Forecasting the Income Statement — Revenue Methods and Margin Assumptions

Revenue is the most important single assumption in any DCF model — more important than the discount rate, more impactful than the terminal growth rate. McKinsey's research confirms that revenue growth, when it occurs at ROIC above WACC, is the dominant driver of enterprise value differences between companies. Getting revenue right requires choosing the appropriate forecasting approach for the business type and grounding every assumption in verifiable data.

Once revenue is forecast, every margin line must be constructed from an explicit assumption about its relationship to revenue or to the absolute cost base. Ratios to revenue (gross margin %, SG&A %) are the most common approach — but they embed a critical assumption that each cost scales proportionally with revenue. Understanding the fixed vs. variable cost structure of the business determines whether this assumption is valid.

• Gross margin forecasting: the gross margin trend reflects pricing power vs. input cost dynamics. For businesses with strong brand pricing power (luxury goods, essential software platforms), gross margins tend to be stable or expanding. For commodity-adjacent businesses, gross margins fluctuate with raw material prices and industry capacity utilization. The forecast should explicitly address: (a) pricing power trajectory — can the company maintain or expand price/cost spread? (b) mix shift — are higher-margin products/customers growing faster than lower-margin ones? (c) supply chain dynamics — is the cost base exposed to commodity inputs or labor cost inflation?
• SG&A margin and operating leverage: as revenue grows, SG&A does not need to grow proportionally — fixed components of SG&A (corporate overhead, executive compensation, shared infrastructure) create operating leverage as volume rises. The forecast should decompose SG&A into its fixed and variable components. For technology companies, SG&A grows with sales headcount (variable) but corporate overhead grows much more slowly (near-fixed). The result: SG&A as a % of revenue should decline as the company scales, reflecting operating leverage in the fixed cost base.
• D&A as a percentage of PP&E (or revenue): depreciation is a function of the asset base, not directly of revenue. The correct forecast: (a) project gross PP&E additions (= capex); (b) compute annual depreciation from the asset base using asset useful life assumptions; (c) D&A as a % of revenue then follows mechanically, not as a direct assumption. A common error is holding D&A/revenue constant, which implicitly assumes capex/revenue stays constant — a reasonable assumption for asset-light businesses but wrong for businesses with changing capital intensity.
• Effective tax rate: the effective tax rate (ETR) is not the statutory rate. Differences arise from: (a) tax credits (R&D credits, renewable energy credits); (b) geographic income mix (profits in low-tax jurisdictions); (c) deferred tax timing. The ETR should be forecast based on the company's specific tax situation, not simply the statutory rate. For US companies post-2017 TCJA, the statutory rate is 21%; most large multinationals have ETRs of 15–22% depending on their international mix and tax planning strategies.

A forecast is internally inconsistent if the implied operational requirements contradict the assumptions. McKinsey's consistency framework requires checking three relationships before accepting any income statement forecast:

• Revenue growth vs. headcount: revenue growing at 15% per year with SG&A growing at only 5% implies revenue per employee is growing at 10% per year — a realistic productivity improvement trajectory or an impossible one? For a scaling software business, 10% revenue per employee growth is achievable (software can be sold to more customers without proportional headcount). For a professional services firm, revenue per employee is tightly capped by utilization rates and billing rates — the SG&A assumption must be revisited.
• NOPAT margin vs. reinvestment rate vs. ROIC: the implied ROIC from the forecast must be achievable and sustainable. If the income statement implies NOPAT margin of 15% and the balance sheet forecast implies invested capital turnover of 2.0×, the implied ROIC is 30%. Is this achievable for this business? If the best-in-class peer ROIC is 20%, a 30% ROIC assumption requires a clear competitive advantage explanation. The consistency check: compute the implied ROIC from the forecasted income statement and balance sheet and verify against the historical trend and peer benchmarks.
• Gross margin vs. capex investment: for businesses where product quality is related to capital investment (semiconductor fabs, pharmaceutical manufacturing), declining capex investment should correlate with declining gross margins as equipment ages. If the model shows falling capex but stable or rising gross margins, the implicit assumption is that the company can maintain product quality with less investment — which requires specific justification.

For businesses with product-level data, decomposing revenue growth into its three components provides a much richer forecasting foundation than a single growth rate assumption: