When uncoupled inductances L₁, L₂, L₃, ... are connected in series, the total inductance L_S is:
L_S = L₁ + L₂ + L₃ + ...

When two coupled inductances L₁ and L₂ with mutual inductance M are connected in series, the total inductance L_S is:
L_S = L₁ + L₂ ± 2M
The plus or minus sign indicates that the coupling is either additive or subtractive, depending on the connection polarity.