# DIY Inductor: Solve for number of turns without knowing the length?

I'm trying to wrap my own inductors (air coil) for an RF circuit that I am making and I am having a little trouble solving for the number of turns I need to reach the desired inductance.

$L=\dfrac{D^2N^2}{18D+40l}$

• L = inductance in uH
• D = coil diameter in inches
• l = length of the coil
• N = number of turns

I'm wondering if there is a way to negate l from the equation using the gauge of the wire. I'm using 32AWG magnet wire and have found that 113 turns is equivalent to 1 inch of l. Can I just replace l with N/113?

My goal is to be able to measure the diameter of the non ferrite core I am going to wrap around and plug in the desired inductance. It shouldn't matter how long the coil is in the end unless I reach the end of the core and wrap another layer on top.

If possible, is there an online calculator that will do this for me?

• You are assuming what is called a close wound coil. That is one possibility, but not the only one - spacing the turns can be one of the easiest ways to adjust the value of the result. – Chris Stratton Mar 22 '13 at 4:46