SaaS Market Sizing: How to Calculate TAM, SAM, and SOM Without Fabricating It

By Yanni Papoutski / Published 19 April 2026 / 12 min read

Market sizing separates fundable businesses from fantasies. Here is how to calculate TAM, SAM, and SOM using both top-down and bottom-up approaches, why your TAM must be large enough to support venture scale, and how to avoid the red flags that make investors dismiss your numbers.

Why Market Size Matters to Investors

Investors use a simple formula to evaluate whether an opportunity is worth their time:

Revenue Opportunity = Market Size × Market Share × Unit Economics

If your TAM is 50 million pounds, your market share is 10 per cent, and your gross margin is 70 per cent, your maximum revenue opportunity is five million pounds. That is a lifestyle business, not a venture-scale business.

If your TAM is five billion pounds, your market share is just 1 per cent, and your gross margin is 70 per cent, your maximum revenue opportunity is 35 million pounds. That is venture-scale.

Venture capitalists are looking for businesses that can reach 100 million pounds of annual recurring revenue or more. That requires a large market. A very large market.

The TAM, SAM, SOM Framework

TAM: Total Addressable Market

TAM is the total market size if you could capture 100 per cent of the addressable market. It answers the question: "If we won the entire market, how much revenue would we generate?"

For a payroll software company targeting the UK SME market, the TAM might be the total amount UK SMEs spend on payroll services per year. If there are 800,000 UK SMEs, and the average SME spends 5,000 pounds per year on payroll services, the TAM is four billion pounds.

TAM should be large. For Series A, your TAM should be at least one billion pounds.

SAM: Serviceable Available Market

SAM is the portion of TAM you can actually serve with your product and your go-to-market strategy.

For the payroll software company, you might have decided to focus only on UK limited companies with 10-100 employees. This segment spends 10 times as much on payroll services as micro-businesses (because they have more employees and more regulatory compliance requirements). Your SAM might be one billion pounds (10 per cent of the total four billion pound market, but for a segment that spends 10x as much).

SAM is where you are realistically competing. It is the market you can reach with your sales and marketing strategy and your product design.

SOM: Serviceable Obtainable Market

SOM is the portion of SAM you can actually capture in the next 5-10 years.

For the payroll company, you might be targeting 5 per cent of the SAM. That is 50 million pounds of annual recurring revenue. This feels achievable if you have a strong team, good capital, and a defensible product.

SOM is your realistic revenue goal. It is what you are actually trying to build.

The Relationship

TAM > SAM > SOM.

TAM is what the market could be. SAM is what you can serve. SOM is what you can realistically capture.

Top-Down Market Sizing

Top-down sizing starts with industry research and works down to your addressable slice.

Step 1: Find Total Market Size

Look for industry reports from Gartner, IDC, or Forrester. These reports typically size the total market for a category.

Example: "Gartner estimates the global SaaS market for HR management software at 20 billion pounds in 2026."

Step 2: Identify Your Geographic/Segment Slice

How much of that market is relevant to you?

Example: "The HR management software market is 20 billion pounds globally. The UK market is 8 per cent of the global market. UK TAM: 1.6 billion pounds."

Or: "The HR management software market is 20 billion pounds globally. SMEs (10-250 employees) represent 40 per cent of the market. SME TAM: 8 billion pounds."

Step 3: Refine Further If Needed

You might refine further based on vertical or feature specialisation.

Example: "The HR software market in the UK for SMEs is 8 billion pounds. Recruiting-specific tools represent 15 per cent of spend. UK SME recruiting software TAM: 1.2 billion pounds."

Top-Down Advantages

Top-Down Disadvantages

Bottom-Up Market Sizing

Bottom-up sizing starts with your customer and works up to market size.

Step 1: Define Your Target Customer

Who specifically are you selling to?

Example: "We are selling to UK limited companies with 10-100 employees that currently use manual payroll processes."

Step 2: Count How Many of These Customers Exist

Use publicly available data, industry estimates, or government statistics.

