Operating Model and FP&A Forecasting for Investment Banking

The operating model is the engine of every valuation in investment banking. Before you can run a DCF, build trading comps, or test an M&A accretion/dilution scenario, you need a credible forecast of revenue, margins, and cash flow. This guide covers the operating model build — from revenue drivers through EBITDA to the unlevered free cash flow that feeds a DCF — with the assumptions and traps that interviewers test at every level.

What an operating model does

An operating model forecasts a company's financial performance over an explicit period (typically 5–10 years). It answers:

• How fast will revenue grow, and what drives that growth?
• What margins will the company earn, and are they sustainable?
• How much capex and working capital does the business require?
• What free cash flow is available to all capital providers?

The output feeds directly into a DCF (as the explicit forecast period), a 3-statement model (as the income statement and cash flow drivers), or an M&A model (as the standalone acquirer or target projections).

Step 1: Revenue build — top-down vs. bottom-up

Top-down approach

Start with market size and the company's share:

Revenue = Total Addressable Market × Market Share × Average Price

Or more simply:

Revenue Growth = GDP Growth + Market Share Gains + Price Increases

Top-down is faster but less precise. Use it for mature, stable businesses where segment-level detail isn't available.

Bottom-up approach (preferred for interviews)

Build revenue from operational drivers:

Revenue = Units Sold × Price per Unit

Or for a multi-segment business:

Total Revenue = Σ (Segment_i Units × Segment_i Price)

Example — SaaS company:

Revenue = Beginning ARR + New Bookings − Churn + Expansion Revenue
Ending ARR = Beginning ARR × (1 + Net Retention Rate) + New Bookings

Example — retail company:

Revenue = Number of Stores × Revenue per Store
New Stores = Prior Stores + Openings − Closures
Same-Store Sales Growth = Price Increase + Traffic Change + Basket Size Change

Bottom-up is more defensible because each assumption is testable. Interviewers prefer it because it shows you understand what actually drives the business, not just that you can apply a growth rate.

Revenue growth assumptions

| Scenario | Typical Range | When to Use | |----------|--------------|-------------| | Mature, stable | 2%–5% | GDP-like growth, market share stable | | Growing market | 8%–15% | Company gaining share in expanding market | | High-growth | 20%+ | Early-stage, land-grab phase | | Declining | Negative | Secular decline, market share loss |

Always sanity-check: is the implied market share change realistic? A company growing 20% in a market growing 3% is gaining significant share every year — is that credible?

Step 2: Margin build — from revenue to EBITDA

Gross margin

Gross Profit = Revenue − COGS
Gross Margin = Gross Profit ÷ Revenue

Forecast gross margin as either:

• Flat (stable business, no mix shift expected)
• Trending (improving with scale, deteriorating with competition)
• Scenario-dependent (base case flat, upside case +200 bps from pricing power)

Operating expenses

SG&A = Revenue × SG&A % (or fixed + variable components)
R&D = Revenue × R&D % (for tech/pharma)
Other OpEx = specific line items

Separate fixed and variable components when possible:

SG&A = Fixed SG&A + (Variable SG&A Rate × Revenue)

Fixed costs create operating leverage: as revenue grows, SG&A as a % of revenue declines, expanding EBITDA margins. This is a key driver in high-growth company models.

EBITDA

EBITDA = Gross Profit − SG&A − R&D − Other OpEx
EBITDA Margin = EBITDA ÷ Revenue

Margin expansion trap: don't assume margins expand forever. Mature companies typically see margins stabilize as competition catches up, growth investments scale, and the law of large numbers applies. A model showing EBITDA margins going from 15% to 35% over 10 years needs a specific, defensible thesis (scale economies, mix shift, pricing power) — not just a trend line.

Step 3: From EBITDA to unlevered free cash flow

EBIT = EBITDA − D&A
Unlevered FCF = EBIT × (1 − Tax Rate) + D&A − Capex − ΔNWC

Each component needs its own forecast:

D&A (Depreciation & Amortization)

D&A = PP&E × Depreciation Rate (or as % of revenue)

Tie D&A to the PP&E schedule: beginning PP&E + Capex − D&A = ending PP&E. D&A should roughly track capex over time for a stable business (capex ≈ D&A means PP&E is flat).

Capex

Two components:

Total Capex = Maintenance Capex + Growth Capex
Maintenance Capex ≈ D&A (replaces worn-out assets)
Growth Capex = incremental investment to support revenue growth

Forecast as % of revenue or absolute dollars tied to growth initiatives:

Capex = Maintenance Capex (% of revenue) + Growth Capex (per new store, per unit, etc.)

