How can I calculate the distance to maintain between induction heating coil and metal parts outside the coil in order to protect them from magnetic field? Is there any formula to calculate the distance?
The best protection is using a shielding material of the magnetic field as can be the permalloy with high magnetic permeability. The calculation of the distance depends of many details.
magnetic field is strong inside the coil not outside. So i guess it is very safe to use coil with few inches of distance between metallic things and induction coil.