You can use the below formula to find the inductance of the air core coil. Air core inductance formula is derived as ((d2) x (n2)) / (18d + 40l). The first step is to square the diameter and Number of turns, multiply the squared value of both. Then, second step is to separately multiply the coil diameter with 18 and multiply 40 with coil length. Add both the obtained values. Finally, divide the step 1 by step 2, the resultant value is the inductance of air coil.
Air core inductance formula is made simple as you will get the inductance of air coil by substituting the values. The disadvantage of air coil inductance is, it is often affected by the mechanical vibrations as it is not rigidly supported by coil form.