Expense Modeling: How to Forecast Costs in a Financial Model
Why Expense Modeling Matters
Revenue gets the headlines, but margins determine value. Two companies with identical revenue can have vastly different valuations if one operates at 30% EBITDA margins and the other at 10%. In a three-statement model, expense assumptions directly drive EBIT, net income, and free cash flow — which flow into every DCF and LBO analysis.
Types of Expenses in a Financial Model
| Expense Category | What It Includes | How to Model It |
|---|---|---|
| Cost of Goods Sold (COGS) | Direct materials, labor, manufacturing overhead | % of revenue (gross margin assumption) or per-unit cost × volume |
| Selling, General & Admin (SG&A) | Sales team, marketing, rent, corporate overhead | Mix of fixed $ amount + variable % of revenue |
| Research & Development (R&D) | Product development, engineering salaries | % of revenue or absolute $ with step-function increases |
| Depreciation & Amortization | Non-cash expense from capital assets | Separate D&A schedule linked to PP&E and intangibles |
| Stock-Based Compensation (SBC) | Equity awards to employees | % of revenue or $ per employee × headcount |
| Interest Expense | Cost of debt financing | Debt schedule: average balance × interest rate |
Fixed vs. Variable vs. Semi-Variable Costs
| Characteristic | Fixed Costs | Variable Costs |
|---|---|---|
| Behavior | Constant regardless of volume | Scales proportionally with revenue/volume |
| Examples | Rent, executive salaries, insurance | Raw materials, shipping, sales commissions |
| Modeling approach | Flat dollar amount, growing with inflation | Percentage of revenue or per-unit cost |
| Impact on margins | Operating leverage — margins expand as revenue grows | Margins stay relatively constant |
Semi-variable costs have both components. A sales team has a fixed base salary (fixed) plus commissions (variable). Model these with a fixed base plus a variable component tied to revenue or volume.
Step-by-Step: Modeling Expenses
Step 1 — Analyze Historical Cost Structure
Calculate each expense line as a percentage of revenue over 3–5 years. Identify trends: is the gross margin expanding or compressing? Is SG&A leverage improving as the company scales? Look for one-time items (restructuring charges, litigation) that should be excluded from run-rate analysis.
Step 2 — Classify Costs as Fixed, Variable, or Semi-Variable
Plot each cost line against revenue to see the relationship. Truly variable costs will show a near-linear relationship. Fixed costs will be flat. Semi-variable costs will show a base level with some revenue sensitivity.
Step 3 — Set Margin Assumptions
For variable costs, project the margin percentage. For COGS, this is your gross margin assumption. Consider input cost inflation, pricing power, product mix shifts, and economies of scale. For fixed costs, project the dollar amount with an inflation escalator (typically 2–3% annually).
Step 4 — Build in Operating Leverage
As revenue grows, fixed costs get spread over a larger base, expanding margins. This is operating leverage. Your model should show this naturally: if SG&A is $50M fixed + 10% variable, SG&A as a % of revenue declines as revenue scales from $200M to $500M.
Step 5 — Model Step-Function Costs
Some costs are fixed within a range but jump at capacity thresholds. A warehouse supports $100M of revenue; at $110M, you need a second warehouse. A factory runs one shift up to 80% utilization; above that, you add a second shift at higher cost. These step functions matter for accurate expense modeling.
Common Expense Modeling Mistakes
Modeling all expenses as % of revenue. Not every cost scales with sales. Rent, senior management salaries, and insurance are largely fixed. If you model them as a percentage, you’ll overstate costs at low revenue and understate them at high revenue.
Ignoring SBC. Stock-based compensation is a real expense that dilutes shareholders. Excluding it from operating expenses makes margins look artificially high. Include SBC above the EBIT line.
Assuming infinite margin expansion. There are natural floors to costs. A retailer can’t have 0% COGS. An asset manager can’t have 0% compliance costs. Build margin ceilings into your model based on best-in-class peers.
Forgetting one-time vs. recurring. Strip out restructuring charges, litigation settlements, and asset write-downs from your run-rate analysis. Project recurring expenses only, and flag one-time items separately.
Key Takeaways
- Classify every expense as fixed, variable, or semi-variable — this determines how margins behave as revenue scales.
- COGS is typically modeled as a gross margin percentage; SG&A as a mix of fixed dollars and variable percentage.
- Operating leverage means fixed costs spread over growing revenue, naturally expanding margins.
- Watch for step-function costs (new warehouses, additional shifts) that create margin inflection points.
- Always include stock-based compensation as a real operating expense — excluding it inflates margins and misleads investors.
Frequently Asked Questions
Should I model COGS as a percentage of revenue or on a per-unit basis?
Per-unit is more accurate when you have the data. It captures changes in product mix, input cost inflation, and volume discounts that a flat percentage misses. For a quick model or when segment data isn’t available, a gross margin percentage works fine.
How do I model operating leverage in a financial model?
Split expenses into fixed and variable components. As revenue grows, fixed costs stay constant in dollar terms, so they shrink as a percentage of revenue. The result is expanding operating margins. Model this by keeping fixed-cost lines in absolute dollars and variable costs as percentages.
What’s a typical SG&A percentage for different industries?
It varies widely. Software companies may run 40–60% SG&A as a % of revenue (heavy sales and marketing). Manufacturers are often 10–20%. Retailers run 20–30%. Always benchmark against industry peers rather than applying universal rules.
How do I handle restructuring charges in my expense model?
Exclude one-time restructuring charges from your operating expense projections. They distort run-rate profitability. However, model the benefits of restructuring (headcount reduction savings, facility closures) in your forward expense assumptions. Show the savings in your forecast, just not the one-time charge.
Should stock-based compensation be included in operating expenses?
Yes. SBC is a real cost to shareholders because it creates dilution. Include it in operating expenses to show true profitability. You can add it back for cash flow purposes (it’s non-cash), but don’t hide it from the margin analysis. Most institutional investors adjust for SBC when evaluating companies.