Example: "There are 800,000 UK limited companies. 60 per cent have 10-100 employees. That is 480,000 companies. 40 per cent of those still use manual or spreadsheet-based payroll. That is 192,000 potential customers."

Step 3: Estimate Average Contract Value (ACV)

How much will each customer spend per year?

Example: "The average limited company with 10-100 employees currently spends 10,000 pounds per year on payroll services (either through Sage or an accountant). They would pay 5,000 pounds per year for a modern SaaS payroll product (lower cost, less integration effort, but newer market)."

Step 4: Calculate TAM

Customer Count × ACV = TAM

Example: "192,000 potential customers × 5,000 pounds ACV = 960 million pounds TAM."

Bottom-Up Advantages

Bottom-Up Disadvantages

Top-Down and Bottom-Up Should Converge

The strongest market sizing uses both top-down and bottom-up approaches. If they converge to a similar number, you have credibility. If they diverge widely, you need to figure out why.

Example:

Top-Down: Global SaaS market is 200 billion pounds. HR software is 10 per cent of that. UK is 8 per cent of global. HR software UK TAM: 1.6 billion pounds.

Bottom-Up: 192,000 potential customers × 5,000 pounds ACV = 960 million pounds.

Top-down says 1.6 billion, bottom-up says 960 million. They are in the same ballpark (within 2x). This is acceptable. The top-down number may be higher because it includes all companies that might spend on payroll services, not just those currently using manual processes.

If top-down said 1.6 billion and bottom-up said 50 million, you have a problem. The numbers do not converge. You need to figure out why.

SAM: What Is Actually Addressable

SAM is the portion of TAM you can actually address with your go-to-market strategy and product design.

Factors That Limit SAM

Calculating SAM

Start with TAM and apply filters for each of the above factors.

Example:

TAM: 1.6 billion pounds (all HR software in UK)

Apply geographic filter: Already UK-only, so no change: 1.6 billion pounds

Apply company size filter: You are targeting companies with 10-500 employees, which represent 60 per cent of the market: 960 million pounds

Apply feature specificity filter: You are focusing on payroll, which represents 40 per cent of HR software spending: 384 million pounds

SAM: 384 million pounds

SOM: What You Can Actually Capture

SOM is your realistic revenue goal for the next 5-10 years.

Conservative SOM: 2-5 per cent of SAM

A conservative SOM assumes you capture 2-5 per cent of your SAM. This is a reasonable goal for a well-executed company with good capital and a defensible product.

Example: "Our SAM is 384 million pounds. We aim to capture 5 per cent of SAM in 10 years. SOM: 19.2 million pounds."

Aggressive SOM: 5-15 per cent of SAM

An aggressive SOM assumes you capture 5-15 per cent of your SAM. This is achievable if you have strong founder expertise, a large market that is consolidated (easy to take share from competitors), or a unique go-to-market advantage.

Example: "Our SAM is 384 million pounds. We have domain expertise and existing customer relationships in payroll. We aim to capture 15 per cent of SAM in 10 years. SOM: 57.6 million pounds."

What to Show in Your Pitch Deck

In your pitch deck, show a conservative SOM. It is easier to over-deliver than to under-deliver. If you say you are aiming for 19 million pounds and you hit 50 million, you are a hero. If you say you are aiming for 100 million and you hit 19, you have disappointed.

Three Worked Examples

Example 1: B2B SaaS for Accountancy Firms

Company: InvoiceOS (invoice management for mid-market accounting firms)

Top-Down:

Global accounting services market: 500 billion pounds. Digital transformation and software adoption is driving 15 per cent of spending toward software services. Software component: 75 billion pounds. UK is 6 per cent of global accounting market: 4.5 billion pounds. UK software component (15 per cent of 4.5 billion): 675 million pounds.

Bottom-Up:

UK mid-market accounting firms (20-100 partners): 3,500 firms. Annual invoicing spend per firm: 150,000 pounds. Total market: 525 million pounds.

