When resistances R₁, R₂, R₃, ... are connected in parallel, the total resistance R_P is:

1 / R_P = 1 / R₁ + 1 / R₂ + 1 / R₃ + ...

Alternatively, when conductances G₁, G₂, G₃, ... are connected in parallel, the total conductance G_P is:

G_P = G₁ + G₂ + G₃ + ...

where G_n = 1 / R_n

For two resistances R₁ and R₂ connected in parallel, the total resistance R_P is:

R_P = R₁R₂ / (R₁ + R₂)

R_P = product / sum

The resistance R₂ to be connected in parallel with resistance R₁ to give a total resistance R_P is:

R₂ = R₁R_P / (R₁ - R_P)