Common DCF Assumption Errors That Can Distort Valuation

A DCF model rarely goes wrong because someone made a mistake in the formula. More often, the problem is with one or two assumptions behind the formula. If those assumptions are unrealistic, inconsistent, or never properly stress tested, the final valuation can end up being significantly off.

Here are some of the most common DCF errors and why they can have a bigger impact on valuation than they may initially appear to.

1. Extrapolating an Unsustainable Growth Rate

A company that grows 40% this year, particularly from a small revenue base, is unlikely to keep growing at 40% for the next five years as the business gets larger.Growth usually slows as a company scales, markets become more saturated, and competitors catch up. Simply carrying today's 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 easy for a model to assume that margins will keep improving year after year. But unless there is a clear reason for that improvement, such as meaningful cost efficiencies, pricing power, or operating leverage, those assumptions can quickly become overly optimistic. For many businesses, margins should eventually move toward more sustainable, industry appropriate levels rather than continuing to expand indefinitely.

3. Mixing Nominal and Real Assumptions

This is an easy one to miss because the model can still calculate perfectly.If your projected cash flows include inflation, they are nominal cash flows. The discount rate and terminal growth rate therefore need to be nominal as well. Mixing a real discount rate with nominal cash flows, or the other way around, can introduce an inflation related distortion without making the error obvious in the final output.

4. Setting a Terminal Growth Rate That Is Too High

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

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

A generic market average WACC does not necessarily reflect the risk of the company being valued. Size, leverage, industry exposure, customer concentration, and other business specific factors can all affect the appropriate discount rate. This becomes particularly important for private companies, where beta is not directly observable and typically has to be estimated using public comparable companies.

6. Double Counting Risk

Risk can also get counted twice without anyone realizing it.For example, a model might already use conservative revenue growth assumptions and then apply a significantly higher discount rate to account for the same uncertainty. Doing both can unnecessarily push the valuation down.The important thing is to be clear about where risk is being reflected and to avoid penalizing the same risk twice.

7. Capex and Depreciation Never Converge

The relationship between capital expenditures and depreciation becomes particularly important in the terminal period.For a mature business that is maintaining its existing asset base, capex 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 asset base will continue shrinking.

8. Working Capital Does Not Keep Pace With Revenue

Fast growing companies usually need more working capital to support that growth. They may need more inventory, carry higher receivables, or invest more in other operating assets. Simply carrying forward a fixed working capital assumption without looking at the company's historical relationship between revenue growth and working capital can therefore understate the cash required to support future growth. Working capital should not be treated as a minor plug just to make the model balance.

9. Treating Comparable Companies as Truly Comparable

Just because a company operates in the same industry does not mean it is a good comparable.Differences in company size, leverage, margins, growth prospects, geographic exposure, and competitive position can make a meaningful difference. Using a beta, growth rate, or margin from a "similar" company without considering those differences can make an assumption look objective simply because it came from a market source.

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 far outside the range observed for comparable public companies or 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 methods can produce different results. But a large, unexplained gap is a reason to investigate the assumptions rather than simply dismissing the market based evidence because the DCF looks more detailed. Most DCF errors are not really math errors. They are assumption errors.

A model can have perfect formulas and still produce a misleading valuation if the growth, margins, discount rate, terminal assumptions, or comparable companies have not been properly tested. The real value of a DCF comes from understanding whether the assumptions make sense, both individually and when viewed together, rather than simply trusting the precision of the spreadsheet.