The net present value is a summation of the present value of all cash inflows
and outflows minus the initial project cost (C_{0}). To include the
effects of taxation (essentially a business expense), all actual cash flows for
tax-paying organizations are reduced by the formula given by the amount
after taxes = C·(1 -T_{x}).

Depreciation allowances are an excellent example of where such tax savings
are possible, they are treated similarly to income. All expenses allowed to be
charged against income for tax purposes, but not representing actual cash flow,
are modified by the following formula: Cash flow =
non-cash expense charge (T_{x})

The PV of the tax savings cash flow from a depreciation expense series (DES)
of an original cost is given by: PV_{DES} = C_{0}(Q,
i, N_{Q})·T_{x}

- where Q is the present value factor for a cash flow stream and
- N
_{Q}is the time span of the depreciation expense stream

Combining these definitions, one can obtain the fundamental NPV equation that includes the tax effects: