polygons in general, and circles, you can determine their side (radius in the case of circles) from the perimeter and then proceed to compute the area.

What is the perimeter of a 20m by 15m rectangular building?

The perimeter is 70 m. To arrive at this result, you need to add together the length of all four sides of the building. Two sides of length 20 m added together give 40 m, while the other two sides of length 15 m added together give 30 m. Together, we get 40 m + 30 m = 70 m, as claimed.