How to Calculate WACC and Use It to Size New Debt

WACC is the weighted blend of your cost of equity and your after-tax cost of debt, where the weights are each source's share of your total capital. Cost of equity is what your investors require as a return for the risk of holding your stock instead of something safer. Cost of debt is what your lenders charge, reduced by the tax benefit of deducting interest.

The formula itself is simple once you have both inputs; the real work is estimating cost of equity for a company that doesn't trade on an exchange, since there's no market price doing that estimation for you automatically.

Public companies can back out an implied cost of equity from their stock price and a model like CAPM. Private companies usually build the number up instead: start with a risk-free rate, add a general equity risk premium for holding stock instead of bonds, then add a size premium and a company-specific risk premium reflecting how much riskier your business is than a large, established public company.

If your equity investors have already stated a required return, whether that's a venture fund's target or a family office's minimum, that stated number is often the more practical starting point than building one up from scratch, since it reflects what your actual investors expect rather than a generic industry estimate.

Take the interest rate on your loan and reduce it by your effective tax rate, since interest payments are typically deductible. That after-tax number, not the stated rate on the term sheet, is what belongs in the WACC calculation, because it reflects what the debt actually costs your company once the tax benefit is accounted for.

If you're comparing multiple debt options, run this same adjustment on each one before comparing rates, since a loan with a slightly higher stated rate but the same tax treatment can still come out cheaper or more expensive than another option once fees and any warrant coverage are factored in.

Say your company is financed entirely with equity that investors expect to return an amount well above what a lender would charge on a term loan. Your WACC in that all-equity case simply equals your cost of equity, since there's no debt in the mix to weigh it down.

Now say you add a term loan that makes up a third of your total capital, at a rate meaningfully below your cost of equity, and after-tax the effective cost of that debt is lower still because the interest is deductible. Blend the two: the equity share of your cost of equity plus the debt share of that after-tax debt cost pulls your overall WACC down from where it stood as an all-equity company, because you've replaced some of the more expensive capital with cheaper debt.

To calculate WACC for your own company, work through it in this order:

1. Estimate your cost of equity by starting with a risk free rate and adding an equity risk premium, then adjust for the added risk of holding private company stock.
2. Take the interest rate on your loan and reduce it by your effective tax rate to get the after-tax cost of debt.
3. Work out the weights by dividing your equity and your debt by your total capital.
4. Multiply each cost by its weight and add the two results to get your blended cost of capital.
5. Recalculate after any financing that meaningfully changes your capital structure, since added debt shifts both the weights and possibly the rates.

Adding debt only lowers your blended cost of capital while lenders and equity holders aren't demanding more for the added risk. Beyond a certain point, more debt makes both your remaining equity riskier, because equity holders are now behind more debt in a downturn, and your next lender pricier, because your existing debt load has already used up some of the company's cushion.

There's no fixed threshold where this happens; it depends on your industry, your cash flow stability, and how much debt your lenders and investors already expect a company like yours to carry. The practical signal is when your next dollar of debt starts coming with a materially higher rate or tighter covenants than the last dollar did, which is the market telling you that additional debt is no longer cheap capital.