Day 10: Build a Simple Financial Model From Scratch

The Point Of Today

A financial model is not a forecast of the future.

It is a map of your assumptions. It shows how the business might behave if certain things are true: customers pay this much, churn is this high, acquisition costs this amount, hiring happens in this month, and expenses grow at this rate. The model will be wrong. That is fine. The value is not in predicting perfectly. The value is in seeing which assumptions matter before the business pays for them.

Many entrepreneurs avoid financial modeling because they imagine a complicated spreadsheet with dozens of tabs. You do not need that yet. You need a simple model that answers three questions:

• How does the business make money?
• How long can it operate with current cash?
• Which assumptions most affect survival and growth?

Today you will use AI to create the first version. Then you will keep it simple enough to update monthly.

Start With The Business Model

The model should match how money actually enters the business.

A subscription company needs monthly recurring revenue, churn, expansion, new customers, and customer acquisition cost. A services company needs project size, delivery capacity, utilization, margin, and payment timing. A marketplace needs supply, demand, take rate, transaction volume, and liquidity. A usage-based product needs active usage, unit price, cost to serve, and variability.

Do not force your business into a SaaS template if it is not a SaaS business. The wrong model creates false confidence.

Ask AI to explain the revenue logic in plain English before it builds the numbers. If you cannot describe the model in one paragraph, the spreadsheet will not make it clearer.

Separate Assumptions From Calculations

A good model keeps assumptions visible.

Assumptions are inputs you can change: price, conversion rate, churn, gross margin, acquisition cost, hiring cost, payment timing, growth rate. Calculations use those inputs to produce outputs: revenue, expenses, runway, margin, cash balance, and break-even timing.

Put assumptions at the top of the spreadsheet. Use a different color if helpful. Then build calculations below. This makes the model easier to maintain and easier to discuss with advisors or investors.

The discipline is simple: never bury an assumption inside a formula.

If the growth rate is hidden in cell G47, nobody will notice when the model quietly depends on unrealistic growth. If it is visible at the top, the conversation becomes honest.

Runway Is A Decision Metric

Runway is the number of months you can keep operating before cash runs out.

Founders often know this number vaguely. They say, "We have about a year." But hiring, marketing spend, slower sales cycles, delayed invoices, and lower conversion can change runway quickly. A founder who checks runway only when the bank account feels uncomfortable has already lost optionality.

Runway supports decisions:

• When should we start fundraising?
• Can we hire now?
• Can we afford a paid acquisition experiment?
• Do we need to reduce expenses?
• How many months do we have to prove the next milestone?

If fundraising may take four to six months, a company with eight months of runway is already inside the planning window. The model should make that visible early.

Unit Economics Tell You Whether Growth Helps

Growth is not automatically good.

If the business loses money on each customer and has no credible path to improving that, growth accelerates the problem. Unit economics help you understand whether each customer is economically healthy.

For a simple model, focus on:

• Average revenue per customer
• Gross margin
• Customer acquisition cost
• Retention or repeat purchase
• Lifetime value
• Payback period

At the earliest stage, some of these numbers will be estimates. That is acceptable if they are labeled honestly. The model can also show what the numbers need to become.

For example, if the business works only when acquisition cost is below $100, but your first paid experiment suggests $350, that is not a reason to panic. It is a reason to rethink channel strategy, pricing, onboarding, or target customer before spending more.

Use Scenarios Instead Of One Fantasy

One projection is a story. Three scenarios are a planning tool.

Build:

• Conservative: growth is slower, costs are higher, sales take longer.
• Base: your current best estimate.
• Optimistic: things go well but remain believable.

The conservative case is especially useful. It tells you what decisions are required if reality is harder than expected. If the conservative case shows you running out of cash before a key milestone, you need to change the plan now.

Optimistic cases can be useful too, but only if they are grounded. A fantasy model with a perfect upward curve does not help the founder or impress investors.

Today's Practice

Run the prompt and build the first spreadsheet.

Do not wait for perfect numbers. Use current facts where you have them and labeled assumptions where you do not. Then identify the three assumptions with the biggest effect on the model.

Those assumptions become your operating questions for the next month. You might need to test willingness to pay, conversion rate, acquisition cost, churn, or delivery margin.

The model is not a finance assignment. It is a founder control panel. When maintained regularly, it gives you more time to make difficult decisions calmly, before the business forces them urgently.