  We provide several formulas that determine the optimal number of entangled
bits (ebits) that a general entanglement-assisted quantum code requires. Our
first theorem gives a formula that applies to an arbitrary
entanglement-assisted block code. Corollaries of this theorem give formulas
that apply to a code imported from two classical binary block codes, to a code
imported from a classical quaternary block code, and to a continuous-variable
entanglement-assisted quantum block code. Finally, we conjecture two formulas
that apply to entanglement-assisted quantum convolutional codes.
