Weighted average cost of capital (WACC) is the average aftertax cost of a company’s various capital sources, including common stock, preferred stock, bonds and any other longterm debt. By taking the weighted average, the WACC shows how much interest the company pays for every dollar it finances.
The internal rate of return (IRR), on the other hand, is the discount rate used in capital budgeting that makes the net present value (NPV) of all cash flows (both inflow and outflow) from a particular project equal to zero. It is used by companies to compare and decide between capital projects. For example, a company may evaluate an investment in a new plant versus expanding an existing plant based on the IRR of each project.
The primary difference between WACC and IRR is that where WACC is the expected average future costs of funds (from both debt and equity sources), IRR is an investment analysis technique used by companies to decide if a project should be undertaken. A close relationship exists between WACC and IRR, however, because together these concepts make up the decision for IRR calculations. In general, the IRR method indicates that a project whose IRR is greater than or equal to the firm's cost of capital should be accepted, and a project whose IRR is less than the firm's cost of capital should be rejected.

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What is the formula for calculating internal rate of return (IRR) in Excel?
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How do interest rates affect the weighted average cost of capital (WACC) calculation?
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Pooled Internal Rate Of Return  PIRR
A method of calculating the overall internal rate of return (IRR) ... 
Weighted Average Cost Of Capital  WACC
Weighted average cost of capital (WACC) is a calculation of a ... 
Net Present Value  NPV
Net Present Value (NPV) is the difference between the present ... 
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A calculation used, either by a firm or investors, to determine ... 
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