Choose Your Beachhead: Market Size Without the Fantasy

By Kasper-Tanakrit Sermsuksan · Global Venture I: Playbook · Block C

You are here: C-02 · Block C · Understanding the Market · article 2 of 9 · Form C · Next: C-03 The Interview Masterclass

In short: A beachhead market is the first narrow customer segment a startup can win completely; you size it bottom-up (TAM, SAM, SOM) from real customers and prices, not from a percentage of a huge market. The people in it share the same problem, buy in a similar way and talk to each other. Choose it by listing possible segments, scoring them honestly, and describing the exact person you serve. Narrow first is not thinking small. It is how you get big without running out of money.

TAM SAM SOM คือการประเมินขนาดตลาดสามระดับ TAM (Total Addressable Market) คือรายได้ต่อปีถ้าทุกคนในตลาดซื้อจากเรา SAM (Serviceable Available Market) คือส่วนที่เราเข้าถึงและให้บริการได้ ส่วน SOM (Serviceable Obtainable Market) คือส่วนแบ่งที่เราน่าจะครองได้จริงในหนึ่งถึงสามปี ทั้งหมดควรคำนวณจากจำนวนลูกค้าและราคาจริงแบบล่างขึ้นบน (bottom-up) ไม่ใช่หยิบตัวเลขตลาดใหญ่มาคิดเปอร์เซ็นต์

Try it in 10 minutes: Write three possible segments for your idea, one concrete line each — not "elderly people" but "adult children in Bangkok caring for a parent after a fall." For the one you like most, write the bottom-up formula with your best guesses (customers × price per year) and circle the number you would check first.

Every year, at least one team tells me their market is "everyone in Thailand over 60."

Then they show me a slide. A big report says the market is worth billions of baht. They say they only need 1%. The room nods. And I ask one question: who, exactly, is your first paying customer, and where will you meet them on Monday?

Silence.

That silence is the gap between a market size and a market you can actually win.

I want to help you close it.

What is a beachhead market?

Bill Aulet, managing director of the Martin Trust Center for MIT Entrepreneurship¹, calls it the beachhead market². The image comes from military landings: you do not invade a whole country at once. You take one beach, secure it, and use it to move inland.

In startup terms, a beachhead is a group of customers who:

• buy similar things,
• in a similar way, and
• talk to each other — so a win with one helps you win the next.

In my experience, that third point is the one most founders miss.

If your customers never talk to each other, every sale starts from zero. If they do, every happy customer becomes a door.

Look at how narrow Kulina, my food delivery startup, was. We specialized in catering meals for office workers in Jakarta. Customers ordered a day in advance. And we gave a group discount when people in the same building ordered together.³

Same building. Same way of buying. Colleagues pulling colleagues in.

Looking back, that was our beachhead.

What are TAM, SAM and SOM, and how do you calculate them?

Investors often talk about market size in three layers. I use the same three:

• TAM — Total Addressable Market: The revenue per year if every possible customer in the market bought from you.
• SAM — Serviceable Available Market: The part of TAM you can actually serve with your product, channel and location.
• SOM — Serviceable Obtainable Market: The part of SAM you can realistically win in the next one to three years.

For an early venture, I think the most important number is the TAM of your beachhead — not of the whole industry. And the way you get it matters more than the number itself.

You will calculate all three in Step 4 below, after you have chosen the beachhead.

Why should a startup start with a narrow market?

Because you are small. And small things win by being focused.

A narrow market means you can learn fast. Five interviews in one tight group tell you far more than five interviews spread across five groups.

It means your message is specific, so the right people recognize themselves in it. It means your first customers can refer each other. And it means you can be the best option for someone, instead of an average option for everyone.

I would always rather see a team be the best for a few than average for everyone.

My favorite picture of this comes from Geoffrey Moore. In Crossing the Chasm, he compares it to bowling⁴: hit the head pin squarely and it knocks down the pins behind it. Miss it, and you spend your money rolling balls down the side.

Narrow first is not thinking small. It is the order of operations for thinking big.

How do you choose a beachhead market? Four steps

This follows the first four steps of Aulet's Disciplined Entrepreneurship, in the way I teach them.

Step 1 — List your possible segments

Brainstorm every group that might have the problem. I tell teams not to judge yet. Aim for five to ten. Be concrete: not "elderly people" but "retirees living alone in Bangkok condos," "adult children caring for a parent after a hospital stay," "private hospitals' rehab departments."

Step 2 — Pick one beachhead

Score each segment honestly. A simplified version of Aulet's checklist²:

1. Pain: do they have a strong reason to buy — evidence from interviews, not your guess?
2. Money: can they pay, or does someone pay for them?
3. Reach: can you actually get to them in the next month, with the network you have?
4. Deliver: can you give them a complete solution now, alone or with partners?
5. Competition: is there an entrenched player who will block you?
6. Next step: will winning them help you enter the next segment?
7. Fit: does your team care about these people enough to spend years with them?

