# Circle surface area

1. Nov 21, 2014

### magmash

Hi

How is circular surface area values square rooted ??. I am using this formula r2Xπ=mm2 to calculate the surface area of a circle but i want to know if there is a formula to get back to the diameter or the radius of the circle ?.

example i have a circular surface area of 5.26mm2 and i want the diameter of that value, how do i solve this ?

2. Nov 21, 2014

### ShayanJ

$S=\pi r^2 \Rightarrow r^2=\frac{S}{\pi} \Rightarrow r=\sqrt{\frac{S}{\pi}}$

3. Nov 21, 2014

### magmash

Thanks for the replay Shyan.

a circel with a diameter of 1.29mm 0.6452π = 1.30 The formula you mentioned √s/π gives a value of 0.362.. ???

Could you show me an example, I would be very thanks full

4. Nov 21, 2014

### ShayanJ

Careful boy! Its not $r=\frac{\sqrt{S}}{\pi}$, its $r=\sqrt{\frac{S}{\pi}}$.

5. Nov 21, 2014

### magmash

:D

Thanks

6. Nov 21, 2014

### Staff: Mentor

We don't call it "surface area" - just area. Surface area applies to surfaces in three dimensions. For objects in the plane (two dimensions), we talk about the area.
The actual formula is $A = \pi r^2$. If r is given in mm., then the units of area will be square millimeters or mm2. The formula shouldn't explicitly show units.
From the equation $A = \pi r^2$, to get the radius, solve for r. To do this, divide both sides of the equation by $\pi$, then take the square root. Once you have the radius r, you can get the diameter using d = 2r.