Magnetic field at center of 60-turn coil with 0.2m radius and 4A current

[pooka]

What is the magnetic field at the center of a 60-turn coil carrying a current of 4 A, if it has a radius of 0.2 m?

I was thinking of using the equation B=(mu0)(N/l)(current)

But I was wondering do I use the radius for l or do I use the diameter?

 

[rock.freak667]

I think you need to consider it as flat circular coil instead of a solenoid

$$B=\frac{\mu_0 NI}{2r}$$

 

[pooka]

thanks!