How to Build a Simple 13-Week Cash Flow Model for Your Startup

Why is 13 Weeks the Right Starting Point?

For startups, cash is not just a number - it is survival, the lifeblood of their business. Many founders keep an eye on their bank balance but do not have a clear picture of where that number is heading in the weeks ahead. That is where a 13-week cash flow model comes in.

Why 13 weeks?

• It is long enough to spot a cash issue before it becomes a crisis

• It is short enough to provide actionable insights and adjust quickly

• It aligns with most business cycles (quarterly planning, investor updates, etc.)

A well built 13-week cash flow forecast is not just a spreadsheet. It is a decision making tool that gives you the confidence to manage hiring, vendor payments and growth investments.

Step 1: Start with Your Beginning Cash Balance

Your model starts with one number: the amount of cash you have in the bank today. If you have multiple accounts (e.g. an operating account and a savings account), consolidate those into a single starting figure. This is your foundation.

Step 2: Forecast Cash Inflows

List all the cash you expect to receive each week over the next 13 weeks. For most startups, inflows come from:

• Customer payments: Look at open invoices and payment history. Be realistic about when customers actually pay (not just looking at the invoice due dates).

• Other inflows: This might include loans, grants, tax credits or investor funding.

• Miscellaneous: Refunds or reimbursements.

Pro tip: Always be conservative with timing, assume some payments will slip by a week or two. The key is to have a realistic or conservative view of the business, and when payments come in ahead of schedule, you have some flexibility vs. the alternative of needing to scramble to push payments out if you are waiting for payments to come in.

Step 3: Forecast Cash Outflows

Next, list all the cash you plan to spend each week. Breaking this down into key categories can be done a few different of ways:

• High level: By looking at fixed vs. variable costs.

• Detailed: Breaking out the major expense buckets, such as:

  • Payroll and Benefits: Usually the largest expense and often biweekly or semi-monthly.

  • Rent and Utilities: Fixed recurring costs.

  • Vendors and Contractors: Be mindful of annual or quarterly payments that might hit suddenly.

  • Software Subscriptions: These can add up and some renew annually, which can be large outflows.

  • Miscellaneous Costs: Marketing campaigns, travel or unexpected expenses.

Step 4: Build Your 13-Week Model

Create a simple spreadsheet with 13 columns (one for each week) and rows for each inflow and outflow category. At the bottom, calculate:

• Total inflows per week

• Total outflows per week

• Net cash flow (inflows - outflows)

• Ending cash balance (previous week’s ending cash + net cash flow)

Step 5: Update Weekly

Your 13-week cash flow isn not “set and forget.” Update it every week by replacing forecasts with actual numbers. Then roll it forward another week so you are always looking 13 weeks ahead.

Step 6: Use It to Make Decisions

The real power of this model is how it guides your decisions. It can answer questions like:

• Do we have enough cash to hire this new role?

• Should we delay that vendor payment or negotiate terms?

• Do we need to speed up collections or start a fundraising process now?

A 13-week cash flow forecast gives you the time to course correct before problems hit. Cash flow management is not glamorous, but it is the foundation of every successful business.

Excelerate’s Role

At Excelerate, we provide fractional Finance and HR leadership for early stage startups. We help founders build the operational foundation for scale without the cost of full time executives.

As Co-Founder and a Fractional Chief Finance Officer I work with early stage teams to gain visibility, build discipline and make better decisions through strategic finance. Some of the things we can do to help are:

• Setting up tools and processes that founders can maintain.

• Ensuring cash flow connects with broader business goals (growth, margin, runway).

• Translating data into strategy.

You do not need a full time Finance team to do this well. At Excelerate we thrive in using advanced forecasting methods to help bridge the gap between art and science and make your models more actionable.

Let’s Talk

If you do not have a rolling 13 week cash flow forecast today, start now. It is one of the highest ROI activities a founder can do.

If you need help getting there, that is exactly what we do at Excelerate. Let’s talk, even if it is just to benchmark where you stand today. Reach us at hello@excelerate.work and check out our website, www.excelerate.work.