Pick one. Not two.

If you cannot choose, my honest answer is usually that you need more interviews, not a bigger market.

If you are reading this before your first customer interviews, that is normal. Score on the best evidence you have — entrepreneur interviews, reviews, dated data. Mark each score "assumed." Then score again after your first three customer interviews.

Step 3 — Describe the end user

I ask teams to write down the exact person in their beachhead. Age range, where they live, what they do, what triggers the problem, what they do about it today, what they already spend, where they get information, who influences them. This is your Angel Customer™ at the center of the beachhead.

Step 4 — Size the beachhead, bottom-up

Now — and only now — do the numbers, using the TAM, SAM and SOM layers above. I insist on this order. Start bottom-up.

Top-down or bottom-up: which market sizing should you use?

Top-down starts with a big published number and takes a slice. "The market is ฿50 billion. We take 1%. That's ฿500 million."

It feels rigorous. It is fantasy.

Nobody buys 1% of anything. The 1% does not tell you who the customers are, how you will reach them, or why they would choose you.

Bottom-up starts with real customers and counts up:

Number of customers in your beachhead × what each one would pay per year = beachhead TAM

Every number in that sentence must be something you can defend: a count from a dated source, a share you saw in your interviews or in credible data, a price based on what people already spend today. My rule: if you cannot name where a number came from, it is not a number.

It is a hope.

I use top-down only as a sanity check at the end: does your bottom-up number look plausible next to the big published figure?

Who are you counting?

The count depends on who pays.

• B2C: people or households who have the problem.
• B2B: companies, branches or outlets that have the problem and a budget for it.
• B2G: agencies, local governments, public hospitals or schools, and the budget line they would pay from.
• B2B2C: both the partners you sell through and the end users they reach. Size the revenue on whoever pays.

Not sure which you are? Business Models explains the types, and Who Are Your Customers? helps you find the payer.

A worked example (illustrative)

Here is how I would walk a team through all four steps.

All numbers in this example are invented to show the method. Do not quote them. Replace each one with a number you can source.

Challenge: helping older people recover safely at home after a fall.

Step 1 — Segments: (a) older people living alone in Bangkok; (b) working adult children in Bangkok caring for a parent after a fall; (c) private hospitals' rehab departments; (d) condo management offices; (e) retirement communities in Chiang Mai.

Step 2 — Beachhead: (b). Interviews showed strong pain and worry; they already pay out of pocket; the team can reach them through two hospital discharge nurses they know; they talk to each other in caregiver LINE groups.

Step 3 — End user: a working daughter or son, 30–50, in Bangkok; parent aged 70+ living with or near them; a fall in the past 12 months; pays for some physiotherapy; worries during the workday.

Step 4 — Bottom-up TAM:

• Bangkok households with a parent aged 70+: 300,000 (invented — find an official count)
• Share where the parent had a fall in the past year: 20% → 60,000 households (invented — find a health source)
• What they would pay per year (based on what interviewees spend today on physio and help): ฿8,000 (invented — use your interview data)
• Beachhead TAM: 60,000 × ฿8,000 = ฿480 million per year

SAM: households the team can actually reach through its first channel (three partner hospitals' discharge programs): 6,000 households → ฿48 million per year.

SOM: what the team could realistically win in two years: 5% of SAM = 300 households → ฿2.4 million per year.

Sanity check: ฿480 million is a small fraction of the national elder-care spending figure the team found in a report. Plausible. And ฿2.4 million is a real, testable first target — 300 families you can name the route to.

Notice what happened. The fantasy number disappeared. Something more useful appeared: a list of numbers you now have to go and check.

That list is your research plan for the next few weeks.

What if my beachhead turns out to be too small?

Good. You found out cheaply. I would much rather you find out now than after a year of building.

A beachhead can be too small to sustain a company — but it is supposed to be small enough to win. If it is truly tiny, look at the next segment on your list and ask whether winning the first one makes the second easier. That chain — first pin, next pin — is your growth story.

If your beachhead looks too big, that is usually a sign it is not one segment. Split it until the people inside it really do buy the same way and talk to each other.

And you are allowed to change your beachhead. Write down why. Changing it because of evidence is progress. Changing it because a bigger number looks better on a slide is not.

Reality check

Choosing one beachhead feels like giving up on everyone else. I understand the feeling. Most teams resist it. They keep two or three segments "just in case," split their customer interviews across all of them, and end up with no clear pattern anywhere.

The numbers are hard too. Official data is often old, in the wrong format, or simply not there for your exact group. That is normal. Use the best dated source you can find, write down what you assumed, and say so out loud when you present it. I respect an honest "assumed" far more than a confident guess.

