Data & spreadsheets · DCF Calculator
Why Your DCF Assumptions Matter More Than the Formula Itself
· Why it matters
dcf valuation cash-flow
The discounting arithmetic is settled; the assumptions are not. This post explains which inputs carry the most weight, how to document them, and why a transparent educational model is a better place to learn than a black box.
Two people, one formula, wildly different values — why the disagreement is never about the maths
Two analysts can use the same equations and produce widely separated values because the equation is the common part. Forecast growth, reinvestment, discount rate and terminal conditions carry the disagreement. A calculator resolves neither case; it deterministically converts each case into figures that can be compared and challenged.
The assumptions that drive everything — growth, margins, reinvestment, discount rate and terminal growth, and how each enters the calculation
Growth compounds the starting FCF, margins and reinvestment determine the cash flow before it reaches this tool, the discount rate reduces every future amount and terminal growth changes the perpetuity denominator. Each assumption enters at a different point, so documenting only the final rate leaves most of the economic story invisible.
Garbage in, precision out — why a calculator produces a confident number from any inputs, and why that confidence is not evidence
A precise result can be produced from arbitrary valid inputs. ToolAcre prevents NaN, infinity and invalid perpetuity combinations, but those guards establish mathematical coherence rather than factual quality. Two decimal places are formatting; they are not evidence that revenue, capital spending or long-run growth can occur as modelled.
Documenting the case — writing down why each assumption is what it is, so the model can be challenged
Record the source, date, definition and rationale beside every assumption, including whether cash flows are FCFF, whether rates are nominal, and which share scale is used. A useful case can be falsified: a reviewer should be able to identify what changed, why it changed and which output rows moved as a result.
Worked example — a base case and an alternative with modest, defensible changes to three inputs, showing how far apart the two values land
For a hypothetical comparison, hold the balance-sheet bridge fixed and alter forecast growth, discount rate and terminal growth modestly between two cases. The repository’s own scenario tests show the expected ordering: lower-growth and lower-terminal cases value below base, while the opposite case values above it. The distance is conditional, not a probability range.
What this does not cover — the calculator does not suggest, validate or source assumptions; that judgement is the whole job
The tool supplies no filings, market prices, beta, premium or forecast. Its WACC helper computes from inputs the user provides, and its validation cannot say that those inputs are current or suitable. Sourcing and defending assumptions is not preparatory work around the model; it is the central analytical task.
The model is the easy part — how the ToolAcre DCF Calculator keeps every input visible and editable so the assumptions stay in view
ToolAcre keeps inputs, yearly rows, terminal details and the net-debt bridge visible and exports assumptions with results. Use that transparency to compare documented cases rather than to choose the most appealing output. A DCF is a model, and no result should be presented as a correct valuation or a price discovered by arithmetic.