What's the Biggest Risk to a DCF?

A DCF values a company as the sum of its discounted explicit-period free cash flows (usually 5-10 years) plus the discounted terminal value — the value of all cash flows beyond the projection window. In a typical model, the terminal value is 60-80% of total enterprise value because it captures the company's entire perpetual future, while the explicit period only covers a handful of years. That concentration means your answer is dominated by assumptions about a far-off period you can least predict. There are two methods: the perpetuity growth (Gordon Growth) method, TV = FCF × (1 + g) / (WACC − g), and the exit-multiple method, TV = final-year EBITDA × an assumed EV/EBITDA multiple. Both bury enormous leverage in a single input.

After terminal value, the discount rate (WACC) is the next biggest lever — it sits in the denominator of every cash flow and inside the perpetuity formula, so a 1% move in WACC reprices the whole thing. WACC is itself an estimate: cost of equity comes from CAPM, which relies on a contested equity risk premium, a beta that varies by data source, and a risk-free rate. Third comes revenue growth and margin assumptions in the explicit period — important, but they compound less than terminal value. The hierarchy to recite: terminal value, then WACC, then operating assumptions (growth and margins).

The mature follow-up is to show how you'd bound the risk. First, run a sensitivity table flexing WACC and terminal growth (or exit multiple) on two axes — a data table in Excel — to produce a value range rather than a single point. Second, sanity-check the implied terminal value: back out the exit multiple from a perpetuity-growth TV (or vice versa) and confirm they're consistent and reasonable versus comparable companies. Third, never anchor on the DCF alone — triangulate against comps and precedent transactions. Saying 'a DCF gives you a range, not a number, and I'd cross-check it with market-based methods' is exactly what interviewers want to hear.

A DCF is least reliable for early-stage companies (cash flows are negative and unknowable, so terminal value is essentially the entire valuation), highly cyclical businesses (the exit year may be a peak or trough), and firms undergoing major change (turnarounds, hyper-growth). It's most reliable for mature, stable, cash-generative businesses where the explicit-period forecasts are credible and terminal value is a smaller share of the total. Flagging this judgment shows you understand the tool's domain, not just its mechanics.