DCF Valuation: Common Errors in Key Assumptions

A DCF model rarely goes wrong because someone entered the wrong formula. More often, the real problem lies in one or two assumptions behind the calculations. If those assumptions are unrealistic, inconsistent, or not properly stress-tested, the resulting valuation can be significantly off.

Here are some of the most common DCF errors and why they can have a much bigger impact on valuation than they might initially seem.

1. Extrapolating an Unsustainable Growth Rate

A company that grows 40% in a particular year, especially from a small revenue base, is unlikely to maintain that same 40% growth rate for the next five years as the business becomes larger.

Growth typically slows as companies scale, markets become more competitive, and competitors catch up. Simply carrying the current growth rate through the entire forecast period is one of the easiest ways to push a DCF valuation too high.

2. Ignoring Margin Fade Over the Forecast Period

It is tempting to assume that profit margins will continue improving year after year. But unless there is a clear reason for that improvement — such as meaningful cost efficiencies, pricing power, or operating leverage — these assumptions can quickly become overly optimistic.

For many businesses, margins should eventually settle at more sustainable, industry-appropriate levels rather than continuing to expand indefinitely.

3. Mixing Nominal and Real Assumptions

This is an easy mistake to overlook because the model can still calculate without showing any obvious error.

If projected cash flows include inflation, they are nominal cash flows. The discount rate and terminal growth rate should therefore also be stated in nominal terms. Similarly, if the cash flows are expressed in real terms, the discount rate and terminal growth rate need to be consistent with that approach.

Mixing a real discount rate with nominal cash flows, or vice versa, can introduce an inflation-related distortion that may not be immediately obvious in the final valuation.

4. Setting the Terminal Growth Rate Too High

A company cannot realistically grow faster than the economy forever. The terminal growth rate is intended to represent what the business can sustainably achieve over the long term. It should therefore be based on reasonable expectations for long-term economic growth in the relevant market, rather than simply extending the company's recent high-growth period indefinitely.

A small change in the terminal growth rate can have a significant effect on a DCF, particularly when the terminal value makes up a large portion of the overall valuation.

5. Using a Discount Rate That Does Not Reflect the Company's Risk

Using a generic market-average WACC does not necessarily reflect the risk of the company being valued. Factors such as company size, leverage, industry exposure, customer concentration, geographic risk, and business model can all influence the appropriate discount rate. This is particularly relevant when valuing private companies, where beta is not directly observable and often needs to be estimated using comparable publicly traded companies.

An overly low discount rate can make future cash flows appear more valuable than they really are, while an excessively high rate can have the opposite effect.

6. Double Counting Risk

Risk can sometimes get factored into a DCF twice without anyone noticing.

For example, a model may already use conservative revenue growth assumptions to account for uncertainty and then apply a significantly higher discount rate for the same risk. Doing both can unnecessarily reduce the valuation. The important thing is to understand where each risk is being reflected and make sure the same risk is not being penalized twice.

7. Capex and Depreciation Never Converge

The relationship between capital expenditure and depreciation becomes particularly important when a DCF moves into the terminal period.

For a mature business that is primarily maintaining its existing asset base, capital expenditure and depreciation would generally be expected to move toward similar levels over time. If terminal-year capex remains significantly below depreciation without a clear explanation, the model may effectively be assuming that the company's productive asset base will continue shrinking.

That may be appropriate in some situations, but it should be a deliberate assumption rather than an unintended consequence of the model.

8. Working Capital Does Not Keep Pace With Revenue

Fast-growing companies generally need more working capital to support that growth. They may need to carry more inventory, extend more credit to customers, or invest in other operating assets. Simply carrying forward a fixed working capital assumption can therefore understate the amount of cash required to support future growth.

A better approach is to examine the company's historical relationship between revenue and working capital and consider how that relationship might change as the business scales. Working capital should not simply be treated as a plug used to make the model balance.

9. Treating Comparable Companies as Truly Comparable

Just because two companies operate in the same industry does not mean they are genuinely comparable.

Differences in company size, leverage, margins, growth prospects, geographic exposure, customer mix, and competitive position can all affect valuation. Using the beta, growth rate, or margins of a "similar" company without considering these differences can make an assumption appear objective simply because it came from a market source. Comparable companies are useful benchmarks, but they still require judgment.

10. Never Sanity-Checking the Final Valuation

Even a technically sound DCF should pass a basic reality check. If the valuation implies a multiple that is significantly outside the range observed for comparable public companies or recent transactions, it is worth going back through the assumptions before accepting the result.

The DCF does not have to match a market-based valuation exactly. Different valuation methods can produce different results, and there is nothing inherently wrong with that. But a large and unexplained gap is a reason to investigate the assumptions rather than simply dismissing the market evidence because the DCF appears more detailed.

Most DCF errors are not really math errors. They are assumption errors. A model can have perfectly correct formulas and still produce a misleading valuation if its growth expectations, margins, discount rate, terminal assumptions, or comparable companies have not been properly evaluated.

The real value of a DCF comes from understanding whether the assumptions make sense both individually and when considered together. A spreadsheet can produce a very precise number, but precision in the output does not necessarily mean precision in the underlying valuation.