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.

Which is a better measure for capital budgeting, IRR or NPV?
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How do you use internal rate of return to calculate a capital budget?
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What is the formula for calculating the internal rate of return (IRR)?
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How do you use discounted cash flow to calculate a capital budget?
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What does a high weighted average cost of capital (WACC) signify?
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Internal Rate Of Return  IRR
A metric used in capital budgeting measuring the profitability ... 
IRR Rule
A measure for evaluating whether to proceed with a project or ... 
Modified Internal Rate Of Return  MIRR
While the internal rate of return (IRR) assumes the cash flows ... 
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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1. A performance metric that is equal to the difference between ...