Market sizing is often treated as a numbers game. But for B2B companies, it’s more like solving a multidimensional puzzle—one that requires not just data, but judgment, context, and a deep understanding of how businesses actually buy. Whether you're entering a new vertical, pitching to investors, or prioritizing product development, knowing the true size of your opportunity is essential. And the best way to get there? A hybrid approach that blends top-down analysis, bottom-up validation, and insights from both secondary research and real-world conversations.

Starting from the Top: The Macro View

Top-down market sizing begins with the big picture. You start by identifying the total addressable market (TAM)—the full universe of potential revenue if every possible customer adopted your solution. This often involves combing through industry reports from sources like Gartner, IDC, or Statista, and layering in filters like geography, vertical, and company size.

But raw TAM figures are rarely useful on their own. The real work begins when you start narrowing down to your serviceable available market (SAM)—the subset of companies that could realistically use your product, given current capabilities, regulations, and market readiness. For example, if you're selling a compliance automation tool, your SAM might exclude industries with low regulatory exposure or regions where digital adoption is lagging.

This is where expert interviews become invaluable. Speaking with industry insiders—whether they’re procurement leads, consultants, or former operators—helps you validate assumptions that secondary data can’t capture. They can tell you whether the adoption curve is steep or flat, whether budgets are growing or shrinking, and whether your product is solving a pain point or just a nice-to-have.

Building from the Bottom: Ground-Level Reality

While top-down sizing gives you scale, bottom-up sizing gives you truth. This approach starts with the customer—how much they spend, how often they buy, and how many users they need. You might begin by defining your ideal customer profile (ICP) using firmographic filters: company size, industry, region, and tech stack. Then, through structured interviews or surveys, you gather data on actual purchase behavior.

For example, if you're selling a SaaS platform, you’d want to know the average annual contract value, the number of seats per company, and renewal rates. Multiply that by the number of companies in your target segment, and you start to see a realistic picture of your revenue potential.

Quantitative interviews play a critical role here. They help you understand not just what companies say they need, but what they’re willing to pay for. Segmenting responses by company maturity or region can reveal patterns that secondary data misses—like higher spend among digitally mature firms or lower adoption in emerging markets.

Reconciling the Two: Where Strategy Meets Reality

The most robust market sizing exercises don’t choose between top-down and bottom-up—they triangulate both. If your top-down analysis suggests a $2 billion opportunity, but your bottom-up data points to $500 million, that discrepancy isn’t a failure—it’s a clue. Maybe your product faces adoption barriers. Maybe pricing needs to be rethought. Or maybe the market is still nascent, and growth will come in waves.

Expert interviews are especially powerful in bridging this gap. They can help you understand why certain segments aren’t converting, whether your assumptions about penetration rates are realistic, and how competitors are shaping the market.

Why B2B Is a Different Beast from B2C

It’s tempting to borrow market sizing techniques from consumer markets, but B2B requires a fundamentally different lens. In B2C, you’re often dealing with millions of individuals whose behavior can be modeled using demographic data, retail audits, and large-scale surveys. Purchase decisions are emotional, impulsive, and price-sensitive.

In B2B, the customer is an organization. Buying decisions are made by committees, influenced by ROI calculations, and shaped by procurement processes. Data sources shift from census reports to firmographic databases. Interviews focus on decision-making units (DMUs), not personal preferences. And pricing is rarely fixed—it’s negotiated, tiered, and often bundled with services.

This means your market sizing must account for complexity. You’re not just estimating demand—you’re modeling behavior across layers of stakeholders, budget cycles, and operational constraints.

Market Sizing as Strategic Craft

Done well, market sizing is more than a spreadsheet—it’s a strategic narrative. It tells the story of where your product fits, how big the opportunity is, and what it will take to win. It’s a blend of art and science, requiring both analytical rigor and qualitative depth.

For B2B companies, the key is to embrace the complexity. Use top-down data to understand the scale, bottom-up insights to ground your assumptions, and expert voices to fill in the gaps. Iterate often, visualize clearly, and always ask: not just how big is the market, but how reachable, how ready, and how real.