Calculating Inductance

To calculate the inductance of an Air, one layer coil, use the following formula:

L = 0.001 N2r2 / (228r + 254l)

where L is the inductance in henrys, r is the coil radius in metres, l is the coil length in metres (>0.8r) and N is the number of turns.
(source: http://info.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html)

For example:
N= 7 turns
r = .004 (4mm) - radius of coil
L=.011 (1.1cm)

• .001 * 7 squared * .004 squared / (228 * .004) + (254 * .011)
  
• .000000784 / 3.706
• .00000021154
• convert to micro Henry (multiply by 1,000,000 since micro is a million)
• .21uH Inductance