Converged TAM: 550 million pounds (average of top-down and bottom-up)

SAM: Firms spending 150k+ per year on invoicing: 300 million pounds (60 per cent of TAM). These are the firms with enough invoicing volume to justify dedicated software.

SOM: 5 per cent of SAM over 5 years: 15 million pounds annual recurring revenue.

Example 2: B2B SaaS for Engineering Teams

Company: AccessAI (access management for startups)

Top-Down:

Global security and access management market: 50 billion pounds. UK and EU is 40 per cent of global market: 20 billion pounds. Startup and mid-market segment is 15 per cent of the market: 3 billion pounds.

Bottom-Up:

UK and EU SaaS startups (10-500 people): 25,000 companies. Average annual spend on identity and access tools: 50,000 pounds. Total market: 1.25 billion pounds.

Converged TAM: 2 billion pounds

SAM: Startups that are raising capital and can afford dedicated tools: 800 million pounds (40 per cent of TAM).

SOM: 5 per cent of SAM over 10 years: 40 million pounds annual recurring revenue.

Example 3: Vertical SaaS for Healthcare Providers

Company: PatientOS (patient data management for small practices)

Top-Down:

UK healthcare IT market: 8 billion pounds. Patient management software is 20 per cent of that: 1.6 billion pounds. Small practices (1-5 doctors) represent 30 per cent of the market: 480 million pounds.

Bottom-Up:

UK single-handed GP practices and small practices (1-5 doctors): 8,000 practices. Current annual spend on patient management software: 30,000 pounds per practice. Total market: 240 million pounds.

Converged TAM: 300 million pounds

SAM: Practices with 2-5 doctors that can justify investing in modern software: 150 million pounds (50 per cent of TAM).

SOM: 3 per cent of SAM over 8 years (healthcare buying cycles are long): 4.5 million pounds annual recurring revenue.

Red Flags in Market Sizing

Investors watch for these red flags:

Red Flag 1: TAM Is Too Small

If your TAM is less than 500 million pounds, investors will question whether this can be a venture-scale business. For Series A, aim for at least one billion pounds.

Red Flag 2: Only Top-Down or Only Bottom-Up

Both approaches should be present. If you only use top-down, investors think you have not done customer research. If you only use bottom-up, they think your numbers are not grounded in market reality.

Red Flag 3: Market Sizing Changes Between Versions

If version 1 of your deck says TAM is 500 million and version 2 says 5 billion, investors think you are adjusting the numbers to be impressive, not adjusting the analysis to be more rigorous.

Red Flag 4: You Are Claiming You Can Win 25 per cent of the Market

Most successful SaaS companies capture 5-10 per cent of their SAM. If you are claiming 25 per cent, you need an extraordinary reason (you have 70 per cent of the market already and are growing into white space, or you have a defensible moat that competitors cannot replicate).

Red Flag 5: No Source Citations

If Gartner says the market is X, cite Gartner. If you did bottom-up research with 50 customers, say so. If you made an assumption, show your work.

Red Flag 6: "Everyone With Problem X" Is Your TAM

Bad: "Millions of people struggle with email management. That is our TAM."

This is too broad. You need to define your specific target: "Mid-market SaaS companies with 50-500 people spend 500,000 pounds per year on email and collaboration tools. There are 5,000 such companies in the UK. That is a 2.5 billion pound TAM."

The Venture Math

Here is how a venture capitalist uses market sizing to make a go/no-go decision:

If your company captures 5 per cent of SAM and your gross margin is 70 per cent, what is your potential revenue?

SOM = SAM × 5 per cent = Revenue opportunity

If SOM is 10 million pounds, that is not venture-scale (it is lifestyle business scale).

If SOM is 100 million pounds, that is venture-scale.

The venture capitalist is asking: "If this company executes perfectly, can it become a 100 million pound business?" If the answer is no (because the SAM is too small), they pass. If the answer is yes, they move forward.

This is why market sizing matters. It is not about being perfectly accurate. It is about proving that the market is large enough to support a venture-scale exit.

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