Capital-light businesses (SaaS, services): capex 2%–5% of revenue. Capital-intensive businesses (manufacturing, telecom): capex 10%–20% of revenue.

Change in Net Working Capital (ΔNWC)

NWC = Current Assets (ex-cash) − Current Liabilities (ex-debt)
ΔNWC = NWC_current year − NWC_prior year

Forecast NWC as days outstanding:

Accounts Receivable = Revenue × (DSO ÷ 365)
Inventory = COGS × (DIO ÷ 365)
Accounts Payable = COGS × (DPO ÷ 365)
NWC = AR + Inventory − AP + Other Current Items

ΔNWC is a cash outflow when positive (the company is investing in working capital to support growth). High-growth companies often show large negative FCF despite positive EBITDA because ΔNWC consumes cash.

Negative NWC businesses (SaaS with deferred revenue, retail with fast inventory turns): ΔNWC is a cash inflow, which boosts FCF above EBITDA. This is a sign of a strong business model, not a modeling error.

Step 4: Connect to the DCF

The operating model's unlevered FCF feeds directly into the DCF:

Explicit Period (Years 1–5 or 1–10):
  PV of UFCF = Σ [UFCF_t ÷ (1 + WACC)^t]

Terminal Period:
  TV = Final Year UFCF × (1 + g) ÷ (WACC − g)  [Gordon Growth]
  or
  TV = Final Year EBITDA × Exit Multiple          [Exit Multiple]

Enterprise Value = PV of Explicit UFCF + PV of Terminal Value

The operating model's assumptions (revenue growth, margins, capex, NWC) directly determine the explicit period cash flows. Terminal value assumptions (growth rate or exit multiple) determine everything beyond.

Sensitivity: because terminal value is often 60%–80% of total EV, the operating model's final-year EBITDA and UFCF are critical. Small changes in Year 5 margin assumptions can move the entire valuation.

Step 5: Sanity checks

Run these on every operating model before presenting:

| Check | What to Look For | |-------|-----------------| | Revenue growth vs. market | Is implied market share change realistic? | | Margin trajectory | Are margins expanding without a thesis? | | Capex vs. D&A | Is capex ≥ D&A for a growing business? | | ΔNWC vs. revenue growth | Is NWC growing with revenue (normal) or shrinking (verify why)? | | FCF conversion | FCF/EBITDA should be 50%–80% for most businesses | | Terminal growth | g ≤ long-run GDP (2%–3%); higher implies outgrowing the economy forever | | Implied exit multiple | Gordon Growth TV ÷ Final EBITDA should be reasonable vs. comps |

Common interview questions

"Walk me through an operating model." Revenue drivers → gross margin → opex → EBITDA → D&A → capex → ΔNWC → unlevered FCF. State whether you're using top-down or bottom-up revenue and why.

"How do you forecast capex?" Maintenance capex ≈ D&A; growth capex tied to specific initiatives (new stores, capacity expansion). Total capex as % of revenue, benchmarked against historical and peer levels.

"Why is ΔNWC subtracted in the FCF formula?" An increase in NWC (more AR, more inventory, slower AP) represents cash tied up in operations — a real cash outflow not captured in EBITDA. Subtracting it converts EBITDA-based earnings to actual cash generation.

"What's a good FCF/EBITDA conversion rate?" 50%–80% for most businesses. Below 50%: high capex, high ΔNWC, or both (capital-intensive or high-growth). Above 80%: capital-light business with favorable working capital dynamics (SaaS, marketplaces).

"How does the operating model connect to the DCF?" Operating model produces the explicit period UFCF forecasts. DCF discounts those at WACC and adds terminal value. The operating model's assumptions are the primary driver of DCF output — especially the final-year EBITDA that anchors terminal value.

The build order to practice

1. Revenue build (bottom-up, by segment or driver)
2. Margin build (gross margin → opex → EBITDA)
3. D&A forecast (tied to PP&E schedule)
4. Capex forecast (maintenance + growth)
5. NWC forecast (days outstanding method)
6. Unlevered FCF calculation
7. Sanity checks on all assumptions
8. Feed into DCF or 3-statement model

The operating model is where valuation assumptions live. Get the revenue drivers and margin logic right, and the DCF mostly takes care of itself. Get them wrong, and no amount of WACC precision saves the output.