The discounted sum of a project's present and future cash flows. In theory all projects with positive a NPV should be viable, though it is common experience that this is not the case. The interest rate (r) where the NPV is zero is internal rate of return (IRR).

NPV is generalized:

Σct/(1+rt)t for t = 1..n
is the zero-base number of the period
is the signed cash flow
is the interest rate for period t