If you are struggling at this stage, you are like most people. It means you are doing the real work.

Frequently asked questions

What is a beachhead market?
The first narrow group of customers you focus on winning — people who buy similar things, in a similar way, and talk to each other — so that winning them helps you win the next group.

What is the difference between TAM, SAM and SOM?
TAM is the total yearly revenue if everyone in the market bought from you; SAM is the part you can actually serve; SOM is the part you can realistically win in the next one to three years.

How do you calculate TAM, SAM and SOM bottom-up?
Count the customers in your beachhead and multiply by what each would pay per year: that is the beachhead TAM. Keep only the customers your first channel can actually reach: that is SAM. Estimate the share you can realistically win in one to three years: that is SOM. Write a dated source or "assumed" next to every number.

Why is bottom-up market sizing better than top-down?
Bottom-up starts from real customers and real prices, so every number can be checked. Top-down "1% of a big market" tells you nothing about who will buy or why.

How big should a beachhead market be?
Big enough to matter, small enough to win with the time, money and network you have now. If you cannot name how you will reach the first customers, it is too big.

✦ A Note for Global Venture I Students · Block C: Understanding the Market · Form C

For TLV teams, the worked example above is the Touch track. If you are reading before customer interviews #1–3, mark scores "assumed" and score again after them. Write any change of beachhead, and why, in your Venture Log.

Your beachhead choice is one of the main outputs of Block C. It decides where your five customer interviews go, and it appears in your pitch deck, your Business Model Canvas (Customer Segments) and your 6-page memo.

Bring to the debrief: your segment list, your scoring, your one-sentence beachhead and your bottom-up TAM with a source for every number.

Do this before class:

1. List five to ten segments and score them on the seven questions above. Choose one beachhead and write it in one sentence in Form C.
2. Write the end-user profile and a bottom-up TAM (customers × price per year). Mark every number as "sourced" (with link and date) or "assumed." Put it all in your Venture Log.
3. Plan all five customer interviews inside the beachhead — not across several segments. Interviews #1–3 are due in Form C, #4–5 in Form D.

Pre-class check (in Form C):

1. What three things make a group of customers a beachhead market?
2. Why is "1% of a ฿50 billion market" a weak argument?
3. Which number in your TAM are you least sure about, and how will you check it?

Venture update (Form C): tool applied · what changed in the idea · evidence link · readiness self-score. Each member writes their own update, in their own words (your team keeps one Venture Log), and fills in the form's "AI used for:" field.

Your progress: Business model readiness to level 3 (a defined target market) · Customer readiness to level 3. How we measure progress

Skills you practice: Analytical thinking · spotting opportunities · vision.

Keep reading

• The Interview Masterclass — next in Block C: how to run the interviews inside your beachhead
• Market Research for Startups — Zoom In / Zoom Out before you choose
• First 10 Customers & Angel Customer™ — the person at the center of your beachhead
• Who Are Your Customers? — in Block D: who uses, who pays, who decides

All articles in Block C · Understanding the Market

• C-01 Market Research for Startups
• C-02 Choose Your Beachhead (you are here)
• C-03 The Interview Masterclass
• C-04 How Many Customer Interviews Are Enough?
• C-05 The Interview Log & Evidence Rules
• C-06 First 10 Customers & Angel Customer™
• C-07 Jobs-to-Be-Done
• C-08 The Competitive Reality Map™
• C-09 Double Diamond & Design Thinking

References

1. Martin Trust Center for MIT Entrepreneurship. (n.d.). Bill Aulet [Profile]. https://entrepreneurship.mit.edu/profile/bill-aulet — accessed Nov 2026.
2. Disciplined Entrepreneurship. (n.d.). Step 2: Select a beachhead market. https://www.d-eship.com/step2/ — accessed Nov 2026.
3. Sermsuksan, K.-T. (2023). The Startup Mindset [in Thai], Chapter 7. Wish Publishing (สำนักพิมพ์วิช). English: the author's own translation, not yet published.
4. Lean B2B. (n.d.). How to identify follow-on markets (with a great bowling analogy) (on Moore's bowling-pin strategy). https://leanb2bbook.com/blog/identify-follow-on-markets — accessed Nov 2026.

Further reading

• Aulet, B. (2013). Disciplined entrepreneurship: 24 steps to a successful startup. Wiley. (Steps 1–4.)
• Moore, G. A. (2014). Crossing the chasm (3rd ed.). HarperBusiness.
• Blank, S., & Dorf, B. (2012). The startup owner's manual: The step-by-step guide for building a great company. K&S Ranch.

About the author: Kasper-Tanakrit Sermsuksan is the Founder of SEA Bridge and a Visiting Lecturer in Computer Engineering and Digital Technology at Chulalongkorn University's Faculty of Engineering. Learn more →